io: fix infinite loop bug in MultiReader

If an io.Reader returned (non-zero, EOF), MultiReader would yield
bytes forever.

This bug has existed before Go 1 (!!), introduced in the original
MultiReader implementation in https://golang.org/cl/1764043 and also
survived basically the only update to this code since then
(https://golang.org/cl/17873, git rev ccdca832), which was added in
Go 1.7.

This just bit me when writing a test for some unrelated code.

Fixes #16795

Change-Id: I36e6a701269793935d19a47ac12f67b07179fbff
Reviewed-on: https://go-review.googlesource.com/27397
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/io/multi.go

@@ -18,15 +18,16 @@ func (mr *multiReader) Read(p []byte) (n int, err error) {
 			}
 		}
 		n, err = mr.readers[0].Read(p)
+		if err == EOF {
+			mr.readers = mr.readers[1:]
+		}
 		if n > 0 || err != EOF {
-			if err == EOF {
-				// Don't return EOF yet. There may be more bytes
-				// in the remaining readers.
+			if err == EOF && len(mr.readers) > 0 {
+				// Don't return EOF yet. More readers remain.
 				err = nil
 			}
 			return
 		}
-		mr.readers = mr.readers[1:]
 	}
 	return 0, EOF
 }

src/io/multi_test.go

@@ -196,3 +196,41 @@ func TestMultiReaderFlatten(t *testing.T) {
 			myDepth+2, readDepth)
 	}
 }
+
+// byteAndEOFReader is a Reader which reads one byte (the underlying
+// byte) and io.EOF at once in its Read call.
+type byteAndEOFReader byte
+
+func (b byteAndEOFReader) Read(p []byte) (n int, err error) {
+	if len(p) == 0 {
+		// Read(0 bytes) is useless. We expect no such useless
+		// calls in this test.
+		panic("unexpected call")
+	}
+	p[0] = byte(b)
+	return 1, EOF
+}
+
+// In Go 1.7, this yielded bytes forever.
+func TestMultiReaderSingleByteWithEOF(t *testing.T) {
+	got, err := ioutil.ReadAll(LimitReader(MultiReader(byteAndEOFReader('a'), byteAndEOFReader('b')), 10))
+	if err != nil {
+		t.Fatal(err)
+	}
+	const want = "ab"
+	if string(got) != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}
+
+// Test that a reader returning (n, EOF) at the end of an MultiReader
+// chain continues to return EOF on its final read, rather than
+// yielding a (0, EOF).
+func TestMultiReaderFinalEOF(t *testing.T) {
+	r := MultiReader(bytes.NewReader(nil), byteAndEOFReader('a'))
+	buf := make([]byte, 2)
+	n, err := r.Read(buf)
+	if n != 1 || err != EOF {
+		t.Errorf("got %v, %v; want 1, EOF", n, err)
+	}
+}