Fixed a bug when subscribing and unsubscribing in callback handling

[python36]

Any manipulation of topics (subscription or unsubscription) changes the length of the callback list in the iteration cycle and causes an exception.

For example:

import nmqtt
import asyncdispatch

let ctx = newMqttCtx("test")
ctx.setHost("localhost", 1883)

proc secondOnMsg(topic: string, messageRaw: string) =
  echo "second"

proc subs(topic: string) {.async.} =
  await ctx.subscribe(topic, 0, secondOnMsg)

proc firstOnMsg(topic: string, messageRaw: string) =
  echo "first"
  waitFor subs("second_topic")

proc mqttSub() {.async.} =
  await ctx.start()
  await ctx.subscribe("first_topic", 0, firstOnMsg)

asyncCheck mqttSub()
runForever()

Raise next exception:

Hint: used config file '/home/user/.choosenim/toolchains/nim-2.2.4/config/nim.cfg' [Conf]
Hint: used config file '/home/user/.choosenim/toolchains/nim-2.2.4/config/config.nims' [Conf]
.............................................................................................................................................
/opt/machina/test.nim(7, 7) Hint: 'baseTopicWill' is declared but not used [XDeclaredButNotUsed]
/opt/machina/test.nim(9, 5) Hint: 'log' is declared but not used [XDeclaredButNotUsed]
/opt/machina/test.nim(5, 8) Warning: imported and not used: 'lists' [UnusedImport]
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/system/exceptions.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/private/digitsutils.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/assertions.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/objectdollar.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/system/dollars.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/syncio.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/system.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/math.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/algorithm.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/strutils.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/oserrors.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/private/ospaths2.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/options.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/times.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/envvars.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/cmdline.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/hashes.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/tables.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/heapqueue.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/sets.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/deques.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncfutures.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/std/monotimes.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/nativesockets.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/net.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/selectors.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncmacro.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim
CC: ../../home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncnet.nim
CC: ../../home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim
CC: logger.nim
CC: test.nim
Hint:  [Link]
Hint: mm: orc; threads: on; opt: none (DEBUG BUILD, `-d:release` generates faster code)
70387 lines; 2.221s; 140.688MiB peakmem; proj: /opt/machina/test.nim; out: /opt/machina/test [SuccessX]
Hint: /opt/machina/test [Exec]
first
/opt/machina/test.nim(63) test
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(2018) runForever
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1712) poll
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1453) runOnce
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/threadimpl.nim(269) processPendingCallbacks
/home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1050) runRx (Async)
/home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1024) handle (Async)
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/tables.nim(771) onPublish (Async)
/home/user/.choosenim/toolchains/nim-2.2.4/lib/std/assertions.nim(41) failedAssertImpl
/home/user/.choosenim/toolchains/nim-2.2.4/lib/std/assertions.nim(36) raiseAssert
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/fatal.nim(53) sysFatal
[[reraised from:
/opt/machina/test.nim(63) test
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(2018) runForever
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1712) poll
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1453) runOnce
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/threadimpl.nim(269) processPendingCallbacks
/home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1050) runRx (Async)
/home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1024) handle (Async)
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncfutures.nim(370) read
]]
[[reraised from:
/opt/machina/test.nim(63) test
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(2018) runForever
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1712) poll
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1453) runOnce
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/threadimpl.nim(269) processPendingCallbacks
/home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1050) runRx (Async)
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncfutures.nim(370) read
]]
[[reraised from:
/opt/machina/test.nim(63) test
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(2018) runForever
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1712) poll
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1453) runOnce
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/threadimpl.nim(269) processPendingCallbacks
/home/user/.choosenim/toolchains/nim-2.2.4/lib/std/typedthreads.nim(287) runRx (Async)
]]
[[reraised from:
/opt/machina/test.nim(63) test
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(2018) runForever
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1712) poll
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncdispatch.nim(1453) runOnce
/home/user/.choosenim/toolchains/nim-2.2.4/lib/system/threadimpl.nim(269) processPendingCallbacks
/home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/asyncfutures.nim(428) asyncCheckCallback
]]
Error: unhandled exception: /home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/tables.nim(771, 13) `len(t) == L` the length of the table changed while iterating over it
Async traceback:
  /opt/machina/test.nim(63) test
  /home/user/.choosenim/toolchains/nim-2.2.4/lib/std/typedthreads.nim(287) runRx (Async)
  /home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1050) runRx (Async)
  /home/user/.nimble/pkgs2/nmqtt-1.0.6-926c1e352673c57dcb3cdefc20cf0ca5a454d81e/nmqtt.nim(1024) handle (Async)
  /home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/tables.nim(771) onPublish (Async)
  /home/user/.choosenim/toolchains/nim-2.2.4/lib/std/assertions.nim(41) failedAssertImpl
  /home/user/.choosenim/toolchains/nim-2.2.4/lib/std/assertions.nim(36) raiseAssert
  /home/user/.choosenim/toolchains/nim-2.2.4/lib/system/fatal.nim(53) sysFatal
Exception message: /home/user/.choosenim/toolchains/nim-2.2.4/lib/pure/collections/tables.nim(771, 13) `len(t) == L` the length of the table changed while iterating over it
 [AssertionDefect]
Error: execution of an external program failed: '/opt/machina/test'

[ThomasTJdev]

@zevv - Something looks different to me in the repo settings. Could you try to enable Github Workflows to run on first-time-committers? Or provide me access to those repo-settings?

@python36 - Could you please try to run the same code with Nim v1.6.18 (choosenim 1.6.18)? Sorry for not getting back to you before now.