go/store/datas/pull: pull_chunk_fetcher: Move chunk fetching to a str…

…eaming interface instead of batch.

We want to better pipeline I/O when pulling from a remote, and moving a
streaming interface where storage can see more of the addresses that are needed
at once will allow us to achieve it.

For now, we implement the streaming interface just by calling the existing
batch get interface.

go/store/datas/pull/pull_chunk_fetcher.go

@@ -0,0 +1,174 @@
+// Copyright 2024 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pull
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"golang.org/x/sync/errgroup"
+
+	"github.com/dolthub/dolt/go/store/hash"
+	"github.com/dolthub/dolt/go/store/nbs"
+)
+
+type GetManyer interface {
+	GetManyCompressed(ctx context.Context, hashes hash.HashSet, found func(context.Context, nbs.CompressedChunk)) error
+}
+
+// A ChunkFetcher is a batching, stateful, potentially concurrent interface to
+// fetch lots of chunks from a ChunkStore. A caller is expected to call
+// `Get()` and `Recv()` concurrently. Unless there is an error, for every
+// single Hash passed to Get, a corresponding Recv() call will deliver the
+// contents of the chunk. When a caller is done with a ChunkFetcher, they
+// should call |CloseSend()|. After CloseSend, all requested hashes have been
+// delivered through Recv(), Recv() will return `io.EOF`.
+//
+// A ChunkFetcher should be Closed() when it is no longer needed. In non-error
+// cases, this will typically be after Recv() has delivererd io.EOF. If Close
+// is called before Recv() delivers io.EOF, there is no guarantee that all
+// requested chunks will be delivered through Recv().
+//
+// In contrast to interfaces to like GetManyCompressed on ChunkStore, if the
+// chunk is not in the underlying database, then Recv() will return an
+// nbs.CompressedChunk with its Hash set, but with empty contents.
+//
+// Other than an io.EOF from Recv(), any |error| returned from any method
+// indicates an underlying problem wtih fetching requested chunks from the
+// ChunkStore. A ChunkFetcher is single use and cannot be used effectively
+// after an error is returned.
+type ChunkFetcher interface {
+	Get(ctx context.Context, hashes hash.HashSet) error
+
+	CloseSend() error
+
+	Recv(context.Context) (nbs.CompressedChunk, error)
+
+	Close() error
+}
+
+// A PullChunkFetcher is a simple implementation of |ChunkFetcher| based on
+// calling GetManyCompressed.
+//
+// It only has one outstanding GetManyCompressed call at a time.
+type PullChunkFetcher struct {
+	ctx context.Context
+	eg  *errgroup.Group
+
+	getter GetManyer
+
+	batchCh chan hash.HashSet
+	doneCh  chan struct{}
+	resCh   chan nbs.CompressedChunk
+}
+
+func NewPullChunkFetcher(ctx context.Context, getter GetManyer) *PullChunkFetcher {
+	eg, ctx := errgroup.WithContext(ctx)
+	ret := &PullChunkFetcher{
+		ctx:     ctx,
+		eg:      eg,
+		getter:  getter,
+		batchCh: make(chan hash.HashSet),
+		doneCh:  make(chan struct{}),
+		resCh:   make(chan nbs.CompressedChunk),
+	}
+	ret.eg.Go(func() error {
+		return ret.fetcherThread(func() {
+			close(ret.resCh)
+		})
+	})
+	return ret
+}
+
+func (f *PullChunkFetcher) fetcherThread(finalize func()) error {
+	for {
+		select {
+		case batch, ok := <-f.batchCh:
+			if !ok {
+				finalize()
+				return nil
+			}
+
+			var mu sync.Mutex
+			missing := batch.Copy()
+
+			// Blocking get, no concurrency, only one fetcher.
+			err := f.getter.GetManyCompressed(f.ctx, batch, func(ctx context.Context, chk nbs.CompressedChunk) {
+				mu.Lock()
+				missing.Remove(chk.H)
+				mu.Unlock()
+				select {
+				case <-ctx.Done():
+				case <-f.ctx.Done():
+				case f.resCh <- chk:
+				case <-f.doneCh:
+				}
+			})
+			if err != nil {
+				return err
+			}
+
+			for h := range missing {
+				select {
+				case <-f.ctx.Done():
+					return context.Cause(f.ctx)
+				case f.resCh <- nbs.CompressedChunk{H: h}:
+				case <-f.doneCh:
+					return nil
+				}
+			}
+		case <-f.ctx.Done():
+			return context.Cause(f.ctx)
+		case <-f.doneCh:
+			return nil
+		}
+	}
+}
+
+func (f *PullChunkFetcher) Get(ctx context.Context, hashes hash.HashSet) error {
+	select {
+	case f.batchCh <- hashes:
+		return nil
+	case <-ctx.Done():
+		return context.Cause(ctx)
+	case <-f.ctx.Done():
+		return context.Cause(f.ctx)
+	}
+}
+
+func (f *PullChunkFetcher) CloseSend() error {
+	close(f.batchCh)
+	return nil
+}
+
+func (f *PullChunkFetcher) Close() error {
+	close(f.doneCh)
+	return f.eg.Wait()
+}
+
+func (f *PullChunkFetcher) Recv(ctx context.Context) (nbs.CompressedChunk, error) {
+	select {
+	case res, ok := <-f.resCh:
+		if !ok {
+			return nbs.CompressedChunk{}, io.EOF
+		}
+		return res, nil
+	case <-ctx.Done():
+		return nbs.CompressedChunk{}, context.Cause(ctx)
+	case <-f.ctx.Done():
+		return nbs.CompressedChunk{}, context.Cause(f.ctx)
+	}
+}

go/store/datas/pull/pull_chunk_fetcher_test.go

@@ -0,0 +1,170 @@
+// Copyright 2024 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pull
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/dolthub/dolt/go/store/hash"
+	"github.com/dolthub/dolt/go/store/nbs"
+)
+
+func TestPullChunkFetcher(t *testing.T) {
+	t.Run("ImmediateCloseSend", func(t *testing.T) {
+		f := NewPullChunkFetcher(context.Background(), emptyGetManyer{})
+		assert.NoError(t, f.CloseSend())
+		_, err := f.Recv(context.Background())
+		assert.ErrorIs(t, err, io.EOF)
+		assert.NoError(t, f.Close())
+	})
+	t.Run("CanceledGetCtx", func(t *testing.T) {
+		ctx, c := context.WithCancel(context.Background())
+		gm := blockingGetManyer{make(chan struct{})}
+		f := NewPullChunkFetcher(context.Background(), gm)
+		hs := make(hash.HashSet)
+		var h hash.Hash
+		hs.Insert(h)
+		err := f.Get(ctx, hs)
+		assert.NoError(t, err)
+		c()
+		err = f.Get(ctx, hs)
+		assert.Error(t, err)
+		close(gm.block)
+		assert.NoError(t, f.Close())
+	})
+	t.Run("CanceledRecvCtx", func(t *testing.T) {
+		ctx, c := context.WithCancel(context.Background())
+		f := NewPullChunkFetcher(context.Background(), emptyGetManyer{})
+		c()
+		_, err := f.Recv(ctx)
+		assert.Error(t, err)
+		assert.NoError(t, f.Close())
+	})
+	t.Run("ReturnsDelieveredChunk", func(t *testing.T) {
+		var gm deliveringGetManyer
+		gm.C.FullCompressedChunk = make([]byte, 1024)
+		f := NewPullChunkFetcher(context.Background(), gm)
+		hs := make(hash.HashSet)
+		hs.Insert(gm.C.H)
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			cmp, err := f.Recv(context.Background())
+			assert.NoError(t, err)
+			assert.Equal(t, cmp.H, gm.C.H)
+			assert.Equal(t, cmp.FullCompressedChunk, gm.C.FullCompressedChunk)
+			_, err = f.Recv(context.Background())
+			assert.ErrorIs(t, err, io.EOF)
+			assert.NoError(t, f.Close())
+		}()
+		err := f.Get(context.Background(), hs)
+		assert.NoError(t, err)
+		assert.NoError(t, f.CloseSend())
+		wg.Wait()
+	})
+	t.Run("ReturnsEmptyCompressedChunk", func(t *testing.T) {
+		f := NewPullChunkFetcher(context.Background(), emptyGetManyer{})
+		hs := make(hash.HashSet)
+		var h hash.Hash
+		hs.Insert(h)
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			cmp, err := f.Recv(context.Background())
+			assert.NoError(t, err)
+			assert.Equal(t, cmp.H, h)
+			assert.Nil(t, cmp.FullCompressedChunk)
+			_, err = f.Recv(context.Background())
+			assert.ErrorIs(t, err, io.EOF)
+			assert.NoError(t, f.Close())
+		}()
+		err := f.Get(context.Background(), hs)
+		assert.NoError(t, err)
+		assert.NoError(t, f.CloseSend())
+		wg.Wait()
+	})
+	t.Run("ErrorGetManyer", func(t *testing.T) {
+		f := NewPullChunkFetcher(context.Background(), errorGetManyer{})
+		hs := make(hash.HashSet)
+		var h hash.Hash
+		hs.Insert(h)
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, err := f.Recv(context.Background())
+			assert.Error(t, err)
+			err = f.Close()
+			assert.Error(t, err)
+		}()
+		err := f.Get(context.Background(), hs)
+		assert.NoError(t, err)
+		err = f.Get(context.Background(), hs)
+		assert.Error(t, err)
+		wg.Wait()
+	})
+	t.Run("ClosedFetcherErrorsGet", func(t *testing.T) {
+		f := NewPullChunkFetcher(context.Background(), emptyGetManyer{})
+		assert.NoError(t, f.Close())
+		hs := make(hash.HashSet)
+		var h hash.Hash
+		hs.Insert(h)
+		assert.Error(t, f.Get(context.Background(), hs))
+	})
+}
+
+type emptyGetManyer struct {
+}
+
+func (emptyGetManyer) GetManyCompressed(ctx context.Context, hashes hash.HashSet, found func(context.Context, nbs.CompressedChunk)) error {
+	return nil
+}
+
+type deliveringGetManyer struct {
+	C nbs.CompressedChunk
+}
+
+func (d deliveringGetManyer) GetManyCompressed(ctx context.Context, hashes hash.HashSet, found func(context.Context, nbs.CompressedChunk)) error {
+	for _ = range hashes {
+		found(ctx, d.C)
+	}
+	return nil
+}
+
+type blockingGetManyer struct {
+	block chan struct{}
+}
+
+func (b blockingGetManyer) GetManyCompressed(ctx context.Context, hashes hash.HashSet, found func(context.Context, nbs.CompressedChunk)) error {
+	<-b.block
+	return nil
+}
+
+type errorGetManyer struct {
+}
+
+var getManyerErr = fmt.Errorf("always return an error")
+
+func (errorGetManyer) GetManyCompressed(ctx context.Context, hashes hash.HashSet, found func(context.Context, nbs.CompressedChunk)) error {
+	return getManyerErr
+}