fix: -Wunsafe-buffer-usage warning in WriteAsciiChunk() (#44169)

* fix: -Wunsafe-buffer-usage warning in WriteAsciiChunk()

Co-authored-by: Charles Kerr <[email protected]>

* chore: add // SAFETY comment to explain UNSAFE_BUFFERS() use

Co-authored-by: Charles Kerr <[email protected]>

---------

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <[email protected]>

shell/common/heap_snapshot.cc

@@ -4,6 +4,7 @@
 
 #include "shell/common/heap_snapshot.h"
 
+#include "base/containers/span.h"
 #include "base/files/file.h"
 #include "base/memory/raw_ptr.h"
 #include "v8/include/v8-profiler.h"
@@ -24,8 +25,13 @@ class HeapSnapshotOutputStream : public v8::OutputStream {
   void EndOfStream() override { is_complete_ = true; }
 
   v8::OutputStream::WriteResult WriteAsciiChunk(char* data, int size) override {
-    auto bytes_written = file_->WriteAtCurrentPos(data, size);
-    return bytes_written == size ? kContinue : kAbort;
+    const uint8_t* udata = reinterpret_cast<const uint8_t*>(data);
+    const size_t usize = static_cast<size_t>(std::max(0, size));
+    // SAFETY: since WriteAsciiChunk() only gives us data + size, our
+    // UNSAFE_BUFFERS macro call is unavoidable here. It can be removed
+    // if/when v8 changes WriteAsciiChunk() to pass a v8::MemorySpan.
+    const auto data_span = UNSAFE_BUFFERS(base::span(udata, usize));
+    return file_->WriteAtCurrentPosAndCheck(data_span) ? kContinue : kAbort;
   }
 
  private: