libziparchive: ensure ReadAtOffset is atomic

ag/880725 modified ReadAtOffset to seek then read from the open
file descriptor. Previously pread64 was used to provide atomic
behaviour.

This causes races when multiple threads are trying to access data from
the file. This is supported, so this change reverts the relevant parts
of the above CL to restore the old behaviour.
Bug:27563413

Change-Id: I7bffd78da8c558745dfc3c072ba9691b1b15bb5b

1 files changed, 3 insertions, 0 deletions

diff --git a/include/ziparchive/zip_archive.h b/include/ziparchive/zip_archive.h
index 3591a6beb..7dc60aed2 100644
--- a/include/ziparchive/zip_archive.h
+++ b/include/ziparchive/zip_archive.h
@@ -152,6 +152,9 @@ void CloseArchive(ZipArchiveHandle handle);
  * if this file entry contains a data descriptor footer. To verify crc32s
  * and length, a call to VerifyCrcAndLengths must be made after entry data
  * has been processed.
+ *
+ * On non-Windows platforms this method does not modify internal state and
+ * can be called concurrently.
  */
 int32_t FindEntry(const ZipArchiveHandle handle, const ZipString& entryName,
                   ZipEntry* data);