Treat SkFILEStream's fOriginalOffset as the start

Bug: b/78866720

::rewind() rewinds to fOriginalOffset
::seek(position) seeks to position + fOriginalOffset
::move(offset) will not move < fOriginalOffset
::getPosition() returns position relative to fOriginalOffset
::getLength() returns full size minus fOriginalOffset
::duplicate() and ::fork() pass on fOriginalOffset

Android may create an SkFILEStream using a file descriptor whose offset
is at the beginning of the data that Android cares about. Treat all
positions in SkFILEStream as relative to that original offset.

This allows AnimatedImageDrawable to read directly from the
SkFILEStream, rather than using an SkFrontBufferedStream and forcing
SkGifCodec to cache data for later use.

This fixes a TODO that was introduced in
https://skia-review.googlesource.com/c/skia/+/9498 and takes it a step
further. In that CL, bungeman@ and I discussed the change and decided to
"leave this alone for now to avoid changing behavior". Doing a code
search today, the only two callers want the new behavior.

Change-Id: I9211394d5b730adf528fac0df0af7a664b1295be
Reviewed-on: https://skia-review.googlesource.com/126511
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index e9e88811f3..75fef125d9 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -289,8 +289,10 @@ public:
      */
     explicit SkFILEStream(const char path[] = nullptr);
 
-    /** Initialize the stream with an existing C file stream.
-     *  The C file stream will be closed in the destructor.
+    /** Initialize the stream with an existing C FILE stream.
+     *  The current position of the C FILE stream will be considered the
+     *  beginning of the SkFILEStream.
+     *  The C FILE stream will be closed in the destructor.
      */
     explicit SkFILEStream(FILE* file);