Provoke unreferenced packs using fewer goroutines

TestParallelSaveWithDuplication has been reworked to provoke
unreferenced packs using fewer goroutines than before and create
only one bytes.Reader per blob. This reduces memory usage
significantly.

The following actions have been taken to keep the chance of provoking
unreferenced packs due to #358 high:
 * Interweaved processing of subsequent chunks
 * Delaying each goroutine by a few pseudo-randomly chosen nanoseconds
   (depending on the platform this will most probably only make the os
   yield execution to another thread): together with the interweaved
   processing of subsequent chunks, this ensures a minimalistic delay
   between processing of (some) duplicated chunks
 * Repeating the test 5 times with different seeds

On my test machine, the modified test provoked unreferenced packs 60
times in a row.

archiver_test.go

@@ -4,8 +4,8 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"io"
-	"math"
 	"testing"
+	"time"
 
 	"github.com/restic/chunker"
 	"github.com/restic/restic"
@@ -242,47 +242,48 @@ func BenchmarkLoadTree(t *testing.B) {
 	}
 }
 
-// Saves several identical chunks concurrently and later check that there are no
+// Saves several identical chunks concurrently and later checks that there are no
 // unreferenced packs in the repository. See also #292 and #358.
-// The combination of high duplication and high concurrency should provoke any
-// issues leading to unreferenced packs.
-func TestParallelSaveWithHighDuplication(t *testing.T) {
+func TestParallelSaveWithDuplication(t *testing.T) {
+	for seed := 0; seed < 5; seed++ {
+		testParallelSaveWithDuplication(t, seed)
+	}
+}
+
+func testParallelSaveWithDuplication(t *testing.T, seed int) {
 	repo := SetupRepo()
 	defer TeardownRepo(repo)
 
-	// For every seed a pseudo-random 32Mb blob is generated and split into
-	// chunks. During the test all chunks of all blobs are processed in parallel
-	// goroutines. To increase duplication, each chunk is processed
-	// <duplication> times. Concurrency can be limited by changing <maxParallel>.
-	// Note: seeds 5, 3, 66, 4, 12 produce the most chunks (descending)
-	seeds := []int{5, 3, 66, 4, 12}
-	maxParallel := math.MaxInt32
-	duplication := 15
+	dataSizeMb := 92
+	duplication := 7
 
 	arch := restic.NewArchiver(repo)
-	data := getRandomData(seeds)
+	data, chunks := getRandomData(seed, dataSizeMb*1024*1024)
+	reader := bytes.NewReader(data)
 
-	barrier := make(chan struct{}, maxParallel)
 	errChannels := [](<-chan error){}
 
-	for _, d := range data {
-		for _, c := range d.chunks {
-			for dupIdx := 0; dupIdx < duplication; dupIdx++ {
-				errChan := make(chan error)
-				errChannels = append(errChannels, errChan)
+	// interweaved processing of subsequent chunks
+	maxParallel := 2*duplication - 1
+	barrier := make(chan struct{}, maxParallel)
 
-				go func(buf *[]byte, c *chunker.Chunk, errChan chan<- error) {
-					barrier <- struct{}{}
+	for _, c := range chunks {
+		for dupIdx := 0; dupIdx < duplication; dupIdx++ {
+			errChan := make(chan error)
+			errChannels = append(errChannels, errChan)
 
-					hash := c.Digest
-					id := backend.ID{}
-					copy(id[:], hash)
+			go func(reader *bytes.Reader, c *chunker.Chunk, errChan chan<- error) {
+				barrier <- struct{}{}
 
-					err := arch.Save(pack.Data, id, c.Length, c.Reader(bytes.NewReader(*buf)))
-					<-barrier
-					errChan <- err
-				}(&d.buf, c, errChan)
-			}
+				hash := c.Digest
+				id := backend.ID{}
+				copy(id[:], hash)
+
+				time.Sleep(time.Duration(hash[0]))
+				err := arch.Save(pack.Data, id, c.Length, c.Reader(reader))
+				<-barrier
+				errChan <- err
+			}(reader, c, errChan)
 		}
 	}
 
@@ -297,34 +298,20 @@ func TestParallelSaveWithHighDuplication(t *testing.T) {
 	assertNoUnreferencedPacks(t, chkr)
 }
 
-func getRandomData(seeds []int) []*chunkedData {
-	chunks := []*chunkedData{}
-	sem := make(chan struct{}, len(seeds))
+func getRandomData(seed int, size int) ([]byte, []*chunker.Chunk) {
+	buf := Random(seed, size)
+	chunks := []*chunker.Chunk{}
+	chunker := chunker.New(bytes.NewReader(buf), testPol, sha256.New())
 
-	for seed := range seeds {
-		c := &chunkedData{}
+	for {
+		c, err := chunker.Next()
+		if err == io.EOF {
+			break
+		}
 		chunks = append(chunks, c)
-
-		go func(seed int, data *chunkedData) {
-			data.buf = Random(seed, 32*1024*1024)
-			chunker := chunker.New(bytes.NewReader(data.buf), testPol, sha256.New())
-
-			for {
-				c, err := chunker.Next()
-				if err == io.EOF {
-					break
-				}
-				data.chunks = append(data.chunks, c)
-			}
-
-			sem <- struct{}{}
-		}(seed, c)
 	}
 
-	for i := 0; i < len(seeds); i++ {
-		<-sem
-	}
-	return chunks
+	return buf, chunks
 }
 
 func createAndInitChecker(t *testing.T, repo *repository.Repository) *checker.Checker {