services/content: fix reading a blob which is smaller than the read buffer.

The newly added test fails without this fix in services/content/service.go:

    $ go test -c . && sudo ./containerd.test -test.v -test.root -test.run TestContentClient
    ...
        --- FAIL: TestContentClient/SmallBlob (0.02s)
            provideringester.go:62: rpc error: code = OutOfRange desc = read
    past object length 6 bytes
            helpers.go:67: drwx------       4096
    /tmp/content-suite-ContentClient-286788688
    FAIL

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

content/testsuite/provideringester.go

@@ -0,0 +1,76 @@
+/*
+   Copyright The containerd Authors.
+
+   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 testsuite
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"testing"
+
+	"github.com/containerd/containerd/content"
+	digest "github.com/opencontainers/go-digest"
+)
+
+// ProviderIngester consists of provider and ingester.
+// This interface is used for test cases that does not require
+// complete content.Store implementation.
+type ProviderIngester interface {
+	content.Provider
+	content.Ingester
+}
+
+// TestSmallBlob tests reading a blob which is smaller than the read size.
+// This test is exposed so that it can be used for incomplete content.Store implementation.
+func TestSmallBlob(ctx context.Context, t *testing.T, store ProviderIngester) {
+	blob := []byte(`foobar`)
+	blobSize := int64(len(blob))
+	blobDigest := digest.FromBytes(blob)
+	// test write
+	w, err := store.Writer(ctx, t.Name(), blobSize, blobDigest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := w.Write(blob); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Commit(ctx, blobSize, blobDigest); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	// test read.
+	readSize := blobSize + 1
+	ra, err := store.ReaderAt(ctx, blobDigest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := io.NewSectionReader(ra, 0, readSize)
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := ra.Close(); err != nil {
+		t.Fatal(err)
+	}
+	d := digest.FromBytes(b)
+	if blobDigest != d {
+		t.Fatalf("expected %s (%q), got %s (%q)", blobDigest, string(blob),
+			d, string(b))
+	}
+}

content/testsuite/testsuite.go

@@ -50,6 +50,10 @@ func ContentSuite(t *testing.T, name string, storeFn func(ctx context.Context, r
 
 	t.Run("CrossNamespaceAppend", makeTest(t, name, storeFn, checkCrossNSAppend))
 	t.Run("CrossNamespaceShare", makeTest(t, name, storeFn, checkCrossNSShare))
+
+	t.Run("SmallBlob", makeTest(t, name, storeFn, func(ctx context.Context, t *testing.T, cs content.Store) {
+		TestSmallBlob(ctx, t, cs)
+	}))
 }
 
 // ContextWrapper is used to decorate new context used inside the test

services/content/service.go

@@ -187,6 +187,8 @@ func (s *service) Read(req *api.ReadContentRequest, session api.Content_ReadServ
 
 	var (
 		offset = req.Offset
+		// size is read size, not the expected size of the blob (oi.Size), which the caller might not be aware of.
+		// offset+size can be larger than oi.Size.
 		size = req.Size_
 
 		// TODO(stevvooe): Using the global buffer pool. At 32KB, it is probably
@@ -199,12 +201,12 @@ func (s *service) Read(req *api.ReadContentRequest, session api.Content_ReadServ
 		offset = 0
 	}
 
-	if size <= 0 {
-		size = oi.Size - offset
+	if offset > oi.Size {
+		return status.Errorf(codes.OutOfRange, "read past object length %v bytes", oi.Size)
 	}
 
-	if offset+size > oi.Size {
-		return status.Errorf(codes.OutOfRange, "read past object length %v bytes", oi.Size)
+	if size <= 0 || offset+size > oi.Size {
+		size = oi.Size - offset
 	}
 
 	_, err = io.CopyBuffer(