mime/multipart: fix bug when body contains prefix of the boundary

Fixes #10616

Change-Id: I4ef25eb0be6ccf474976fdb5087dd2c62c66c510
Reviewed-on: https://go-review.googlesource.com/11811Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/multipart/multipart.go

@@ -165,16 +165,18 @@ func (pr partReader) Read(d []byte) (n int, err error) {
 	if peek == nil {
 		panic("nil peek buf")
 	}
-
 	// Search the peek buffer for "\r\n--boundary". If found,
 	// consume everything up to the boundary. If not, consume only
 	// as much of the peek buffer as cannot hold the boundary
 	// string.
 	nCopy := 0
 	foundBoundary := false
-	if idx := bytes.Index(peek, p.mr.nlDashBoundary); idx != -1 {
+	if idx, isEnd := p.mr.peekBufferSeparatorIndex(peek); idx != -1 {
 		nCopy = idx
-		foundBoundary = true
+		foundBoundary = isEnd
+		if !isEnd && nCopy == 0 {
+			nCopy = 1 // make some progress.
+		}
 	} else if safeCount := len(peek) - len(p.mr.nlDashBoundary); safeCount > 0 {
 		nCopy = safeCount
 	} else if unexpectedEOF {
@@ -338,6 +340,33 @@ func (mr *Reader) peekBufferIsEmptyPart(peek []byte) bool {
 	return bytes.HasPrefix(rest, mr.nl)
 }
 
+// peekBufferSeparatorIndex returns the index of mr.nlDashBoundary in
+// peek and whether it is a real boundary (and not a prefix of an
+// unrelated separator). To be the end, the peek buffer must contain a
+// newline after the boundary.
+func (mr *Reader) peekBufferSeparatorIndex(peek []byte) (idx int, isEnd bool) {
+	idx = bytes.Index(peek, mr.nlDashBoundary)
+	if idx == -1 {
+		return
+	}
+	peek = peek[idx+len(mr.nlDashBoundary):]
+	if len(peek) > 1 && peek[0] == '-' && peek[1] == '-' {
+		return idx, true
+	}
+	peek = skipLWSPChar(peek)
+	// Don't have a complete line after the peek.
+	if bytes.IndexByte(peek, '\n') == -1 {
+		return -1, false
+	}
+	if len(peek) > 0 && peek[0] == '\n' {
+		return idx, true
+	}
+	if len(peek) > 1 && peek[0] == '\r' && peek[1] == '\n' {
+		return idx, true
+	}
+	return idx, false
+}
+
 // skipLWSPChar returns b with leading spaces and tabs removed.
 // RFC 822 defines:
 //    LWSP-char = SPACE / HTAB

src/mime/multipart/multipart_test.go

@@ -565,6 +565,58 @@ foo: bar
 		},
 	},
 
+	// Issue 10616; minimal
+	{
+		name: "issue 10616 minimal",
+		sep:  "sep",
+		in: "--sep \r\nFoo: bar\r\n\r\n" +
+			"a\r\n" +
+			"--sep_alt\r\n" +
+			"b\r\n" +
+			"\r\n--sep--",
+		want: []headerBody{
+			{textproto.MIMEHeader{"Foo": {"bar"}}, "a\r\n--sep_alt\r\nb\r\n"},
+		},
+	},
+
+	// Issue 10616; full example from bug.
+	{
+		name: "nested separator prefix is outer separator",
+		sep:  "----=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9",
+		in: strings.Replace(`------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9
+Content-Type: multipart/alternative; boundary="----=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt"
+
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: 8bit
+
+This is a multi-part message in MIME format.
+
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt
+Content-Type: text/html; charset="utf-8"
+Content-Transfer-Encoding: 8bit
+
+html things
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt--
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9--`, "\n", "\r\n", -1),
+		want: []headerBody{
+			{textproto.MIMEHeader{"Content-Type": {`multipart/alternative; boundary="----=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt"`}},
+				strings.Replace(`------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: 8bit
+
+This is a multi-part message in MIME format.
+
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt
+Content-Type: text/html; charset="utf-8"
+Content-Transfer-Encoding: 8bit
+
+html things
+------=_NextPart_4c2fbafd7ec4c8bf08034fe724b608d9_alt--`, "\n", "\r\n", -1),
+			},
+		},
+	},
+
 	roundTripParseTest(),
 }