From e638b46a1303ace658a3f4dc6239dd5ddbb774b4 Mon Sep 17 00:00:00 2001
From: Michael Eischer <michael.eischer@fau.de>
Date: Tue, 6 Oct 2020 15:59:00 +0200
Subject: [PATCH] Embed context into ReaderAt

The io.Reader interface does not support contexts, such that it is
necessary to embed the context into the backendReaderAt struct. This has
the problem that a reader might suddenly stop working when it's
contained context is canceled. However, this is now problem here as the
reader instances never escape the calling function.
---
 cmd/restic/cmd_debug.go           |  2 +-
 internal/pack/pack_test.go        |  4 ++--
 internal/repository/repository.go |  2 +-
 internal/restic/readerat.go       | 15 +++++++++------
 4 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/cmd/restic/cmd_debug.go b/cmd/restic/cmd_debug.go
index c242d0a25..b36eefa07 100644
--- a/cmd/restic/cmd_debug.go
+++ b/cmd/restic/cmd_debug.go
@@ -87,7 +87,7 @@ func printPacks(ctx context.Context, repo *repository.Repository, wr io.Writer)
 	return repo.List(ctx, restic.PackFile, func(id restic.ID, size int64) error {
 		h := restic.Handle{Type: restic.PackFile, Name: id.String()}
 
-		blobs, err := pack.List(repo.Key(), restic.ReaderAt(repo.Backend(), h), size)
+		blobs, err := pack.List(repo.Key(), restic.ReaderAt(ctx, repo.Backend(), h), size)
 		if err != nil {
 			Warnf("error for pack %v: %v\n", id.Str(), err)
 			return nil
diff --git a/internal/pack/pack_test.go b/internal/pack/pack_test.go
index 71aac082d..e40e5bd2a 100644
--- a/internal/pack/pack_test.go
+++ b/internal/pack/pack_test.go
@@ -128,7 +128,7 @@ func TestUnpackReadSeeker(t *testing.T) {
 
 	handle := restic.Handle{Type: restic.PackFile, Name: id.String()}
 	rtest.OK(t, b.Save(context.TODO(), handle, restic.NewByteReader(packData)))
-	verifyBlobs(t, bufs, k, restic.ReaderAt(b, handle), packSize)
+	verifyBlobs(t, bufs, k, restic.ReaderAt(context.TODO(), b, handle), packSize)
 }
 
 func TestShortPack(t *testing.T) {
@@ -141,5 +141,5 @@ func TestShortPack(t *testing.T) {
 
 	handle := restic.Handle{Type: restic.PackFile, Name: id.String()}
 	rtest.OK(t, b.Save(context.TODO(), handle, restic.NewByteReader(packData)))
-	verifyBlobs(t, bufs, k, restic.ReaderAt(b, handle), packSize)
+	verifyBlobs(t, bufs, k, restic.ReaderAt(context.TODO(), b, handle), packSize)
 }
diff --git a/internal/repository/repository.go b/internal/repository/repository.go
index fc58a660e..635b15253 100644
--- a/internal/repository/repository.go
+++ b/internal/repository/repository.go
@@ -679,7 +679,7 @@ func (r *Repository) List(ctx context.Context, t restic.FileType, fn func(restic
 func (r *Repository) ListPack(ctx context.Context, id restic.ID, size int64) ([]restic.Blob, int64, error) {
 	h := restic.Handle{Type: restic.PackFile, Name: id.String()}
 
-	blobs, err := pack.List(r.Key(), restic.ReaderAt(r.Backend(), h), size)
+	blobs, err := pack.List(r.Key(), restic.ReaderAt(ctx, r.Backend(), h), size)
 	if err != nil {
 		return nil, 0, err
 	}
diff --git a/internal/restic/readerat.go b/internal/restic/readerat.go
index 6e945b43a..6ed2e83f5 100644
--- a/internal/restic/readerat.go
+++ b/internal/restic/readerat.go
@@ -9,17 +9,20 @@ import (
 )
 
 type backendReaderAt struct {
-	be Backend
-	h  Handle
+	ctx context.Context
+	be  Backend
+	h   Handle
 }
 
 func (brd backendReaderAt) ReadAt(p []byte, offset int64) (n int, err error) {
-	return ReadAt(context.TODO(), brd.be, brd.h, offset, p)
+	return ReadAt(brd.ctx, brd.be, brd.h, offset, p)
 }
 
-// ReaderAt returns an io.ReaderAt for a file in the backend.
-func ReaderAt(be Backend, h Handle) io.ReaderAt {
-	return backendReaderAt{be: be, h: h}
+// ReaderAt returns an io.ReaderAt for a file in the backend. The returned reader
+// should not escape the caller function to avoid unexpected interactions with the
+// embedded context
+func ReaderAt(ctx context.Context, be Backend, h Handle) io.ReaderAt {
+	return backendReaderAt{ctx: ctx, be: be, h: h}
 }
 
 // ReadAt reads from the backend handle h at the given position.