Make SkCodec support peek() and read()

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 11eb1f98479461b7307695ac49d44cde6f85cecb..27fe88befb07e2ed377a115001b7edb93f897349 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -20,7 +20,7 @@
 #include "SkWebpCodec.h"
 
 struct DecoderProc {
-    bool (*IsFormat)(SkStream*);
+    bool (*IsFormat)(const char*, size_t);
     SkCodec* (*NewFromStream)(SkStream*);
 };
 
@@ -42,23 +42,29 @@ SkCodec* SkCodec::NewFromStream(SkStream* stream,
     }
 
     SkAutoTDelete<SkStream> streamDeleter(stream);
-    
+
+    // 14 is enough to read all of the supported types.

[scroggo, 2015/11/25 22:08:35]

I've convinced myself that 14 bytes is enough to support the largest wbmp we
support, but have failed at writing a test to prove it so.

[msarett, 2015/11/30 14:08:11]

This matches my understanding as well.  Looks like 1 + 1 + 3 (width) + 3
(height) is the most we should need.

[scroggo, 2015/11/30 20:02:30]

Added a test.

+    size_t bytesRead = 14;
+    char buffer[bytesRead];
+    if (!stream->peek(buffer, bytesRead)) {
+        SkCodecPrintf("Could not peek!\n");
+        // It is possible the stream does not support peeking, but does support
+        // rewinding, so try reading.
+        // (Is there a valid image that is less than 14 bytes, causing peek to
+        // fail, even though we read enough?)

[msarett, 2015/11/30 14:08:11]

Good question.  I don't know, but I would guess that there is.  Maybe a 1x1
wbmp?

[scroggo, 2015/11/30 20:02:30]

Yeah, that should do it:

1 + 1 (headers) + 1 (width) + 1 (height) + 1 (1 bit, padded to a byte). I'll
update the comment.

+        bytesRead = stream->read(buffer, bytesRead);
+        if (!stream->rewind()) {
+            return nullptr;
+        }
+    }
     SkAutoTDelete<SkCodec> codec(nullptr);
     // PNG is special, since we want to be able to supply an SkPngChunkReader.
     // But this code follows the same pattern as the loop.
-    const bool isPng = SkPngCodec::IsPng(stream);
-    if (!stream->rewind()) {
-        return NULL;
-    }
-    if (isPng) {
+    if (SkPngCodec::IsPng((const char*)buffer, bytesRead)) {
         codec.reset(SkPngCodec::NewFromStream(streamDeleter.detach(), chunkReader));
     } else {
         for (DecoderProc proc : gDecoderProcs) {
-            const bool correctFormat = proc.IsFormat(stream);
-            if (!stream->rewind()) {
-                return nullptr;
-            }
-            if (correctFormat) {
+            if (proc.IsFormat(buffer, bytesRead)) {
                 codec.reset(proc.NewFromStream(streamDeleter.detach()));
                 break;
             }