api_server.go

package worker

import (
	"bufio"
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"os/user"
	"path"
	"path/filepath"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

	etcd "github.com/coreos/etcd/clientv3"
	"github.com/gogo/protobuf/jsonpb"
	"github.com/gogo/protobuf/proto"
	"github.com/gogo/protobuf/types"
	lru "github.com/hashicorp/golang-lru"
	"golang.org/x/net/context"
	"golang.org/x/sync/errgroup"
	"gopkg.in/go-playground/webhooks.v3/github"
	"gopkg.in/src-d/go-git.v4"
	gitPlumbing "gopkg.in/src-d/go-git.v4/plumbing"
	kube "k8s.io/client-go/kubernetes"
	"k8s.io/client-go/rest"

	"github.com/fsouza/go-dockerclient"
	"github.com/pachyderm/pachyderm/src/client"
	"github.com/pachyderm/pachyderm/src/client/enterprise"
	"github.com/pachyderm/pachyderm/src/client/limit"
	"github.com/pachyderm/pachyderm/src/client/pfs"
	"github.com/pachyderm/pachyderm/src/client/pkg/grpcutil"
	"github.com/pachyderm/pachyderm/src/client/pps"
	"github.com/pachyderm/pachyderm/src/server/pkg/backoff"
	col "github.com/pachyderm/pachyderm/src/server/pkg/collection"
	"github.com/pachyderm/pachyderm/src/server/pkg/exec"
	"github.com/pachyderm/pachyderm/src/server/pkg/hashtree"
	"github.com/pachyderm/pachyderm/src/server/pkg/ppsdb"
	"github.com/pachyderm/pachyderm/src/server/pkg/ppsutil"
	filesync "github.com/pachyderm/pachyderm/src/server/pkg/sync"
	"github.com/pachyderm/pachyderm/src/server/pkg/uuid"
	"github.com/pachyderm/pachyderm/src/server/pkg/watch"
)

const (
	// The maximum number of concurrent download/upload operations
	concurrency = 10
	logBuffer   = 25

	chunksPrefix = "/chunks"
	lockPrefix   = "/locks"
)

var (
	errSpecialFile = errors.New("cannot upload special file")
	statsTagSuffix = "_stats"
)

// APIServer implements the worker API
type APIServer struct {
	pachClient *client.APIClient
	kubeClient *kube.Clientset
	etcdClient *etcd.Client
	etcdPrefix string

	// Information needed to process input data and upload output
	pipelineInfo *pps.PipelineInfo

	// Information attached to log lines
	logMsgTemplate pps.LogMessage

	// The k8s pod name of this worker
	workerName string

	statusMu sync.Mutex

	// The currently running job ID
	jobID string
	// The currently running data
	data []*Input
	// The time we started the currently running
	started time.Time
	// Func to cancel the currently running datum
	cancel func()
	// Stats about the execution of the job
	stats *pps.ProcessStats
	// queueSize is the number of items enqueued
	queueSize int64

	// The total number of workers for this pipeline
	numWorkers int
	// The namespace in which pachyderm is deployed
	namespace string
	// The jobs collection
	jobs col.Collection
	// The pipelines collection
	pipelines col.Collection
	// The progress collection
	chunks col.Collection

	// Only one datum can be running at a time because they need to be
	// accessing /pfs, runMu enforces this
	runMu sync.Mutex

	// datumCache is used by the master to keep track of the datums that
	// have already been processed.
	datumCache *lru.Cache

	// We only export application statistics if enterprise is enabled
	exportStats bool

	uid uint32
	gid uint32
}

type putObjectResponse struct {
	object *pfs.Object
	size   int64
	err    error
}

type taggedLogger struct {
	template     pps.LogMessage
	stderrLog    log.Logger
	marshaler    *jsonpb.Marshaler
	buffer       bytes.Buffer
	putObjClient pfs.ObjectAPI_PutObjectClient
	objSize      int64
	msgCh        chan string
	eg           errgroup.Group
}

// DatumID computes the id for a datum, this value is used in ListDatum and
// InspectDatum.
func (a *APIServer) DatumID(data []*Input) string {
	hash := sha256.New()
	for _, d := range data {
		hash.Write([]byte(d.FileInfo.File.Path))
		hash.Write(d.FileInfo.Hash)
	}
	// InputFileID is a single string id for the data from this input, it's used in logs and in
	// the statsTree
	return hex.EncodeToString(hash.Sum(nil))
}

func (a *APIServer) getTaggedLogger(pachClient *client.APIClient, jobID string, data []*Input, enableStats bool) (*taggedLogger, error) {
	result := &taggedLogger{
		template:  a.logMsgTemplate, // Copy struct
		stderrLog: log.Logger{},
		marshaler: &jsonpb.Marshaler{},
		msgCh:     make(chan string, logBuffer),
	}
	result.stderrLog.SetOutput(os.Stderr)
	result.stderrLog.SetFlags(log.LstdFlags | log.Llongfile) // Log file/line

	// Add Job ID to log metadata
	result.template.JobID = jobID

	// Add inputs' details to log metadata, so we can find these logs later
	for _, d := range data {
		result.template.Data = append(result.template.Data, &pps.InputFile{
			Path: d.FileInfo.File.Path,
			Hash: d.FileInfo.Hash,
		})
	}
	// InputFileID is a single string id for the data from this input, it's used in logs and in
	// the statsTree
	result.template.DatumID = a.DatumID(data)
	if enableStats {
		putObjClient, err := pachClient.ObjectAPIClient.PutObject(pachClient.Ctx())
		if err != nil {
			return nil, err
		}
		result.putObjClient = putObjClient
		result.eg.Go(func() error {
			for msg := range result.msgCh {
				for _, chunk := range grpcutil.Chunk([]byte(msg), grpcutil.MaxMsgSize/2) {
					if err := result.putObjClient.Send(&pfs.PutObjectRequest{
						Value: chunk,
					}); err != nil && err != io.EOF {
						return err
					}
				}
				result.objSize += int64(len(msg))
			}
			return nil
		})
	}
	return result, nil
}

// Logf logs the line Sprintf(formatString, args...), but formatted as a json
// message and annotated with all of the metadata stored in 'loginfo'.
//
// Note: this is not thread-safe, as it modifies fields of 'logger.template'
func (logger *taggedLogger) Logf(formatString string, args ...interface{}) {
	logger.template.Message = fmt.Sprintf(formatString, args...)
	if ts, err := types.TimestampProto(time.Now()); err == nil {
		logger.template.Ts = ts
	} else {
		logger.stderrLog.Printf("could not generate logging timestamp: %s\n", err)
		return
	}
	msg, err := logger.marshaler.MarshalToString(&logger.template)
	if err != nil {
		logger.stderrLog.Printf("could not marshal %v for logging: %s\n", &logger.template, err)
		return
	}
	fmt.Println(msg)
	if logger.putObjClient != nil {
		logger.msgCh <- msg + "\n"
	}
}

func (logger *taggedLogger) Write(p []byte) (_ int, retErr error) {
	// never errors
	logger.buffer.Write(p)
	r := bufio.NewReader(&logger.buffer)
	for {
		message, err := r.ReadString('\n')
		if err != nil {
			if err == io.EOF {
				logger.buffer.Write([]byte(message))
				return len(p), nil
			}
			// this shouldn't technically be possible to hit io.EOF should be
			// the only error bufio.Reader can return when using a buffer.
			return 0, fmt.Errorf("error ReadString: %v", err)
		}
		// We don't want to make this call as:
		// logger.Logf(message)
		// because if the message has format characters like %s in it those
		// will result in errors being logged.
		logger.Logf("%s", strings.TrimSuffix(message, "\n"))
	}
}

func (logger *taggedLogger) Close() (*pfs.Object, int64, error) {
	close(logger.msgCh)
	if logger.putObjClient != nil {
		if err := logger.eg.Wait(); err != nil {
			return nil, 0, err
		}
		object, err := logger.putObjClient.CloseAndRecv()
		// we set putObjClient to nil so that future calls to Logf won't send
		// msg down logger.msgCh as we've just closed that channel.
		logger.putObjClient = nil
		return object, logger.objSize, err
	}
	return nil, 0, nil
}

func (logger *taggedLogger) clone() *taggedLogger {
	return &taggedLogger{
		template:     logger.template, // Copy struct
		stderrLog:    log.Logger{},
		marshaler:    &jsonpb.Marshaler{},
		putObjClient: logger.putObjClient,
		msgCh:        logger.msgCh,
	}
}

func (logger *taggedLogger) userLogger() *taggedLogger {
	result := logger.clone()
	result.template.User = true
	return result
}

// NewAPIServer creates an APIServer for a given pipeline
func NewAPIServer(pachClient *client.APIClient, etcdClient *etcd.Client, etcdPrefix string, pipelineInfo *pps.PipelineInfo, workerName string, namespace string) (*APIServer, error) {
	initPrometheus()
	cfg, err := rest.InClusterConfig()
	if err != nil {
		return nil, err
	}
	kubeClient, err := kube.NewForConfig(cfg)
	if err != nil {
		return nil, err
	}
	datumCache, err := lru.New(numCachedDatums)
	if err != nil {
		return nil, fmt.Errorf("error creating datum cache: %v", err)
	}
	server := &APIServer{
		pachClient:   pachClient,
		kubeClient:   kubeClient,
		etcdClient:   etcdClient,
		etcdPrefix:   etcdPrefix,
		pipelineInfo: pipelineInfo,
		logMsgTemplate: pps.LogMessage{
			PipelineName: pipelineInfo.Pipeline.Name,
			WorkerID:     os.Getenv(client.PPSPodNameEnv),
		},
		workerName: workerName,
		namespace:  namespace,
		jobs:       ppsdb.Jobs(etcdClient, etcdPrefix),
		pipelines:  ppsdb.Pipelines(etcdClient, etcdPrefix),
		chunks:     col.NewCollection(etcdClient, path.Join(etcdPrefix, chunksPrefix), nil, &Chunks{}, nil, nil),
		datumCache: datumCache,
	}
	logger, err := server.getTaggedLogger(pachClient, "", nil, false)
	if err != nil {
		return nil, err
	}
	resp, err := pachClient.Enterprise.GetState(context.Background(), &enterprise.GetStateRequest{})
	if err != nil {
		logger.Logf("failed to get enterprise state with error: %v\n", err)
	} else {
		server.exportStats = resp.State == enterprise.State_ACTIVE
	}
	numWorkers, err := ppsutil.GetExpectedNumWorkers(kubeClient, pipelineInfo.ParallelismSpec)
	if err != nil {
		logger.Logf("error getting number of workers, default to 1 worker: %v", err)
		numWorkers = 1
	}
	server.numWorkers = numWorkers
	var noDocker bool
	if _, err := os.Stat("/var/run/docker.sock"); err != nil {
		noDocker = true
	}
	if pipelineInfo.Transform.Image != "" && !noDocker {
		docker, err := docker.NewClientFromEnv()
		if err != nil {
			return nil, err
		}
		image, err := docker.InspectImage(pipelineInfo.Transform.Image)
		if err != nil {
			return nil, fmt.Errorf("error inspecting image %s: %+v", pipelineInfo.Transform.Image, err)
		}
		if pipelineInfo.Transform.User == "" {
			pipelineInfo.Transform.User = image.Config.User
		}
		if pipelineInfo.Transform.WorkingDir == "" {
			pipelineInfo.Transform.WorkingDir = image.Config.WorkingDir
		}
		if server.pipelineInfo.Transform.Cmd == nil {
			server.pipelineInfo.Transform.Cmd = image.Config.Entrypoint
		}
	}
	if pipelineInfo.Transform.User != "" {
		user, err := lookupUser(pipelineInfo.Transform.User)
		if err != nil && !os.IsNotExist(err) {
			return nil, err
		}
		// if User == "" then uid, and gid don't get set which
		// means they default to a value of 0 which means we run the code as
		// root which is the only sane default.
		if user != nil { // user is nil when os.IsNotExist(err) is true in which case we use root
			uid, err := strconv.ParseUint(user.Uid, 10, 32)
			if err != nil {
				return nil, err
			}
			server.uid = uint32(uid)
			gid, err := strconv.ParseUint(user.Gid, 10, 32)
			if err != nil {
				return nil, err
			}
			server.gid = uint32(gid)
		}
	}
	if pipelineInfo.Service == nil {
		go server.master()
	} else {
		go server.serviceMaster()
	}
	go server.worker()
	return server, nil
}

func (a *APIServer) downloadGitData(pachClient *client.APIClient, dir string, input *Input) error {
	file := input.FileInfo.File
	pachydermRepoName := input.Name
	var rawJSON bytes.Buffer
	err := pachClient.GetFile(pachydermRepoName, file.Commit.ID, "commit.json", 0, 0, &rawJSON)
	if err != nil {
		return err
	}
	var payload github.PushPayload
	err = json.Unmarshal(rawJSON.Bytes(), &payload)
	if err != nil {
		return err
	}
	sha := payload.After
	// Clone checks out a reference, not a SHA
	r, err := git.PlainClone(
		filepath.Join(dir, pachydermRepoName),
		false,
		&git.CloneOptions{
			URL:           payload.Repository.CloneURL,
			SingleBranch:  true,
			ReferenceName: gitPlumbing.ReferenceName(payload.Ref),
		},
	)
	if err != nil {
		return fmt.Errorf("error cloning repo %v at SHA %v from URL %v: %v", pachydermRepoName, sha, input.GitURL, err)
	}
	hash := gitPlumbing.NewHash(sha)
	wt, err := r.Worktree()
	if err != nil {
		return err
	}
	err = wt.Checkout(&git.CheckoutOptions{Hash: hash})
	if err != nil {
		return fmt.Errorf("error checking out SHA %v from repo %v: %v", sha, pachydermRepoName, err)
	}
	return nil
}

func (a *APIServer) reportDownloadSizeStats(downSize float64, logger *taggedLogger) {

	if a.exportStats {
		if hist, err := datumDownloadSize.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get histogram w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			hist.Observe(downSize)
		}
		if counter, err := datumDownloadBytesCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(downSize)
		}
	}
}

func (a *APIServer) reportDownloadTimeStats(start time.Time, stats *pps.ProcessStats, logger *taggedLogger) {
	duration := time.Since(start)
	stats.DownloadTime = types.DurationProto(duration)
	if a.exportStats {
		if hist, err := datumDownloadTime.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get histogram w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			hist.Observe(duration.Seconds())
		}
		if counter, err := datumDownloadSecondsCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(duration.Seconds())
		}
	}
}

func (a *APIServer) downloadData(pachClient *client.APIClient, logger *taggedLogger, inputs []*Input, puller *filesync.Puller, parentTag *pfs.Tag, stats *pps.ProcessStats, statsTree hashtree.OpenHashTree, statsPath string) (_ string, retErr error) {
	defer a.reportDownloadTimeStats(time.Now(), stats, logger)
	logger.Logf("starting to download data")
	defer func(start time.Time) {
		if retErr != nil {
			logger.Logf("errored downloading data after %v: %v", time.Since(start), retErr)
		} else {
			logger.Logf("finished downloading data after %v", time.Since(start))
		}
	}(time.Now())
	dir := filepath.Join(client.PPSScratchSpace, uuid.NewWithoutDashes())
	// Create output directory (currently /pfs/out)
	if err := os.MkdirAll(filepath.Join(dir, "out"), 0777); err != nil {
		return "", err
	}
	var incremental bool
	if parentTag != nil {
		if err := func() error {
			var buffer bytes.Buffer
			if err := pachClient.GetTag(parentTag.Name, &buffer); err != nil {
				// This likely means that the parent job errored in some way,
				// this doesn't prevent us from running the job, it just means
				// we have to run it in an un-incremental fashion, as if this
				// were the first input commit.
				return nil
			}
			incremental = true
			tree, err := hashtree.Deserialize(buffer.Bytes())
			if err != nil {
				return fmt.Errorf("failed to deserialize parent hashtree: %v", err)
			}
			if err := puller.PullTree(pachClient, path.Join(dir, "out"), tree, false, concurrency); err != nil {
				return fmt.Errorf("error pulling output tree: %+v", err)
			}
			return nil
		}(); err != nil {
			return "", err
		}
	}
	for _, input := range inputs {
		if input.GitURL != "" {
			if err := a.downloadGitData(pachClient, dir, input); err != nil {
				return "", err
			}
			continue
		}
		file := input.FileInfo.File
		root := filepath.Join(dir, input.Name, file.Path)
		treeRoot := path.Join(statsPath, input.Name, file.Path)
		if incremental && input.ParentCommit != nil {
			if err := puller.PullDiff(pachClient, root,
				file.Commit.Repo.Name, file.Commit.ID, file.Path,
				input.ParentCommit.Repo.Name, input.ParentCommit.ID, file.Path,
				true, input.Lazy, input.EmptyFiles, concurrency, statsTree, treeRoot); err != nil {
				return "", err
			}
		} else {
			if err := puller.Pull(pachClient, root, file.Commit.Repo.Name, file.Commit.ID, file.Path, input.Lazy, input.EmptyFiles, concurrency, statsTree, treeRoot); err != nil {
				return "", err
			}
		}
	}
	return dir, nil
}

func (a *APIServer) linkData(inputs []*Input, dir string) error {
	for _, input := range inputs {
		src := filepath.Join(dir, input.Name)
		dst := filepath.Join(client.PPSInputPrefix, input.Name)
		if err := os.Symlink(src, dst); err != nil {
			return err
		}
	}
	return os.Symlink(filepath.Join(dir, "out"), filepath.Join(client.PPSInputPrefix, "out"))
}

func (a *APIServer) unlinkData(inputs []*Input) error {
	for _, input := range inputs {
		if err := os.RemoveAll(filepath.Join(client.PPSInputPrefix, input.Name)); err != nil {
			return err
		}
	}
	return os.RemoveAll(filepath.Join(client.PPSInputPrefix, "out"))
}

func (a *APIServer) reportUserCodeStats(logger *taggedLogger) {
	if a.exportStats {
		if counter, err := datumCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID, "started"); err != nil {
			logger.Logf("failed to get histogram w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(1)
		}
	}
}

func (a *APIServer) reportDeferredUserCodeStats(err error, start time.Time, stats *pps.ProcessStats, logger *taggedLogger) {
	duration := time.Since(start)
	stats.ProcessTime = types.DurationProto(duration)
	if a.exportStats {
		state := "errored"
		if err == nil {
			state = "finished"
		}
		if counter, err := datumCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID, state); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) state (%v) with error %v", a.pipelineInfo.ID, a.jobID, state, err)
		} else {
			counter.Add(1)
		}
		if hist, err := datumProcTime.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID, state); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) state (%v) with error %v", a.pipelineInfo.ID, a.jobID, state, err)
		} else {
			hist.Observe(duration.Seconds())
		}
		if counter, err := datumProcSecondsCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(duration.Seconds())
		}
	}
}

// Run user code and return the combined output of stdout and stderr.
func (a *APIServer) runUserCode(ctx context.Context, logger *taggedLogger, environ []string, stats *pps.ProcessStats, rawDatumTimeout *types.Duration) (retErr error) {
	a.reportUserCodeStats(logger)
	defer func(start time.Time) { a.reportDeferredUserCodeStats(retErr, start, stats, logger) }(time.Now())
	logger.Logf("beginning to run user code")
	defer func(start time.Time) {
		if retErr != nil {
			logger.Logf("errored running user code after %v: %v", time.Since(start), retErr)
		} else {
			logger.Logf("finished running user code after %v", time.Since(start))
		}
	}(time.Now())
	if rawDatumTimeout != nil {
		datumTimeout, err := types.DurationFromProto(rawDatumTimeout)
		if err != nil {
			return err
		}
		datumTimeoutCtx, cancel := context.WithTimeout(ctx, datumTimeout)
		defer cancel()
		ctx = datumTimeoutCtx
	}

	// Run user code
	cmd := exec.CommandContext(ctx, a.pipelineInfo.Transform.Cmd[0], a.pipelineInfo.Transform.Cmd[1:]...)
	if a.pipelineInfo.Transform.Stdin != nil {
		cmd.Stdin = strings.NewReader(strings.Join(a.pipelineInfo.Transform.Stdin, "\n") + "\n")
	}
	cmd.Stdout = logger.userLogger()
	cmd.Stderr = logger.userLogger()
	cmd.Env = environ
	cmd.SysProcAttr = &syscall.SysProcAttr{
		Credential: &syscall.Credential{
			Uid: a.uid,
			Gid: a.gid,
		},
	}
	cmd.Dir = a.pipelineInfo.Transform.WorkingDir
	err := cmd.Start()
	if err != nil {
		return fmt.Errorf("error cmd.Start: %v", err)
	}
	// A context w a deadline will successfully cancel/kill
	// the running process (minus zombies)
	state, err := cmd.Process.Wait()
	if err != nil {
		return fmt.Errorf("error cmd.Wait: %v", err)
	}
	if isDone(ctx) {
		if err = ctx.Err(); err != nil {
			return err
		}
	}

	// Because of this issue: https://github.com/golang/go/issues/18874
	// We forked os/exec so that we can call just the part of cmd.Wait() that
	// happens after blocking on the process. Unfortunately calling
	// cmd.Process.Wait() then cmd.Wait() will produce an error. So instead we
	// close the IO using this helper
	err = cmd.WaitIO(state, err)
	// We ignore broken pipe errors, these occur very occasionally if a user
	// specifies Stdin but their process doesn't actually read everything from
	// Stdin. This is a fairly common thing to do, bash by default ignores
	// broken pipe errors.
	if err != nil && !strings.Contains(err.Error(), "broken pipe") {
		// (if err is an acceptable return code, don't return err)
		if exiterr, ok := err.(*exec.ExitError); ok {
			if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
				for _, returnCode := range a.pipelineInfo.Transform.AcceptReturnCode {
					if int(returnCode) == status.ExitStatus() {
						return nil
					}
				}
			}
		}
		return fmt.Errorf("error cmd.WaitIO: %v", err)
	}
	return nil
}

func (a *APIServer) reportUploadStats(start time.Time, stats *pps.ProcessStats, logger *taggedLogger) {
	duration := time.Since(start)
	stats.UploadTime = types.DurationProto(duration)
	if a.exportStats {
		if hist, err := datumUploadTime.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get histogram w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			hist.Observe(duration.Seconds())
		}
		if counter, err := datumUploadSecondsCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(duration.Seconds())
		}
		if hist, err := datumUploadSize.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get histogram w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			hist.Observe(float64(stats.UploadBytes))
		}
		if counter, err := datumUploadBytesCount.GetMetricWithLabelValues(a.pipelineInfo.ID, a.jobID); err != nil {
			logger.Logf("failed to get counter w labels: pipeline (%v) job (%v) with error %v", a.pipelineInfo.ID, a.jobID, err)
		} else {
			counter.Add(float64(stats.UploadBytes))
		}
	}
}

func (a *APIServer) uploadOutput(pachClient *client.APIClient, dir string, tag string, logger *taggedLogger, inputs []*Input, stats *pps.ProcessStats, statsTree hashtree.OpenHashTree, statsRoot string) (retErr error) {
	defer a.reportUploadStats(time.Now(), stats, logger)
	logger.Logf("starting to upload output")
	defer func(start time.Time) {
		if retErr != nil {
			logger.Logf("errored uploading output after %v: %v", time.Since(start), retErr)
		} else {
			logger.Logf("finished uploading output after %v", time.Since(start))
		}
	}(time.Now())
	// hashtree is not thread-safe--guard with 'lock'
	var lock sync.Mutex
	tree := hashtree.NewHashTree()
	outputPath := filepath.Join(dir, "out")

	// Upload all files in output directory
	var g errgroup.Group
	limiter := limit.New(concurrency)
	if err := filepath.Walk(outputPath, func(filePath string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		g.Go(func() (retErr error) {
			limiter.Acquire()
			defer limiter.Release()
			if filePath == outputPath {
				return nil
			}

			relPath, err := filepath.Rel(outputPath, filePath)
			if err != nil {
				return err
			}

			// Put directory. Even if the directory is empty, that may be useful to
			// users
			// TODO(msteffen) write a test pipeline that outputs an empty directory and
			// make sure it's preserved
			if info.IsDir() {
				lock.Lock()
				defer lock.Unlock()
				tree.PutDir(relPath)
				return nil
			}

			// Under some circumstances, the user might have copied
			// some pipes from the input directory to the output directory.
			// Reading from these files will result in job blocking.  Thus
			// we preemptively detect if the file is a named pipe.
			if (info.Mode() & os.ModeNamedPipe) > 0 {
				logger.Logf("cannot upload named pipe: %v", relPath)
				return errSpecialFile
			}

			// If the output file is a symlink to an input file, we can skip
			// the uploading.
			if (info.Mode() & os.ModeSymlink) > 0 {
				realPath, err := os.Readlink(filePath)
				if err != nil {
					return err
				}
				if strings.HasPrefix(realPath, client.PPSInputPrefix) {
					var pathWithInput string
					var err error
					if strings.HasPrefix(realPath, dir) {
						pathWithInput, err = filepath.Rel(dir, realPath)
					} else {
						pathWithInput, err = filepath.Rel(client.PPSInputPrefix, realPath)
					}
					if err == nil {
						// We can only skip the upload if the real path is
						// under /pfs, meaning that it's a file that already
						// exists in PFS.

						// The name of the input
						inputName := strings.Split(pathWithInput, string(os.PathSeparator))[0]
						var input *Input
						for _, i := range inputs {
							if i.Name == inputName {
								input = i
							}
						}
						// this changes realPath from `/pfs/input/...` to `/scratch/<id>/input/...`
						realPath = filepath.Join(dir, pathWithInput)
						if input != nil {
							return filepath.Walk(realPath, func(filePath string, info os.FileInfo, err error) error {
								if err != nil {
									return err
								}
								rel, err := filepath.Rel(realPath, filePath)
								if err != nil {
									return err
								}
								subRelPath := filepath.Join(relPath, rel)
								// The path of the input file
								pfsPath, err := filepath.Rel(filepath.Join(dir, input.Name), filePath)
								if err != nil {
									return err
								}

								if info.IsDir() {
									lock.Lock()
									defer lock.Unlock()
									tree.PutDir(subRelPath)
									return nil
								}

								fc := input.FileInfo.File.Commit
								fileInfo, err := pachClient.InspectFile(fc.Repo.Name, fc.ID, pfsPath)
								if err != nil {
									return err
								}

								lock.Lock()
								defer lock.Unlock()
								if statsTree != nil {
									if err := statsTree.PutFile(path.Join(statsRoot, subRelPath), fileInfo.Objects, int64(fileInfo.SizeBytes)); err != nil {
										return err
									}
								}
								return tree.PutFile(subRelPath, fileInfo.Objects, int64(fileInfo.SizeBytes))
							})
						}
					}
				}
			}

			f, err := os.Open(filePath)
			if err != nil {
				return fmt.Errorf("os.Open(%s): %v", filePath, err)
			}
			defer func() {
				if err := f.Close(); err != nil && retErr == nil {
					retErr = err
				}
			}()

			putObjClient, err := pachClient.ObjectAPIClient.PutObject(pachClient.Ctx())
			if err != nil {
				return err
			}
			size, err := grpcutil.ChunkReader(f, func(chunk []byte) error {
				return putObjClient.Send(&pfs.PutObjectRequest{
					Value: chunk,
				})
			})
			if err != nil {
				return fmt.Errorf("Read(%s): %v", filePath, err)
			}
			object, err := putObjClient.CloseAndRecv()
			if err != nil {
				return err
			}

			lock.Lock()
			defer lock.Unlock()
			atomic.AddUint64(&stats.UploadBytes, uint64(size))
			if statsTree != nil {
				if err := statsTree.PutFile(path.Join(statsRoot, relPath), []*pfs.Object{object}, int64(size)); err != nil {
					return err
				}
			}
			return tree.PutFile(relPath, []*pfs.Object{object}, int64(size))
		})
		return nil
	}); err != nil {
		return fmt.Errorf("error walking output: %v", err)
	}

	if err := g.Wait(); err != nil {
		return err
	}

	finTree, err := tree.Finish()
	if err != nil {
		return err
	}

	treeBytes, err := hashtree.Serialize(finTree)
	if err != nil {
		return err
	}

	if _, _, err := pachClient.PutObject(bytes.NewReader(treeBytes), tag); err != nil {
		return err
	}

	return nil
}

// HashDatum computes and returns the hash of datum + pipeline, with a
// pipeline-specific prefix.
func HashDatum(pipelineName string, pipelineSalt string, data []*Input) string {
	hash := sha256.New()
	for _, datum := range data {
		hash.Write([]byte(datum.Name))
		hash.Write([]byte(datum.FileInfo.File.Path))
		hash.Write(datum.FileInfo.Hash)
	}

	hash.Write([]byte(pipelineName))
	hash.Write([]byte(pipelineSalt))

	return client.DatumTagPrefix(pipelineSalt) + hex.EncodeToString(hash.Sum(nil))
}

// HashDatum15 computes and returns the hash of datum + pipeline for version <= 1.5.0, with a
// pipeline-specific prefix.
func HashDatum15(pipelineInfo *pps.PipelineInfo, data []*Input) (string, error) {
	hash := sha256.New()
	for _, datum := range data {
		hash.Write([]byte(datum.Name))
		hash.Write([]byte(datum.FileInfo.File.Path))
		hash.Write(datum.FileInfo.Hash)
	}

	// We set env to nil because if env contains more than one elements,
	// since it's a map, the output of Marshal() can be non-deterministic.
	env := pipelineInfo.Transform.Env
	pipelineInfo.Transform.Env = nil
	defer func() {
		pipelineInfo.Transform.Env = env
	}()
	bytes, err := pipelineInfo.Transform.Marshal()
	if err != nil {
		return "", err
	}
	hash.Write(bytes)
	hash.Write([]byte(pipelineInfo.Pipeline.Name))
	hash.Write([]byte(pipelineInfo.ID))
	hash.Write([]byte(strconv.Itoa(int(pipelineInfo.Version))))

	// Note in 1.5.0 this function was called HashPipelineID, it's now called
	// HashPipelineName but it has the same implementation.
	return client.DatumTagPrefix(pipelineInfo.ID) + hex.EncodeToString(hash.Sum(nil)), nil
}

// Status returns the status of the current worker.
func (a *APIServer) Status(ctx context.Context, _ *types.Empty) (*pps.WorkerStatus, error) {
	a.statusMu.Lock()
	defer a.statusMu.Unlock()
	started, err := types.TimestampProto(a.started)
	if err != nil {
		return nil, err
	}
	result := &pps.WorkerStatus{
		JobID:     a.jobID,
		WorkerID:  a.workerName,
		Started:   started,
		Data:      a.datum(),
		QueueSize: atomic.LoadInt64(&a.queueSize),
	}
	return result, nil
}

// Cancel cancels the currently running datum
func (a *APIServer) Cancel(ctx context.Context, request *CancelRequest) (*CancelResponse, error) {
	a.statusMu.Lock()
	defer a.statusMu.Unlock()
	if request.JobID != a.jobID {
		return &CancelResponse{Success: false}, nil
	}
	if !MatchDatum(request.DataFilters, a.datum()) {
		return &CancelResponse{Success: false}, nil
	}
	a.cancel()
	// clear the status since we're no longer processing this datum
	a.jobID = ""
	a.data = nil
	a.started = time.Time{}
	a.cancel = nil
	return &CancelResponse{Success: true}, nil
}

func (a *APIServer) datum() []*pps.InputFile {
	var result []*pps.InputFile
	for _, datum := range a.data {
		result = append(result, &pps.InputFile{
			Path: datum.FileInfo.File.Path,
			Hash: datum.FileInfo.Hash,
		})
	}
	return result
}

func (a *APIServer) userCodeEnv(jobID string, data []*Input) []string {
	result := os.Environ()
	for _, input := range data {
		result = append(result, fmt.Sprintf("%s=%s", input.Name, filepath.Join(client.PPSInputPrefix, input.Name, input.FileInfo.File.Path)))
	}
	result = append(result, fmt.Sprintf("PACH_JOB_ID=%s", jobID))
	return result
}

// deleteJob is identical to updateJobState, except that jobPtr points to a job
// that should be deleted rather than marked failed. Jobs may be deleted if
// their output commit is deleted.
func (a *APIServer) deleteJob(stm col.STM, jobPtr *pps.EtcdJobInfo) error {
	pipelinePtr := &pps.EtcdPipelineInfo{}
	if err := a.pipelines.ReadWrite(stm).Update(jobPtr.Pipeline.Name, pipelinePtr, func() error {
		if pipelinePtr.JobCounts == nil {
			pipelinePtr.JobCounts = make(map[int32]int32)
		}
		if pipelinePtr.JobCounts[int32(jobPtr.State)] != 0 {
			pipelinePtr.JobCounts[int32(jobPtr.State)]--
		}
		return nil
	}); err != nil {
		return err
	}
	return a.jobs.ReadWrite(stm).Delete(jobPtr.Job.ID)
}

type processResult struct {
	failedDatumID   string
	datumsProcessed int64
	datumsSkipped   int64
	datumsFailed    int64
}

type processFunc func(low, high int64) (*processResult, error)

func (a *APIServer) acquireDatums(ctx context.Context, jobID string, chunks *Chunks, logger *taggedLogger, process processFunc) error {
	complete := false
	for !complete {
		// func to defer cancel in
		if err := func() error {
			ctx, cancel := context.WithCancel(ctx)
			defer cancel()
			var low int64
			var high int64
			var found bool
			if _, err := col.NewSTM(ctx, a.etcdClient, func(stm col.STM) error {
				// Reinitialize closed upon variables.
				low, high = 0, 0
				found = false
				locks := a.locks(jobID).ReadWrite(stm)
				// we set complete to true and then unset it if we find an incomplete chunk
				complete = true
				for _, high = range chunks.Chunks {
					var chunkState ChunkState
					if err := locks.Get(fmt.Sprint(high), &chunkState); err != nil {
						if col.IsErrNotFound(err) {
							found = true
						} else {
							return err
						}
					}
					// This gets triggered either if we found a chunk that wasn't
					// complete or if we didn't find a chunk at all.
					if chunkState.State == ChunkState_RUNNING {
						complete = false
					}
					if found {
						break
					}
					low = high
				}
				if found {
					return locks.PutTTL(fmt.Sprint(high), &ChunkState{State: ChunkState_RUNNING}, ttl)
				}
				return nil
			}); err != nil {
				return err
			}
			if found {
				go func() {
				Renew:
					for {
						select {
						case <-time.After((time.Second * time.Duration(ttl)) / 2):
						case <-ctx.Done():
							break Renew
						}
						if _, err := col.NewSTM(ctx, a.etcdClient, func(stm col.STM) error {
							locks := a.locks(jobID).ReadWrite(stm)
							var chunkState ChunkState
							if err := locks.Get(fmt.Sprint(high), &chunkState); err != nil {
								return err
							}
							if chunkState.State == ChunkState_RUNNING {
								return locks.PutTTL(fmt.Sprint(high), &chunkState, ttl)
							}
							return nil
						}); err != nil {
							cancel()
							logger.Logf("failed to renew lock: %v", err)
						}
					}
				}()
				// process the datums in newRange
				processResult, err := process(low, high)
				if err != nil {
					return err
				}

				if _, err := col.NewSTM(ctx, a.etcdClient, func(stm col.STM) error {
					jobs := a.jobs.ReadWrite(stm)
					jobPtr := &pps.EtcdJobInfo{}
					if err := jobs.Update(jobID, jobPtr, func() error {
						jobPtr.DataProcessed += processResult.datumsProcessed
						jobPtr.DataSkipped += processResult.datumsSkipped
						jobPtr.DataFailed += processResult.datumsFailed
						return nil
					}); err != nil {
						return err
					}
					locks := a.locks(jobID).ReadWrite(stm)
					if processResult.failedDatumID != "" {
						return locks.Put(fmt.Sprint(high), &ChunkState{
							State:   ChunkState_FAILED,
							DatumID: processResult.failedDatumID,
						})
					}
					return locks.Put(fmt.Sprint(high), &ChunkState{State: ChunkState_COMPLETE})
				}); err != nil {
					return err
				}
			}
			return nil
		}(); err != nil {
			return err
		}
	}
	return nil
}

func isDone(ctx context.Context) bool {
	select {
	case <-ctx.Done():
		return true
	default:
		return false
	}
}

// cancelCtxIfJobFails watches jobID's JobPtr, and if its state is changed to a
// terminal state (KILLED, FAILED, or SUCCESS) cancel the jobCtx so we kill any
// user processes
func (a *APIServer) cancelCtxIfJobFails(jobCtx context.Context, jobCancel func(), jobID string) {
	logger := a.getWorkerLogger() // this worker's formatting logger

	backoff.RetryNotify(func() error {
		// Check if job was cancelled while backoff was sleeping
		if isDone(jobCtx) {
			return nil
		}

		// Start watching for job state changes
		watcher, err := a.jobs.ReadOnly(jobCtx).WatchOne(jobID)
		if err != nil {
			if col.IsErrNotFound(err) {
				jobCancel() // job deleted before we started--cancel the job ctx
				return nil
			}
			return fmt.Errorf("worker: could not create state watcher for job %s, err is %v", jobID, err)
		}

		// If any job events indicate that the job is done, cancel jobCtx
		for {
			select {
			case e := <-watcher.Watch():
				switch e.Type {
				case watch.EventPut:
					var jobID string
					jobPtr := &pps.EtcdJobInfo{}
					if err := e.Unmarshal(&jobID, jobPtr); err != nil {
						return fmt.Errorf("worker: error unmarshalling while watching job state (%v)", err)
					}
					if ppsutil.IsTerminal(jobPtr.State) {
						jobCancel() // cancel the job
					}
				case watch.EventDelete:
					jobCancel() // cancel the job
				case watch.EventError:
					return fmt.Errorf("job state watch error: %v", e.Err)
				}
			case <-jobCtx.Done():
				break
			}
		}
	}, backoff.NewInfiniteBackOff(), func(err error, d time.Duration) error {
		if jobCtx.Err() == context.Canceled {
			return err // worker is done, nothing else to do
		}
		logger.Logf("worker: error watching job %s (%v); retrying in %v", jobID, err, d)
		return nil
	})
}

// worker does the following:
//  - watches for new jobs (jobInfos in the jobs collection)
//  - claims chunks from the chunk layout it finds in the chunks collection
//  - claims those chunks with acquireDatums
//  - processes the chunks with processDatums
func (a *APIServer) worker() {
	logger := a.getWorkerLogger() // this worker's formatting logger

	// Process incoming jobs
	backoff.RetryNotify(func() error {
		retryCtx, retryCancel := context.WithCancel(a.pachClient.Ctx())
		defer retryCancel()
		watcher, err := a.jobs.ReadOnly(retryCtx).WatchByIndex(ppsdb.JobsPipelineIndex, a.pipelineInfo.Pipeline)
		if err != nil {
			return fmt.Errorf("error creating watch: %v", err)
		}
		defer watcher.Close()
	NextJob:
		for e := range watcher.Watch() {
			if e.Type == watch.EventError {
				return fmt.Errorf("worker watch error: %v", e.Err)
			} else if e.Type == watch.EventDelete {
				// Job was deleted, e.g. because input commit was deleted. This is
				// handled by cancelCtxIfJobFails goro, which was spawned when job was
				// created. Nothing to do here
				continue
			}

			// 'e' is a Put event -- new job
			var jobID string
			jobPtr := &pps.EtcdJobInfo{}
			if err := e.Unmarshal(&jobID, jobPtr); err != nil {
				return fmt.Errorf("error unmarshalling: %v", err)
			}
			if ppsutil.IsTerminal(jobPtr.State) {
				// previously-created job has finished, or job was finished during backoff
				// or in the 'watcher' queue
				logger.Logf("skipping job %v as it is already in state %v", jobID, jobPtr.State)
				continue NextJob
			}

			// create new ctx for this job, and don't use retryCtx as the
			// parent. Just because another job's etcd write failed doesn't
			// mean this job shouldn't run
			jobCtx, jobCancel := context.WithCancel(a.pachClient.Ctx())
			defer jobCancel() // cancel the job ctx
			pachClient := a.pachClient.WithCtx(jobCtx)

			//  Watch for any changes to EtcdJobInfo corresponding to jobID; if
			// the EtcdJobInfo is marked 'FAILED', call jobCancel().
			// ('watcher' above can't detect job state changes--it's watching
			// an index and so only emits when jobs are created or deleted).
			go a.cancelCtxIfJobFails(jobCtx, jobCancel, jobID)

			// Inspect the job and make sure it's relevant, as this worker may be old
			jobInfo, err := pachClient.InspectJob(jobID, false)
			if err != nil {
				if !col.IsErrNotFound(err) {
					continue NextJob // job was deleted--no sense retrying
				}
				return fmt.Errorf("error from InspectJob(%v): %+v", jobID, err)
			}
			if jobInfo.PipelineVersion < a.pipelineInfo.Version {
				continue
			}
			if jobInfo.PipelineVersion > a.pipelineInfo.Version {
				return fmt.Errorf("job %s's version (%d) greater than pipeline's "+
					"version (%d), this should automatically resolve when the worker "+
					"is updated", jobID, jobInfo.PipelineVersion, a.pipelineInfo.Version)
			}

			// Read the chunks laid out by the master and create the datum factory
			chunks := &Chunks{}
			if err := a.chunks.ReadOnly(jobCtx).GetBlock(jobInfo.Job.ID, chunks); err != nil {
				return err
			}
			df, err := NewDatumFactory(pachClient, jobInfo.Input)
			if err != nil {
				return fmt.Errorf("error from NewDatumFactory: %v", err)
			}

			// If a datum fails, acquireDatums updates the relevant lock in
			// etcd, which causes the master to fail the job (which is
			// handled above in the JOB_FAILURE case). There's no need to
			// handle failed datums here, just failed etcd writes.
			if err := a.acquireDatums(
				jobCtx, jobID, chunks, logger,
				func(low, high int64) (*processResult, error) {
					processResult, err := a.processDatums(pachClient, logger, jobInfo, df, low, high)
					if err != nil {
						return nil, err
					}
					return processResult, nil
				},
			); err != nil {
				if jobCtx.Err() == context.Canceled {
					continue NextJob // job cancelled--don't restart, just wait for next job
				}
				return fmt.Errorf("acquire/process datums for job %s exited with err: %v", jobID, err)
			}
		}
		return fmt.Errorf("worker: jobs.WatchByIndex(pipeline = %s) closed unexpectedly", a.pipelineInfo.Pipeline.Name)
	}, backoff.NewInfiniteBackOff(), func(err error, d time.Duration) error {
		logger.Logf("worker: watch closed or error running the worker process: %v; retrying in %v", err, d)
		return nil
	})
}

// processDatums processes datums from low to high in df, if a datum fails it
// returns the id of the failed datum it also may return a variety of errors
// such as network errors.
func (a *APIServer) processDatums(pachClient *client.APIClient, logger *taggedLogger, jobInfo *pps.JobInfo, df DatumFactory, low, high int64) (*processResult, error) {
	ctx := pachClient.Ctx()
	stats := &pps.ProcessStats{}
	var statsMu sync.Mutex
	result := &processResult{}
	var eg errgroup.Group
	limiter := limit.New(int(a.pipelineInfo.MaxQueueSize))
	for i := low; i < high; i++ {
		i := i

		limiter.Acquire()
		atomic.AddInt64(&a.queueSize, 1)
		eg.Go(func() (retErr error) {
			defer limiter.Release()
			defer atomic.AddInt64(&a.queueSize, -1)

			data := df.Datum(int(i))
			logger, err := a.getTaggedLogger(pachClient, jobInfo.Job.ID, data, a.pipelineInfo.EnableStats)
			if err != nil {
				return err
			}
			// Hash inputs
			tag := HashDatum(a.pipelineInfo.Pipeline.Name, a.pipelineInfo.Salt, data)
			if _, err := pachClient.InspectTag(ctx, &pfs.Tag{tag}); err == nil {
				atomic.AddInt64(&result.datumsSkipped, 1)
				logger.Logf("skipping datum")
				return nil
			}
			var statsTag *pfs.Tag
			if a.pipelineInfo.EnableStats {
				statsTag = &pfs.Tag{tag + statsTagSuffix}
			}
			subStats := &pps.ProcessStats{}
			statsPath := path.Join("/", logger.template.DatumID)
			var statsTree hashtree.OpenHashTree
			if a.pipelineInfo.EnableStats {
				statsTree = hashtree.NewHashTree()
				if err := statsTree.PutFile(path.Join(statsPath, fmt.Sprintf("job:%s", jobInfo.Job.ID)), nil, 0); err != nil {
					logger.stderrLog.Printf("error from hashtree.PutFile for job object: %s\n", err)
				}
				defer func() {
					if retErr != nil {
						return
					}
					finStatsTree, err := statsTree.Finish()
					if err != nil {
						retErr = err
						return
					}
					statsTreeBytes, err := hashtree.Serialize(finStatsTree)
					if err != nil {
						retErr = err
						return
					}
					if _, _, err := pachClient.PutObject(bytes.NewReader(statsTreeBytes), statsTag.Name); err != nil {
						retErr = err
						return
					}
				}()
				defer func() {
					object, size, err := logger.Close()
					if err != nil && retErr == nil {
						retErr = err
						return
					}
					if object != nil && a.pipelineInfo.EnableStats {
						if err := statsTree.PutFile(path.Join(statsPath, "logs"), []*pfs.Object{object}, size); err != nil && retErr == nil {
							retErr = err
							return
						}
					}
				}()
				defer func() {
					marshaler := &jsonpb.Marshaler{}
					statsString, err := marshaler.MarshalToString(subStats)
					if err != nil {
						logger.stderrLog.Printf("could not serialize stats: %s\n", err)
						return
					}
					object, size, err := pachClient.PutObject(strings.NewReader(statsString))
					if err != nil {
						logger.stderrLog.Printf("could not put stats object: %s\n", err)
						return
					}
					if err := statsTree.PutFile(path.Join(statsPath, "stats"), []*pfs.Object{object}, size); err != nil {
						logger.stderrLog.Printf("could not put-file stats object: %s\n", err)
						return
					}
				}()
			}
			parentTag, err := a.parentTag(pachClient, jobInfo, data)
			if err != nil {
				return err
			}

			env := a.userCodeEnv(jobInfo.Job.ID, data)
			var dir string
			var failures int64
			if err := backoff.RetryNotify(func() error {
				if isDone(ctx) {
					return ctx.Err() // timeout or cancelled job--don't run datum
				}
				// Download input data
				puller := filesync.NewPuller()
				// TODO parent tag shouldn't be nil
				var err error
				dir, err = a.downloadData(pachClient, logger, data, puller, parentTag, subStats, statsTree, path.Join(statsPath, "pfs"))
				// We run these cleanup functions no matter what, so that if
				// downloadData partially succeeded, we still clean up the resources.
				defer func() {
					if err := os.RemoveAll(dir); err != nil && retErr == nil {
						retErr = err
					}
				}()
				// It's important that we run puller.CleanUp before os.RemoveAll,
				// because otherwise puller.Cleanup might try tp open pipes that have
				// been deleted.
				defer func() {
					if _, err := puller.CleanUp(); err != nil && retErr == nil {
						retErr = err
					}
				}()
				if err != nil {
					return fmt.Errorf("error downloadData: %v", err)
				}
				a.runMu.Lock()
				defer a.runMu.Unlock()
				// shadow ctx and pachClient for the context of processing this one datum
				ctx, cancel := context.WithCancel(ctx)
				pachClient := pachClient.WithCtx(ctx)
				func() {
					a.statusMu.Lock()
					defer a.statusMu.Unlock()
					a.jobID = jobInfo.Job.ID
					a.data = data
					a.started = time.Now()
					a.cancel = cancel
					a.stats = stats
				}()
				if err := os.MkdirAll(client.PPSInputPrefix, 0777); err != nil {
					return err
				}
				if err := a.linkData(data, dir); err != nil {
					return fmt.Errorf("error linkData: %v", err)
				}
				defer func() {
					if err := a.unlinkData(data); err != nil && retErr == nil {
						retErr = fmt.Errorf("error unlinkData: %v", err)
					}
				}()
				if a.pipelineInfo.Transform.User != "" {
					filepath.Walk("/pfs", func(name string, info os.FileInfo, err error) error {
						if err == nil {
							err = os.Chown(name, int(a.uid), int(a.gid))
						}
						return err
					})
				}
				if err := a.runUserCode(ctx, logger, env, subStats, jobInfo.DatumTimeout); err != nil {
					return fmt.Errorf("error runUserCode: %v", err)
				}
				// CleanUp is idempotent so we can call it however many times we want.
				// The reason we are calling it here is that the puller could've
				// encountered an error as it was lazily loading files, in which case
				// the output might be invalid since as far as the user's code is
				// concerned, they might've just seen an empty or partially completed
				// file.
				downSize, err := puller.CleanUp()
				if err != nil {
					logger.Logf("puller encountered an error while cleaning up: %+v", err)
					return err
				}
				atomic.AddUint64(&subStats.DownloadBytes, uint64(downSize))
				a.reportDownloadSizeStats(float64(downSize), logger)
				return a.uploadOutput(pachClient, dir, tag, logger, data, subStats, statsTree, path.Join(statsPath, "pfs", "out"))
			}, &backoff.ZeroBackOff{}, func(err error, d time.Duration) error {
				if isDone(ctx) {
					return ctx.Err() // timeout or cancelled job, err out and don't retry
				}
				failures++
				if failures >= jobInfo.DatumTries {
					logger.Logf("failed to process datum with error: %+v", err)
					if statsTree != nil {
						object, size, err := pachClient.PutObject(strings.NewReader(err.Error()))
						if err != nil {
							logger.stderrLog.Printf("could not put error object: %s\n", err)
						} else {
							if err := statsTree.PutFile(path.Join(statsPath, "failure"), []*pfs.Object{object}, size); err != nil {
								logger.stderrLog.Printf("could not put-file error object: %s\n", err)
							}
						}
					}
					return err
				}
				logger.Logf("failed processing datum: %v, retrying in %v", err, d)
				return nil
			}); err != nil {
				result.failedDatumID = a.DatumID(data)
				atomic.AddInt64(&result.datumsFailed, 1)
				return nil
			}
			statsMu.Lock()
			defer statsMu.Unlock()
			if err := mergeStats(stats, subStats); err != nil {
				logger.Logf("failed to merge Stats: %v", err)
			}
			return nil
		})
	}
	if err := eg.Wait(); err != nil {
		return nil, err
	}
	if _, err := col.NewSTM(ctx, a.etcdClient, func(stm col.STM) error {
		jobs := a.jobs.ReadWrite(stm)
		jobID := jobInfo.Job.ID
		jobPtr := &pps.EtcdJobInfo{}
		if err := jobs.Get(jobID, jobPtr); err != nil {
			return err
		}
		if jobPtr.Stats == nil {
			jobPtr.Stats = &pps.ProcessStats{}
		}
		if err := mergeStats(jobPtr.Stats, stats); err != nil {
			logger.Logf("failed to merge Stats: %v", err)
		}
		return jobs.Put(jobID, jobPtr)
	}); err != nil {
		return nil, err
	}
	result.datumsProcessed = high - low - result.datumsSkipped - result.datumsFailed
	return result, nil
}

func (a *APIServer) parentTag(pachClient *client.APIClient, jobInfo *pps.JobInfo, files []*Input) (*pfs.Tag, error) {
	if !jobInfo.Incremental {
		return nil, nil // don't bother downloading the parent for non-incremental jobs
	}
	ci, err := pachClient.InspectCommit(jobInfo.OutputCommit.Repo.Name, jobInfo.OutputCommit.ID)
	if err != nil {
		return nil, fmt.Errorf("could not get output commit's commitInfo: %v", err)
	}
	if ci.ParentCommit == nil {
		return nil, nil
	}
	parentCi, err := pachClient.InspectCommit(ci.ParentCommit.Repo.Name, ci.ParentCommit.ID)
	if err != nil {
		return nil, fmt.Errorf("could not get output commit's parent's commitInfo: %v", err)
	}

	// compare provenance of ci and parentCi to figure out which commit is new
	key := path.Join
	parentProv := make(map[string]*pfs.Commit)
	for i, commit := range parentCi.Provenance {
		parentProv[key(commit.Repo.Name, parentCi.BranchProvenance[i].Name)] = commit
	}
	newInputCommits := make(map[string]*pfs.Commit)
	for i, c := range ci.Provenance {
		pc, ok := parentProv[key(c.Repo.Name, ci.BranchProvenance[i].Name)]
		if !ok {
			return nil, nil // 'c' has no parent, so there's no parent tag
		}
		if pc.ID != c.ID {
			newInputCommits[key(c.Repo.Name, ci.BranchProvenance[i].Name)] = c
		}
	}
	var parentFiles []*Input // the equivalent datum to 'files', which the parent job processed
	var newFiles int
	for _, file := range files {
		parentFile := proto.Clone(file).(*Input)
		newInputCommit, ok := newInputCommits[key(file.FileInfo.File.Commit.Repo.Name, file.Branch)]
		if ok && file.FileInfo.File.Commit.Repo.Name == newInputCommit.Repo.Name && file.FileInfo.File.Commit.ID == newInputCommit.ID {
			newInputCommitParent, ok := parentProv[key(file.FileInfo.File.Commit.Repo.Name, file.Branch)]
			if !ok {
				// This should be impossible since we'll only add
				// newInputCommit to newInputCommits if the parent exists.
				return nil, fmt.Errorf("parent expected but not found (this is likely a bug)")
			}
			newFiles++
			// 'file' from datumFactory is in the new input commit
			parentFileInfo, err := pachClient.InspectFile(
				newInputCommitParent.Repo.Name, newInputCommitParent.ID, file.FileInfo.File.Path)
			if err != nil {
				if !isNotFoundErr(err) {
					return nil, err
				}
				// we didn't find a match for this file,
				// so we know there's no matching datum
				break
			}
			parentFile.FileInfo = parentFileInfo
			// also tell downloadData to make a diff for 'file'
			// side effect of the main work (derive _parentOutputTag)
			file.ParentCommit = newInputCommitParent
		}
		parentFiles = append(parentFiles, parentFile)
	}
	if newFiles > 1 {
		// Multiple new files means we can't do incremental and must run everything from scratch.
		return nil, nil
	}

	// We have derived what files the parent saw -- compute the tag
	if len(parentFiles) == len(files) {
		_parentOutputTag := HashDatum(a.pipelineInfo.Pipeline.Name, a.pipelineInfo.Salt, parentFiles)
		return &pfs.Tag{Name: _parentOutputTag}, nil
	}
	return nil, nil
}

// mergeStats merges y into x
func mergeStats(x, y *pps.ProcessStats) error {
	var err error
	if x.DownloadTime, err = plusDuration(x.DownloadTime, y.DownloadTime); err != nil {
		return err
	}
	if x.ProcessTime, err = plusDuration(x.ProcessTime, y.ProcessTime); err != nil {
		return err
	}
	if x.UploadTime, err = plusDuration(x.UploadTime, y.UploadTime); err != nil {
		return err
	}
	x.DownloadBytes += y.DownloadBytes
	x.UploadBytes += y.UploadBytes
	return nil
}

// lookupUser is a reimplementation of user.Lookup that doesn't require cgo.
func lookupUser(name string) (_ *user.User, retErr error) {
	passwd, err := os.Open("/etc/passwd")
	if err != nil {
		return nil, err
	}
	defer func() {
		if err := passwd.Close(); err != nil && retErr == nil {
			retErr = err
		}
	}()
	scanner := bufio.NewScanner(passwd)
	for scanner.Scan() {
		parts := strings.Split(scanner.Text(), ":")
		if parts[0] == name {
			return &user.User{
				Username: parts[0],
				Uid:      parts[2],
				Gid:      parts[3],
				Name:     parts[4],
				HomeDir:  parts[5],
			}, nil
		}
	}
	if err := scanner.Err(); err != nil {
		log.Fatal(err)
	}
	return nil, fmt.Errorf("user %s not found", name)
}