Fixes for SkRWBuffer

Do not call SkBufferHead::validate in SkROBuffer's destructor, which
may be called in a separate thread from SkRWBuffer::append. validate()
reads SkBufferBlock::fUsed, and append() writes to it, resulting in
a data race.

Update some comments to be more clear about how it is safe to use
these classes across threads.

Test the readers in separate threads.

In addition, make sure it is safe to create a reader even when no
data has been appended. Add tests for this case.

Mark a parameter to SkBufferHead::validate() as const, reflecting
its use.

BUG=chromium:601578
BUG=chromium:605479

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1871953002

Review URL: https://codereview.chromium.org/1871953002

1 files changed, 8 insertions, 5 deletions

diff --git a/include/core/SkRWBuffer.h b/include/core/SkRWBuffer.h
index d054e0a5fd..9626d28df9 100644
--- a/include/core/SkRWBuffer.h
+++ b/include/core/SkRWBuffer.h
@@ -53,22 +53,25 @@ public:
     private:
         const SkBufferBlock* fBlock;
         size_t               fRemaining;
+        const SkROBuffer*    fBuffer;
     };
 
 private:
-    SkROBuffer(const SkBufferHead* head, size_t available);
+    SkROBuffer(const SkBufferHead* head, size_t available, const SkBufferBlock* fTail);
     virtual ~SkROBuffer();
 
-    const SkBufferHead* fHead;
-    const size_t        fAvailable;
+    const SkBufferHead*     fHead;
+    const size_t            fAvailable;
+    const SkBufferBlock*    fTail;
 
     friend class SkRWBuffer;
 };
 
 /**
  *  Accumulates bytes of memory that are "appended" to it, growing internal storage as needed.
- *  The growth is done such that at any time, a RBuffer or StreamAsset can be snapped off, which
- *  can see the previously stored bytes, but which will be unaware of any future writes.
+ *  The growth is done such that at any time in the writer's thread, an RBuffer or StreamAsset
+ *  can be snapped off (and safely passed to another thread). The RBuffer/StreamAsset snapshot
+ *  can see the previously stored bytes, but will be unaware of any future writes.
  */
 class SK_API SkRWBuffer {
 public: