From a12f493bd316adbf48aacdb453c64f365fe89cb0 Mon Sep 17 00:00:00 2001
From: Derek McGowan <derek@mcgstyle.net>
Date: Thu, 25 Jan 2018 17:15:32 -0800
Subject: [PATCH] Update copy to discard over truncate

Prevents the copy method from calling discard on the
writer when the reader is not seekable. Instead,
the copy method will discard up to the offset.
Truncate is a more expensive operation since any
bytes that are truncated already have their hash calculated
and are stored on disk in the backend. Re-writing bytes
which were truncated requires transfering the data over
GRPC again and re-computing the hash up to the point of
truncation.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>
---
 content/helpers.go             | 33 +++++++++++++++++----------------
 content/helpers_test.go        |  6 +++---
 content/testsuite/testsuite.go |  1 -
 3 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/content/helpers.go b/content/helpers.go
index 83c31d917..c125e2a2b 100644
--- a/content/helpers.go
+++ b/content/helpers.go
@@ -3,6 +3,7 @@ package content
 import (
 	"context"
 	"io"
+	"io/ioutil"
 	"sync"
 
 	"github.com/containerd/containerd/errdefs"
@@ -76,14 +77,7 @@ func Copy(ctx context.Context, cw Writer, r io.Reader, size int64, expected dige
 	if ws.Offset > 0 {
 		r, err = seekReader(r, ws.Offset, size)
 		if err != nil {
-			if !isUnseekable(err) {
-				return errors.Wrapf(err, "unable to resume write to %v", ws.Ref)
-			}
-
-			// reader is unseekable, try to move the writer back to the start.
-			if err := cw.Truncate(0); err != nil {
-				return errors.Wrapf(err, "content writer truncate failed")
-			}
+			return errors.Wrapf(err, "unable to resume write to %v", ws.Ref)
 		}
 	}
 
@@ -103,14 +97,9 @@ func Copy(ctx context.Context, cw Writer, r io.Reader, size int64, expected dige
 	return nil
 }
 
-var errUnseekable = errors.New("seek not supported")
-
-func isUnseekable(err error) bool {
-	return errors.Cause(err) == errUnseekable
-}
-
 // seekReader attempts to seek the reader to the given offset, either by
-// resolving `io.Seeker` or by detecting `io.ReaderAt`.
+// resolving `io.Seeker`, by detecting `io.ReaderAt`, or discarding
+// up to the given offset.
 func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
 	// attempt to resolve r as a seeker and setup the offset.
 	seeker, ok := r.(io.Seeker)
@@ -134,5 +123,17 @@ func seekReader(r io.Reader, offset, size int64) (io.Reader, error) {
 		return sr, nil
 	}
 
-	return r, errors.Wrapf(errUnseekable, "seek to offset %v failed", offset)
+	// well then, let's just discard up to the offset
+	buf := bufPool.Get().(*[]byte)
+	defer bufPool.Put(buf)
+
+	n, err := io.CopyBuffer(ioutil.Discard, io.LimitReader(r, offset), *buf)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to discard to offset")
+	}
+	if n != offset {
+		return nil, errors.Errorf("unable to discard to offset")
+	}
+
+	return r, nil
 }
diff --git a/content/helpers_test.go b/content/helpers_test.go
index b9de4dcb2..4daccfd8f 100644
--- a/content/helpers_test.go
+++ b/content/helpers_test.go
@@ -42,9 +42,9 @@ func TestCopy(t *testing.T) {
 		},
 		{
 			name:     "copy with offset from unseekable source",
-			source:   copySource{reader: bytes.NewBufferString("foo"), size: 3},
-			writer:   fakeWriter{status: Status{Offset: 8}},
-			expected: "foo",
+			source:   copySource{reader: bytes.NewBufferString("foobar"), size: 6},
+			writer:   fakeWriter{status: Status{Offset: 3}},
+			expected: "bar",
 		},
 		{
 			name:   "commit already exists",
diff --git a/content/testsuite/testsuite.go b/content/testsuite/testsuite.go
index 8d6f35de5..7292d48ac 100644
--- a/content/testsuite/testsuite.go
+++ b/content/testsuite/testsuite.go
@@ -403,7 +403,6 @@ func checkResume(rf func(context.Context, content.Writer, []byte, int64, int64,
 				if err := rf(ctx, w, b, limit, size, d); err != nil {
 					t.Fatalf("Resume failed: %+v", err)
 				}
-
 				postCommit := time.Now()
 
 				if err := w.Close(); err != nil {