abandon write

Summary: Lets you bail from writing to cursor without causing an exception. Useful when you have an error and don't want to continue writing.

Reviewed By: yfeldblum

Differential Revision: D71008395

fbshipit-source-id: 0fa60d87a3e8694c8e3be1b7f742b226e20a8836

Author: Robert Roeser

thrift/lib/cpp2/protocol/CursorBasedSerializer.h

@@ -186,6 +186,18 @@ class CursorSerializationWrapper {
     done();
   }
 
+  /**
+   * Allows writing to be aborted. This is useful when you have an error
+   * condition and you want to avoid writing to the buffer. Prevents
+   * CursorSerializationWrapper dtor from throwing if the write was not
+   * completed.
+   */
+  void abandonWrite(StructuredCursorWriter<Tag>&& writer) {
+    writer.abandon();
+    queue_.reset();
+    done();
+  }
+
   /** Access to serialized data */
   const folly::IOBuf& serializedData() const& {
     checkHasData();

thrift/lib/cpp2/protocol/detail/CursorBasedSerialization.h

@@ -160,6 +160,7 @@ class BaseCursorWriter {
     Active,
     Child,
     Done,
+    Abandonded,
   };
   State state_ = State::Active;
 
@@ -183,7 +184,8 @@ class BaseCursorWriter {
   }
 
   ~BaseCursorWriter() {
-    DCHECK(state_ == State::Done) << "Writer must be passed to endWrite";
+    DCHECK(state_ == State::Done || state_ == State::Abandonded)
+        << "Writer must be passed to endWrite";
   }
 
   BaseCursorWriter(BaseCursorWriter&& other) noexcept {
@@ -193,7 +195,8 @@ class BaseCursorWriter {
   }
   BaseCursorWriter& operator=(BaseCursorWriter&& other) noexcept {
     if (this != &other) {
-      DCHECK(state_ == State::Done) << "Writer must be passed to endWrite";
+      DCHECK(state_ == State::Done || state_ == State::Abandonded)
+          << "Writer must be passed to endWrite";
       protocol_ = std::move(other.protocol_);
       state_ = other.state_;
       other.state_ = State::Done;
@@ -207,6 +210,8 @@ class BaseCursorWriter {
 
   template <typename T>
   friend class StructuredCursorWriter;
+
+  void abandon() { state_ = State::Abandonded; }
 };
 
 // std::swap isn't constexpr until C++20 so we need to reimplement :(

thrift/lib/cpp2/protocol/test/CursorBasedSerializerTest.cpp

@@ -765,3 +765,10 @@ TEST(CursorBasedSerializer, ConcurrentAccess) {
   EXPECT_THROW(wrapper.beginWrite(), std::runtime_error);
   wrapper.endRead(std::move(reader));
 }
+
+TEST(CursorSerializer, AbandonedWrite) {
+  CursorSerializationWrapper<Qualifiers> wrapper;
+  auto writer = wrapper.beginWrite();
+  writer.write<ident::opt>(3);
+  wrapper.abandonWrite(std::move(writer));
+}