[GitHub] [thrift] cfriedt commented on a change in pull request #2007: THRIFT-5093: TMemoryBuffer fix wpos for policy == OBSERVER

[GitBox <gi...@apache.org>]

cfriedt commented on a change in pull request #2007:
URL: https://github.com/apache/thrift/pull/2007#discussion_r443396568



##########
File path: lib/cpp/src/thrift/transport/TBufferTransports.h
##########
@@ -587,11 +587,9 @@ class TMemoryBuffer : public TVirtualTransport<TMemoryBuffer, TBufferBase> {
   void resetBuffer() {
     rBase_ = buffer_;
     rBound_ = buffer_;
-    wBase_ = buffer_;
-    // It isn't safe to write into a buffer we don't own.
-    if (!owner_) {
-      wBound_ = wBase_;
-      bufferSize_ = 0;
+    if (owner_) {
+      wBase_ = buffer_;
+      wBound_ = buffer_ + bufferSize_;

Review comment:
       If the buffer is not owned (is read-only, i.e. was created with `policy == OBSERVE`), `wBase_` is initialized to `wBase_ = buffer_ + wPos` (the end of the buffer) inside of `initCommon()` and never updated.
   
   Throughout the lifetime of the buffer, if it is not owned (read-only), `wBound_ == wBase_`, meaning there is no space to write.
   
   Only if the buffer is owned (writeable) should `wbase_` and `wbound_` be reset to the beginning / end of the buffer, in `resetBuffer()`.
   
   If the buffer is not owned (i.e. is read-only), it was created with `policy == OBSERVE`.




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