std/channels example fails

[timotheecour]

Example 1

taken from nim-lang/website#274

when true:
  import std/times
  var
    sender: array[10, Thread[void]]
    receiver: array[5, Thread[void]] 
  import std/[channels, isolation]
  var chan = newChannel[seq[string]](40)
  proc sendHandler() =
    chan.send(isolate(@["Hello, Nim"]))
  proc recvHandler() =
    var x: seq[string]
    chan.recv(x)
    discard x

  template benchmark() =
    for t in mitems(sender):
      t.createThread(sendHandler)
    joinThreads(sender)
    for i in 0 .. receiver.high:
      createThread(receiver[i], recvHandler)
    let start = now()
    joinThreads(receiver)
    let t2 = now()
    let t3 = t2 - start
    echo t3
  benchmark()

nim c --threads --gc:arc -g main.nim
lldb ./main

Current Output

run it a few times, and eventually you get a crash in lldb, or a hang outside of lldb:

(lldb) r
Process 68773 launched: '/Users/timothee/git_clone/nim/timn/tests/nim/all/t12015' (x86_64)
Process 68773 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x0000000100007d0d start-addr:0x0000000100007d03  t12015 eqdestroy___dS1BF3Vxjg9aJMmmhVJKSpQ(dest=0x00000001001720e8) + 45 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/fatal.nim:53:36
   50
   51    else:
   52      proc sysFatal(exceptn: typedesc, message: string) {.inline, noreturn.} =
-> 53        raise (ref exceptn)(msg: message)
   54
   55      proc sysFatal(exceptn: typedesc, message, arg: string) {.inline, noreturn.} =
   56        raise (ref exceptn)(msg: message & arg)
Target 0: (t12015) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
  * frame #0: 0x0000000100007d0d start-addr:0x0000000100007d03  t12015 eqdestroy___dS1BF3Vxjg9aJMmmhVJKSpQ(dest=0x00000001001720e8) + 45 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/fatal.nim:53:36
    frame #1: 0x0000000100010333 start-addr:0x000000010001032b  t12015 eqdestroy___0RiuPw9cXhtLB9a2rQ2jA69cg(dest=0x00007ffeefbfac80) + 83 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2247:4
    frame #2: 0x0000000100019a68 start-addr:0x0000000100019a46  t12015 dollar___PRCiermVoUNsHtsSaiiDLw(dur=(seconds = 0, nanosecond = 427000)) + 968 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2247:2
    frame #3: 0x000000010001da42 start-addr:0x000000010001da2f  t12015 NimMainModule + 1346 at  /Users/timothee/git_clone/nim/timn/tests/nim/all/t12015.nim:154:15
    frame #4: 0x000000010001d4f9 start-addr:0x000000010001d4f4  t12015 NimMainInner + 9 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2210:2
    frame #5: 0x000000010001db9a start-addr:0x000000010001db98  t12015 NimMain + 42 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2218:2
    frame #6: 0x000000010001dbee start-addr:0x000000010001dbe9  t12015 main(argc=1, args=0x00007ffeefbfae90, env=0x00007ffeefbfaea0) + 62 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2225:2
  frame #7: 0x00007fff6a526cc9 libdyld.dylib`start + 1
(lldb) ^D

Expected Output

works

Example 2

same as Example 1, but replace

  benchmark()

by

  for i in 0..<100: benchmark()

then it crashes even more reliably

Additional Information

• nim: 1.5.1 95697d0
• tested on OSX
• lldb isn't needed to reproduce the crash; just run it a few (~5) times
• I can still reproduce the crash if i use var chan = newChannel[seq[string]](4000)

/cc @xflywind

[timotheecour]

here's a stacktrace with this diff:

diff --git a/lib/std/channels.nim b/lib/std/channels.nim
index 1b0844c48..805eb99c1 100644
--- a/lib/std/channels.nim
+++ b/lib/std/channels.nim
@@ -456,7 +456,9 @@ proc `=`*[T](dest: var Channel[T], src: Channel[T]) =
 proc channelSend[T](chan: Channel[T], data: sink T, size: int, nonBlocking: bool): bool {.inline.} =
   ## Send item to the channel (FIFO queue)
   ## (Insert at last)
-  sendMpmc(chan.d, data.unsafeAddr, size, nonBlocking)
+  # sendMpmc(chan.d, data.unsafeAddr, size, nonBlocking)
+  result = sendMpmc(chan.d, data.unsafeAddr, size, nonBlocking)
+  wasMoved(result)

 proc channelReceive[T](chan: Channel[T], data: ptr T, size: int, nonBlocking: bool): bool {.inline.} =
   ## Receive an item from the channel

(lldb) r
Process 70873 launched: '/Users/timothee/git_clone/nim/timn/build/nimcache/t12015' (x86_64)
Process 70873 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xc)
    frame #0: 0x000000010000ec02 start-addr:0x000000010000ec02  t12015 rawAlloc__mE4QEVyMvGRVliDWDngZCQ(a=0x0000000100027010, requestedSize=25) + 738 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim:787:116
   784           result = c.freeList
   785           when not defined(gcDestructors):
   786             sysAssert(c.freeList.zeroField == 0, "rawAlloc 8")
-> 787           c.freeList = c.freeList.next
   788         dec(c.free, size)
   789         sysAssert((cast[ByteAddress](result) and (MemAlign-1)) == 0, "rawAlloc 9")
   790         sysAssert(allocInv(a), "rawAlloc: end c != nil")
Target 0: (t12015) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xc)
  * frame #0: 0x000000010000ec02 start-addr:0x000000010000ec02  t12015 rawAlloc__mE4QEVyMvGRVliDWDngZCQ(a=0x0000000100027010, requestedSize=25) + 738 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim:787:116
    frame #1: 0x00000001000090a5 start-addr:0x00000001000090a0  t12015 alloc__UxtcZ3QOXKsB7mMchxUf9cg(allocator=0x0000000100027010, size=25) + 101 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim:955:11
    frame #2: 0x000000010000a973 start-addr:0x000000010000a964  t12015 allocSharedImpl(size=25) + 51 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim:1075:11
    frame #3: 0x000000010000975d start-addr:0x0000000100009758  t12015 allocShared0Impl__KzdpcuLT9aef9bsiSHlIu9aFg(size=25) + 29 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim:1081:11
    frame #4: 0x000000010000f527 start-addr:0x000000010000f522  t12015 nimAsgnStrV2(a=0x00007ffeefbfaba0, b=NimStringV2 @ 0x00007ffeefbfaa40) + 679 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/strs_v2.nim:151:30
    frame #5: 0x000000010000f625 start-addr:0x000000010000f618  t12015 eqcopy___aBBXmHFBEivKqERloP6zmA(dest=0x00007ffeefbfaba0, src=NimStringV2 @ 0x00007ffeefbfaa70) + 37 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/fatal.nim:53:2
    frame #6: 0x00000001000190f7 start-addr:0x00000001000190e6  t12015 humanizeParts__AO89azFsQMOpWkka1p4T7pQ(parts=tySequence__sM4lkSb7zS6F7OVMvW9cffQ @ 0x00007ffeefbfab90) + 343 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system/fatal.nim:53:3
    frame #7: 0x00000001000199fd start-addr:0x00000001000199f0  t12015 dollar___PRCiermVoUNsHtsSaiiDLw(dur=(seconds = 0, nanosecond = 539000)) + 877 at  /Users/timothee/git_clone/nim/Nim_prs/lib/pure/times.nim:732:11
    frame #8: 0x000000010001da32 start-addr:0x000000010001da1f  t12015 NimMainModule + 1346 at  /Users/timothee/git_clone/nim/timn/tests/nim/all/t12015.nim:183:15
    frame #9: 0x000000010001d4e9 start-addr:0x000000010001d4e4  t12015 NimMainInner + 9 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2210:2
    frame #10: 0x000000010001db8a start-addr:0x000000010001db88  t12015 NimMain + 42 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2218:2
    frame #11: 0x000000010001dbde start-addr:0x000000010001dbd9  t12015 main(argc=1, args=0x00007ffeefbfae88, env=0x00007ffeefbfae98) + 62 at  /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim:2225:2
  frame #12: 0x00007fff6a526cc9 libdyld.dylib`start + 1
  frame #13: 0x00007fff6a526cc9 libdyld.dylib`start + 1


(lldb) fr v
(tyObject_MemRegion__x81NhDv59b8ercDZ9bi85jyg *) a = 0x0000000100027010
(NI) requestedSize = 25
(void *) result = 0x000000000000000c
(NI) size = 32
(TFrame) FR_ = {
  prev = 0x00007ffeefbfa948
  procname = 0x000000010001e57c "rawAlloc"
  line = 787
  filename = 0x000000010001e264 "/Users/timothee/git_clone/nim/Nim_prs/lib/system/alloc.nim"
  len = 0
  calldepth = 4
  frameMsgLen = 0
}
(NI) s = 2
(tyObject_SmallChunk__tXn60W2f8h3jgAYdEmy5NQ *) c = 0x0000000100172000

[timotheecour]

my bad with the diff, seems to work with wasMoved(data)