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{
  "commit": "3a914a9ad18d8db13147595985dda70602b7328f",
  "tree": "45fa5f21be970281fd6598fb132b54701a619b8e",
  "parents": [
    "a995d36a1e4a208300fae7e1b4eba44465dd0e98"
  ],
  "author": {
    "name": "S Ashwin Balaji",
    "email": "sashwinbalaji@google.com",
    "time": "Tue Aug 18 02:36:25 2026 +0100"
  },
  "committer": {
    "name": "S Ashwin Balaji",
    "email": "sashwinbalaji@google.com",
    "time": "Tue Sep 01 16:49:56 2026 +0100"
  },
  "message": "tracing: add the v2 shared-ring chunk protocol\n\nAdd the producer-local shared ring used by tracing v2. This change contains\nthe shared-memory ABI, one writer per TraceWriter, the single reader, and the\nprotocol tests. SDK and service wiring follow in separate changes.\n\n- Define a 64-byte ring header:\n  - an always-lock-free atomic64 packs write_pos in the high half and read_pos\n    in the low half;\n  - num_writers_waiting avoids a wake syscall when no writer is asleep;\n  - a zero-filled mapping is a valid empty ring.\n\n- Define one atomic32 state word at the start of each chunk:\n  - Free(wrap) exposes the chunk to one ring traversal;\n  - BeingWritten identifies the writer and its published fragment prefix;\n  - Complete says the writer has finished touching the chunk, although it may\n    take the chunk back to append more fragments;\n  - RewriteRequested says the reader took the published prefix and the writer\n    must move any newer suffix;\n  - RewriteAcknowledged says the writer released the old chunk and the reader\n    may return it to Free.\n\n- Reserve uint32 positions in FIFO order and map them onto a power-of-two\n  number of physical chunks. Free carries a 16-bit wrap count so a delayed\n  writer cannot claim the same chunk during a later traversal.\n\n- Keep ownership rules local to the chunk word:\n  - one reservation permits one exact claim attempt;\n  - only the reader writes Free;\n  - the reader never waits for a writer;\n  - a lost reader CAS leaves read_pos unchanged and retries the position in a\n    later drain.\n\n- Store target BufferID after the state word. Payload fragments grow towards\n  higher addresses while their sizes grow down from the end of the chunk as\n  shortest-form protobuf varints. Fragment counts publish an append-only\n  prefix. If the reader reaches an active writer, it copies that prefix and\n  asks the writer to move only the unpublished suffix.\n\n- Keep the initial 256-byte minimum chunk size and require a four-byte-aligned\n  chunk stride. Check ring geometry and allocation arithmetic before mapping\n  memory. Require atomic32 and atomic64 to be always lock-free.\n\n- Honor BufferExhaustedPolicy in SharedRingBufferWriter. Linux and Android\n  writers can wait on the read_pos half of rw_positions with a futex. Other\n  platforms report that blocking is unavailable instead of spinning. The\n  waiter hint and its seq_cst fence pair prevent a missed wake across the two\n  atomic words.\n\n- Test ABI encoding, position and wrap rollover, malformed chunk input, every\n  state transition, failed claims, prefix copying, suffix relocation, futex\n  wake races, and concurrent multi-writer stress.\n\nBug: 536851377\n",
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