)]}'
{
  "commit": "92e78f7e8d1bc92cb91d18e912ee20076a4747aa",
  "tree": "bc6e299753df710f29fc5d97fe54bc406607d29f",
  "parents": [
    "c4e9966a32beb2d1d5fca4cbbe8c7e6cd484f976"
  ],
  "author": {
    "name": "Austin Clements",
    "email": "austin@google.com",
    "time": "Fri Mar 15 13:29:40 2019 -0400"
  },
  "committer": {
    "name": "Brad Fitzpatrick",
    "email": "bradfitz@golang.org",
    "time": "Fri Apr 05 18:47:42 2019 +0000"
  },
  "message": "[release-branch.go1.12] runtime: fix write barrier on wasm\n\nThe current wasm write barrier implementation incorrectly implements\nthe \"deletion\" part of the barrier. It correctly greys the new value\nof the pointer, but rather than also greying the old value of the\npointer, it greys the object containing the slot (which, since the old\nvalue was just overwritten, is not going to contain the old value).\n\nThis can lead to unmarked, reachable objects.\n\nOften, this is masked by other marking activity, but one specific\nsequence that can lead to an unmarked object because of this bug is:\n\n1. Initially, GC is off, object A is reachable from just one pointer\nin the heap.\n\n2. GC starts and scans the stack of goroutine G.\n\n3. G copies the pointer to A on to its stack and overwrites the\npointer to A in the heap. (Now A is reachable only from G\u0027s stack.)\n\n4. GC finishes while A is still reachable from G\u0027s stack.\n\nWith a functioning deletion barrier, step 3 causes A to be greyed.\nWithout a functioning deletion barrier, nothing causes A to be greyed,\nso A will be freed even though it\u0027s still reachable from G\u0027s stack.\n\nThis CL fixes the wasm write barrier.\n\nFixes #30873.\n\nChange-Id: I8a74ee517facd3aa9ad606e5424bcf8f0d78e754\nReviewed-on: https://go-review.googlesource.com/c/go/+/167743\nRun-TryBot: Austin Clements \u003caustin@google.com\u003e\nReviewed-by: Cherry Zhang \u003ccherryyz@google.com\u003e\n(cherry picked from commit d9db9e32e924a60bbfbb15cc0dd7cfaaf8a62a3b)\nReviewed-on: https://go-review.googlesource.com/c/go/+/167745\nReviewed-by: Katie Hockman \u003ckatie@golang.org\u003e\nTryBot-Result: Gobot Gobot \u003cgobot@golang.org\u003e\n",
  "tree_diff": [
    {
      "type": "modify",
      "old_id": "374b9f73dbf3ec0463bcb20c7e16fb31d46ff4f1",
      "old_mode": 33188,
      "old_path": "src/runtime/asm_wasm.s",
      "new_id": "6ce1c7070a22fe00ae6b371ef1c7ba0d7740f83c",
      "new_mode": 33188,
      "new_path": "src/runtime/asm_wasm.s"
    }
  ]
}