net/http: Transport should return an error when response body ends early

If a server response contains a Content-Length and the body is short,
the Transport should end in io.ErrUnexpectedEOF, not io.EOF.

Fixes #5738

R=golang-dev, kevlar, r
CC=golang-dev
https://golang.org/cl/10237050

src/pkg/net/http/response_test.go

@@ -565,3 +565,29 @@ func TestResponseStatusStutter(t *testing.T) {
 		t.Errorf("stutter in status: %s", buf.String())
 	}
 }
+
+func TestResponseContentLengthShortBody(t *testing.T) {
+	const shortBody = "Short body, not 123 bytes."
+	br := bufio.NewReader(strings.NewReader("HTTP/1.1 200 OK\r\n" +
+		"Content-Length: 123\r\n" +
+		"\r\n" +
+		shortBody))
+	res, err := ReadResponse(br, &Request{Method: "GET"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.ContentLength != 123 {
+		t.Fatalf("Content-Length = %d; want 123", res.ContentLength)
+	}
+	var buf bytes.Buffer
+	n, err := io.Copy(&buf, res.Body)
+	if n != int64(len(shortBody)) {
+		t.Errorf("Copied %d bytes; want %d, len(%q)", n, len(shortBody), shortBody)
+	}
+	if buf.String() != shortBody {
+		t.Errorf("Read body %q; want %q", buf.String(), shortBody)
+	}
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("io.Copy error = %#v; want io.ErrUnexpectedEOF", err)
+	}
+}

src/pkg/net/http/transfer.go

@@ -532,13 +532,22 @@ func (b *body) Read(p []byte) (n int, err error) {
 	}
 	n, err = b.Reader.Read(p)
 
-	// Read the final trailer once we hit EOF.
-	if err == io.EOF && b.hdr != nil {
-		if e := b.readTrailer(); e != nil {
-			err = e
+	if err == io.EOF {
+		// Chunked case. Read the trailer.
+		if b.hdr != nil {
+			if e := b.readTrailer(); e != nil {
+				err = e
+			}
+			b.hdr = nil
+		} else {
+			// If the server declared the Content-Length, our body is a LimitedReader
+			// and we need to check whether this EOF arrived early.
+			if lr, ok := b.Reader.(*io.LimitedReader); ok && lr.N > 0 {
+				err = io.ErrUnexpectedEOF
+			}
 		}
-		b.hdr = nil
 	}
+
 	return n, err
 }