From: Nigel Tao <nigeltao@golang.org>
Date: Thu, 27 Oct 2016 02:36:48 +0000 (+1100)
Subject: image/png: allow tRNS chunk without a PLTE chunk.
X-Git-Tag: go1.8beta1~570
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=de4b06559119eac499906aaa4d42046b3beed52d;p=gostls13.git

image/png: allow tRNS chunk without a PLTE chunk.

While https://www.w3.org/TR/PNG/#5ChunkOrdering says that tRNS's
ordering constraint is "After PLTE; before IDAT", it is legal for a tRNS
chunk to occur without a PLTE chunk at all, for greyscale and truecolor
transparency as opposed to palette-based transparency. See
https://www.w3.org/TR/PNG/#11transinfo

Fixes #17511.

Change-Id: I047b0b01d78a1cda65e00eeac229bb972cda431d
Reviewed-on: https://go-review.googlesource.com/32139
Reviewed-by: Rob Pike <r@golang.org>
---

diff --git a/src/image/png/reader.go b/src/image/png/reader.go
index 2dd5ed8073..44214eda29 100644
--- a/src/image/png/reader.go
+++ b/src/image/png/reader.go
@@ -709,7 +709,11 @@ func (d *decoder) parseChunk() error {
 		d.stage = dsSeenPLTE
 		return d.parsePLTE(length)
 	case "tRNS":
-		if d.stage != dsSeenPLTE {
+		if cbPaletted(d.cb) {
+			if d.stage != dsSeenPLTE {
+				return chunkOrderError
+			}
+		} else if d.stage != dsSeenIHDR {
 			return chunkOrderError
 		}
 		d.stage = dsSeentRNS