[Support] Fix thread safety issue in raw_null_ostream (llvm#162787)

The global raw_null_ostream singleton returned by llvm::nulls() is
marked as InternalBuffer rather than Unbuffered, causing it to
allocate a buffer when first written to. In multithreaded environments,
multiple threads can simultaneously trigger buffer allocation via
SetBuffered(), leading to race conditions on the buffer pointer
fields (OutBufCur, OutBufEnd).

For example:

raw_ostream::write(const char *Ptr, size_t Size)
->
raw_ostream::SetBuffered()
->
raw_ostream::SetBufferSize(size_t Size)
->
raw_ostream::SetBufferAndMode(char *BufferStart, size_t Size,
                              BufferKind Mode)

This can manifest as a heap corruption when multiple threads write to
the
null stream concurrently, as the buffer pointers will become corrupted
during the race.

The fix is to explicitly pass Unbuffered=true to the raw_pwrite_stream
constructor, ensuring the null stream never allocates a buffer and
all writes go directly to the no-op write_impl().

For example, this can fix multithreaded applications using MCELFStreamer
where getCommentOS() returns the shared nulls() singleton.

Author: scottp101

llvm/include/llvm/Support/raw_ostream.h

@@ -739,7 +739,7 @@ class LLVM_ABI raw_null_ostream : public raw_pwrite_stream {
   uint64_t current_pos() const override;
 
 public:
-  explicit raw_null_ostream() = default;
+  explicit raw_null_ostream() : raw_pwrite_stream(/*Unbuffered=*/true) {}
   ~raw_null_ostream() override;
 };
 

llvm/unittests/Support/raw_ostream_test.cpp

@@ -626,6 +626,11 @@ TEST(raw_ostreamTest, writeToDevNull) {
   EXPECT_TRUE(DevNullIsUsed);
 }
 
+TEST(raw_ostreamTest, nullStreamZeroBufferSize) {
+  raw_ostream &NullStream = nulls();
+  EXPECT_EQ(NullStream.GetBufferSize(), 0u);
+}
+
 TEST(raw_ostreamTest, writeToStdOut) {
   outs().flush();
   testing::internal::CaptureStdout();