internal/repository: Fix LoadBlob + fuzz test

When given a buf that is big enough for a compressed blob but not its
decompressed contents, the copy at the end of LoadBlob would skip the
last part of the contents.

Fixes #3783.

internal/repository/fuzz_test.go

@@ -0,0 +1,43 @@
+//go:build go1.18
+// +build go1.18
+
+package repository
+
+import (
+	"context"
+	"testing"
+
+	"github.com/restic/restic/internal/backend/mem"
+	"github.com/restic/restic/internal/restic"
+)
+
+// Test saving a blob and loading it again, with varying buffer sizes.
+// Also a regression test for #3783.
+func FuzzSaveLoadBlob(f *testing.F) {
+	f.Fuzz(func(t *testing.T, blob []byte, buflen uint) {
+		if buflen > 64<<20 {
+			// Don't allocate enormous buffers. We're not testing the allocator.
+			t.Skip()
+		}
+
+		id := restic.Hash(blob)
+		repo, _ := TestRepositoryWithBackend(t, mem.New(), 2)
+
+		_, _, err := repo.SaveBlob(context.TODO(), restic.DataBlob, blob, id, false)
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = repo.Flush(context.TODO())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		buf, err := repo.LoadBlob(context.TODO(), restic.DataBlob, id, make([]byte, buflen))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if restic.Hash(buf) != id {
+			t.Fatal("mismatch")
+		}
+	})
+}