Fix invalid length bug with some registries

Ensures that the client can handle cases where the
registry ignores content length.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

remotes/docker/fetcher_test.go

@@ -0,0 +1,94 @@
+/*
+   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 docker
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"math/rand"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestFetcherOpen(t *testing.T) {
+	content := make([]byte, 128)
+	rand.New(rand.NewSource(1)).Read(content)
+	start := 0
+
+	s := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		if start > 0 {
+			rw.Header().Set("content-range", fmt.Sprintf("bytes %d-127/128", start))
+		}
+		rw.Header().Set("content-length", fmt.Sprintf("%d", len(content[start:])))
+		rw.Write(content[start:])
+	}))
+	defer s.Close()
+
+	f := dockerFetcher{&dockerBase{
+		client: s.Client(),
+	}}
+	ctx := context.Background()
+
+	checkReader := func(o int64) {
+		t.Helper()
+		rc, err := f.open(ctx, s.URL, "", o)
+		if err != nil {
+			t.Fatalf("failed to open: %+v", err)
+		}
+		b, err := ioutil.ReadAll(rc)
+		if err != nil {
+			t.Fatal(err)
+		}
+		expected := content[o:]
+		if len(b) != len(expected) {
+			t.Errorf("unexpected length %d, expected %d", len(b), len(expected))
+			return
+		}
+		for i, c := range expected {
+			if b[i] != c {
+				t.Errorf("unexpected byte %x at %d, expected %x", b[i], i, c)
+				return
+			}
+		}
+
+	}
+
+	checkReader(0)
+
+	// Test server ignores content range
+	checkReader(25)
+
+	// Use content range on server
+	start = 20
+	checkReader(20)
+
+	// Check returning just last byte and no bytes
+	start = 127
+	checkReader(127)
+	start = 128
+	checkReader(128)
+
+	// Check that server returning a different content range
+	// then requested errors
+	start = 30
+	_, err := f.open(ctx, s.URL, "", 20)
+	if err == nil {
+		t.Fatal("expected error opening with invalid server response")
+	}
+}