rtc::Buffer: Handle move self-assignment

The object should end up in a valid state, just like after being moved
from.

Bug: webrtc:9857
Change-Id: Ia11f9b8e3191ffe749e4a0640cad946038f494a4
Reviewed-on: https://webrtc-review.googlesource.com/c/106701
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25233}

rtc_base/buffer.h

diff --git a/rtc_base/buffer.h b/rtc_base/buffer.h
index 9bf96cd..2707cdf 100644
--- a/rtc_base/buffer.h
+++ b/rtc_base/buffer.h
@@ -152,7 +152,9 @@
     RTC_DCHECK(buf.IsConsistent());
     size_ = buf.size_;
     capacity_ = buf.capacity_;
-    data_ = std::move(buf.data_);
+    using std::swap;
+    swap(data_, buf.data_);
+    buf.data_.reset();
     buf.OnMovedFrom();
     return *this;
   }
@@ -399,6 +401,7 @@
   // Called when *this has been moved from. Conceptually it's a no-op, but we
   // can mutate the state slightly to help subsequent sanity checks catch bugs.
   void OnMovedFrom() {
+    RTC_DCHECK(!data_);  // Our heap block should have been stolen.
 #if RTC_DCHECK_IS_ON
     // Ensure that *this is always inconsistent, to provoke bugs.
     size_ = 1;

rtc_base/buffer_unittest.cc

diff --git a/rtc_base/buffer_unittest.cc b/rtc_base/buffer_unittest.cc
index 1c3abfd..b2f47c1 100644
--- a/rtc_base/buffer_unittest.cc
+++ b/rtc_base/buffer_unittest.cc
@@ -185,6 +185,17 @@
   EXPECT_TRUE(buf1.empty());
 }
 
+TEST(BufferTest, TestMoveAssignSelf) {
+  // Move self-assignment isn't required to produce a meaningful state, but
+  // should not leave the object in an inconsistent state. (Such inconsistent
+  // state could be caught by the DCHECKs and/or by the leak checker.) We need
+  // to be sneaky when testing this; if we're doing a too-obvious
+  // move-assign-to-self, clang's -Wself-move triggers at compile time.
+  Buffer buf(kTestData, 3, 40);
+  Buffer* buf_ptr = &buf;
+  buf = std::move(*buf_ptr);
+}
+
 TEST(BufferTest, TestSwap) {
   Buffer buf1(kTestData, 3);
   Buffer buf2(kTestData, 6, 40);