Rework process collector

This unifies both constructors in one with an options argument.

The options argument allows to switch on error reporting, as discussed
in #219.

The change of the contructor signature is breaking, but only mildly
so. Plus, the process collector is rarely used explicitly. I used
Sourcegraph to search for public usages, with the following results:

- 2 occurrences of NewProcessCollectorPIDFn, once in @discordianfish's
  glimpse, once in @fabxc's etcd_exporter (deprecated anyway). Both
  are Prom veterans and will simply do the one line change if needed.
- 8 occurrences of NewProcessCollector, of which 7 are of the form
    NewProcessCollector(os.Getpid(), "")
  Thus, it's a very easy change, which I even hinted at in the doc
  comment.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/process_collector.go

@@ -13,46 +13,74 @@
 
 package prometheus
 
-import "github.com/prometheus/procfs"
+import (
+	"errors"
+	"os"
+
+	"github.com/prometheus/procfs"
+)
 
 type processCollector struct {
 	collectFn       func(chan<- Metric)
 	pidFn           func() (int, error)
+	reportErrors    bool
 	cpuTotal        *Desc
 	openFDs, maxFDs *Desc
 	vsize, maxVsize *Desc
 	rss             *Desc
 	startTime       *Desc
 }
 
+// ProcessCollectorOpts defines the behavior of a process metrics collector
+// created with NewProcessCollector.
+type ProcessCollectorOpts struct {
+	// PidFn returns the PID of the process the collector collects metrics
+	// for. It is called upon each collection. By default, the PID of the
+	// current process is used, as determined on construction time by
+	// calling os.Getpid().
+	PidFn func() (int, error)
+	// If non-empty, each of the collected metrics is prefixed by the
+	// provided string and an underscore ("_").
+	Namespace string
+	// If true, any error encountered during collection is reported as an
+	// invalid metric (see NewInvalidMetric). Otherwise, errors are ignored
+	// and the collected metrics will be incomplete. (Possibly, no metrics
+	// will be collected at all.) While that's usually not desired, it is
+	// appropriate for the common "mix-in" of process metrics, where process
+	// metrics are nice to have, but failing to collect them should not
+	// disrupt the collection of the remaining metrics.
+	ReportErrors bool
+}
+
 // NewProcessCollector returns a collector which exports the current state of
 // process metrics including CPU, memory and file descriptor usage as well as
-// the process start time for the given process ID under the given namespace.
+// the process start time. The detailed behavior is defined by the provided
+// ProcessCollectorOpts. The zero value of ProcessCollectorOpts creates a
+// collector for the current process with an empty namespace string and no error
+// reporting.
 //
 // Currently, the collector depends on a Linux-style proc filesystem and
 // therefore only exports metrics for Linux.
-func NewProcessCollector(pid int, namespace string) Collector {
-	return NewProcessCollectorPIDFn(
-		func() (int, error) { return pid, nil },
-		namespace,
-	)
-}
-
-// NewProcessCollectorPIDFn works like NewProcessCollector but the process ID is
-// determined on each collect anew by calling the given pidFn function.
-func NewProcessCollectorPIDFn(
-	pidFn func() (int, error),
-	namespace string,
-) Collector {
+//
+// Note: An older version of this function had the following signature:
+//
+//     NewProcessCollector(pid int, namespace string) Collector
+//
+// Most commonly, it was called as
+//
+//     NewProcessCollector(os.Getpid(), "")
+//
+// The following call of the current version is equivalent to the above:
+//
+//     NewProcessCollector(ProcessCollectorOpts{})
+func NewProcessCollector(opts ProcessCollectorOpts) Collector {
 	ns := ""
-	if len(namespace) > 0 {
-		ns = namespace + "_"
+	if len(opts.Namespace) > 0 {
+		ns = opts.Namespace + "_"
 	}
 
-	c := processCollector{
-		pidFn:     pidFn,
-		collectFn: func(chan<- Metric) {},
-
+	c := &processCollector{
+		reportErrors: opts.ReportErrors,
 		cpuTotal: NewDesc(
 			ns+"process_cpu_seconds_total",
 			"Total user and system CPU time spent in seconds.",
@@ -90,12 +118,23 @@ func NewProcessCollectorPIDFn(
 		),
 	}
 
+	if opts.PidFn == nil {
+		pid := os.Getpid()
+		c.pidFn = func() (int, error) { return pid, nil }
+	} else {
+		c.pidFn = opts.PidFn
+	}
+
 	// Set up process metric collection if supported by the runtime.
 	if _, err := procfs.NewStat(); err == nil {
 		c.collectFn = c.processCollect
+	} else {
+		c.collectFn = func(ch chan<- Metric) {
+			c.reportError(ch, nil, errors.New("process metrics not supported on this platform"))
+		}
 	}
 
-	return &c
+	return c
 }
 
 // Describe returns all descriptions of the collector.
@@ -114,16 +153,16 @@ func (c *processCollector) Collect(ch chan<- Metric) {
 	c.collectFn(ch)
 }
 
-// TODO(ts): Bring back error reporting by reverting 7faf9e7 as soon as the
-// client allows users to configure the error behavior.
 func (c *processCollector) processCollect(ch chan<- Metric) {
 	pid, err := c.pidFn()
 	if err != nil {
+		c.reportError(ch, nil, err)
 		return
 	}
 
 	p, err := procfs.NewProc(pid)
 	if err != nil {
+		c.reportError(ch, nil, err)
 		return
 	}
 
@@ -133,15 +172,33 @@ func (c *processCollector) processCollect(ch chan<- Metric) {
 		ch <- MustNewConstMetric(c.rss, GaugeValue, float64(stat.ResidentMemory()))
 		if startTime, err := stat.StartTime(); err == nil {
 			ch <- MustNewConstMetric(c.startTime, GaugeValue, startTime)
+		} else {
+			c.reportError(ch, c.startTime, err)
 		}
+	} else {
+		c.reportError(ch, nil, err)
 	}
 
 	if fds, err := p.FileDescriptorsLen(); err == nil {
 		ch <- MustNewConstMetric(c.openFDs, GaugeValue, float64(fds))
+	} else {
+		c.reportError(ch, c.openFDs, err)
 	}
 
 	if limits, err := p.NewLimits(); err == nil {
 		ch <- MustNewConstMetric(c.maxFDs, GaugeValue, float64(limits.OpenFiles))
 		ch <- MustNewConstMetric(c.maxVsize, GaugeValue, float64(limits.AddressSpace))
+	} else {
+		c.reportError(ch, nil, err)
+	}
+}
+
+func (c *processCollector) reportError(ch chan<- Metric, desc *Desc, err error) {
+	if !c.reportErrors {
+		return
+	}
+	if desc == nil {
+		desc = NewInvalidDesc(err)
 	}
+	ch <- NewInvalidMetric(desc, err)
 }

prometheus/process_collector_test.go

@@ -17,12 +17,15 @@ package prometheus
 
 import (
 	"bytes"
+	"errors"
 	"os"
 	"regexp"
 	"testing"
 
 	"github.com/prometheus/common/expfmt"
 	"github.com/prometheus/procfs"
+
+	dto "github.com/prometheus/client_model/go"
 )
 
 func TestProcessCollector(t *testing.T) {
@@ -31,12 +34,14 @@ func TestProcessCollector(t *testing.T) {
 	}
 
 	registry := NewRegistry()
-	if err := registry.Register(NewProcessCollector(os.Getpid(), "")); err != nil {
+	if err := registry.Register(NewProcessCollector(ProcessCollectorOpts{})); err != nil {
 		t.Fatal(err)
 	}
-	if err := registry.Register(NewProcessCollectorPIDFn(
-		func() (int, error) { return os.Getpid(), nil }, "foobar"),
-	); err != nil {
+	if err := registry.Register(NewProcessCollector(ProcessCollectorOpts{
+		PidFn:        func() (int, error) { return os.Getpid(), nil },
+		Namespace:    "foobar",
+		ReportErrors: true, // No errors expected, just to see if none are reported.
+	})); err != nil {
 		t.Fatal(err)
 	}
 
@@ -72,4 +77,27 @@ func TestProcessCollector(t *testing.T) {
 			t.Errorf("want body to match %s\n%s", re, buf.String())
 		}
 	}
+
+	brokenProcessCollector := NewProcessCollector(ProcessCollectorOpts{
+		PidFn:        func() (int, error) { return 0, errors.New("boo") },
+		ReportErrors: true,
+	})
+
+	ch := make(chan Metric)
+	go func() {
+		brokenProcessCollector.Collect(ch)
+		close(ch)
+	}()
+	n := 0
+	for m := range ch {
+		n++
+		pb := &dto.Metric{}
+		err := m.Write(pb)
+		if err == nil {
+			t.Error("metric collected from broken process collector is unexpectedly valid")
+		}
+	}
+	if n != 1 {
+		t.Errorf("%d metrics collected, want 1", n)
+	}
 }

prometheus/registry.go

@@ -16,7 +16,6 @@ package prometheus
 import (
 	"bytes"
 	"fmt"
-	"os"
 	"runtime"
 	"sort"
 	"strings"
@@ -54,7 +53,7 @@ var (
 )
 
 func init() {
-	MustRegister(NewProcessCollector(os.Getpid(), ""))
+	MustRegister(NewProcessCollector(ProcessCollectorOpts{}))
 	MustRegister(NewGoCollector())
 }