disallow reentrancy

Reentrancy causes event reordering and stack explosions, in addition to
leading to hard-to-debug scenarios.

Since none of chronos is actually tested under reentrant conditions (ie
that all features such as exceptions, cancellations, buffer operations,
timers etc work reliably when the loop is reentered), this is hard to
support over time and prevents useful optimizations - this PR simply
detects and disallows the behaviour to remove the uncertainty,
simplifying reasoning about the event loop in general.

chronos/internal/asyncengine.nim

@@ -64,20 +64,23 @@ type
     ticks*: Deque[AsyncCallback]
     trackers*: Table[string, TrackerBase]
     counters*: Table[string, TrackerCounter]
-
-proc sentinelCallbackImpl(arg: pointer) {.gcsafe, noreturn.} =
-  raiseAssert "Sentinel callback MUST not be scheduled"
-
-const
-  SentinelCallback = AsyncCallback(function: sentinelCallbackImpl,
-                                   udata: nil)
-
-proc isSentinel(acb: AsyncCallback): bool =
-  acb == SentinelCallback
+    polling*: bool
+      ## The event loop is currently running
 
 proc `<`(a, b: TimerCallback): bool =
   result = a.finishAt < b.finishAt
 
+template preparePoll(loop: PDispatcherBase) =
+  # If you hit this assert, you've called `poll`, `runForever` or `waitFor`
+  # from within an async function which is not supported due to the difficulty
+  # to control stack depth and event ordering
+  # If you're using `waitFor`, switch to `await` and / or propagate the
+  # up the call stack.
+  doAssert not loop.polling, "The event loop and chronos functions in general are not reentrant"
+
+  loop.polling = true
+  defer: loop.polling = false
+
 func getAsyncTimestamp*(a: Duration): auto {.inline.} =
   ## Return rounded up value of duration with milliseconds resolution.
   ##
@@ -142,10 +145,10 @@ template processTicks(loop: untyped) =
     loop.callbacks.addLast(loop.ticks.popFirst())
 
 template processCallbacks(loop: untyped) =
-  while true:
-    let callable = loop.callbacks.popFirst()  # len must be > 0 due to sentinel
-    if isSentinel(callable):
-      break
+  # Process existing callbacks but not those that follow, to allow the network
+  # to regain control regularly
+  for _ in 0..<loop.callbacks.len():
+    let callable = loop.callbacks.popFirst()
     if not(isNil(callable.function)):
       callable.function(callable.udata)
 
@@ -337,7 +340,6 @@ elif defined(windows):
       trackers: initTable[string, TrackerBase](),
       counters: initTable[string, TrackerCounter]()
     )
-    res.callbacks.addLast(SentinelCallback)
     initAPI(res)
     res
 
@@ -585,16 +587,13 @@ elif defined(windows):
 
   proc poll*() =
     let loop = getThreadDispatcher()
+    loop.preparePoll()
+
     var
       curTime = Moment.now()
       curTimeout = DWORD(0)
       events: array[MaxEventsCount, osdefs.OVERLAPPED_ENTRY]
 
-    # On reentrant `poll` calls from `processCallbacks`, e.g., `waitFor`,
-    # complete pending work of the outer `processCallbacks` call.
-    # On non-reentrant `poll` calls, this only removes sentinel element.
-    processCallbacks(loop)
-
     # Moving expired timers to `loop.callbacks` and calculate timeout
     loop.processTimersGetTimeout(curTimeout)
 
@@ -660,14 +659,10 @@ elif defined(windows):
     # We move tick callbacks to `loop.callbacks` always.
     processTicks(loop)
 
-    # All callbacks which will be added during `processCallbacks` will be
-    # scheduled after the sentinel and are processed on next `poll()` call.
-    loop.callbacks.addLast(SentinelCallback)
+    # Process the callbacks currently scheduled - new callbacks scheduled during
+    # callback execution will run in the next poll iteration
     processCallbacks(loop)
 
-    # All callbacks done, skip `processCallbacks` at start.
-    loop.callbacks.addFirst(SentinelCallback)
-
   proc closeSocket*(fd: AsyncFD, aftercb: CallbackFunc = nil) =
     ## Closes a socket and ensures that it is unregistered.
     let loop = getThreadDispatcher()
@@ -758,7 +753,6 @@ elif defined(macosx) or defined(freebsd) or defined(netbsd) or
       trackers: initTable[string, TrackerBase](),
       counters: initTable[string, TrackerCounter]()
     )
-    res.callbacks.addLast(SentinelCallback)
     initAPI(res)
     res
 
@@ -1014,14 +1008,11 @@ elif defined(macosx) or defined(freebsd) or defined(netbsd) or
   proc poll*() {.gcsafe.} =
     ## Perform single asynchronous step.
     let loop = getThreadDispatcher()
+    loop.preparePoll()
+
     var curTime = Moment.now()
     var curTimeout = 0
 
-    # On reentrant `poll` calls from `processCallbacks`, e.g., `waitFor`,
-    # complete pending work of the outer `processCallbacks` call.
-    # On non-reentrant `poll` calls, this only removes sentinel element.
-    processCallbacks(loop)
-
     # Moving expired timers to `loop.callbacks` and calculate timeout.
     loop.processTimersGetTimeout(curTimeout)
 
@@ -1068,14 +1059,10 @@ elif defined(macosx) or defined(freebsd) or defined(netbsd) or
     # We move tick callbacks to `loop.callbacks` always.
     processTicks(loop)
 
-    # All callbacks which will be added during `processCallbacks` will be
-    # scheduled after the sentinel and are processed on next `poll()` call.
-    loop.callbacks.addLast(SentinelCallback)
+    # Process the callbacks currently scheduled - new callbacks scheduled during
+    # callback execution will run in the next poll iteration
     processCallbacks(loop)
 
-    # All callbacks done, skip `processCallbacks` at start.
-    loop.callbacks.addFirst(SentinelCallback)
-
 else:
   proc initAPI() = discard
   proc globalInit() = discard

tests/testbugs.nim

@@ -101,40 +101,6 @@ suite "Asynchronous issues test suite":
 
     result = r1 and r2
 
-  proc createBigMessage(size: int): seq[byte] =
-    var message = "MESSAGE"
-    var res = newSeq[byte](size)
-    for i in 0 ..< len(result):
-      res[i] = byte(message[i mod len(message)])
-    res
-
-  proc testIndexError(): Future[bool] {.async.} =
-    var server = createStreamServer(initTAddress("127.0.0.1:0"),
-                                    flags = {ReuseAddr})
-    let messageSize = DefaultStreamBufferSize * 4
-    var buffer = newSeq[byte](messageSize)
-    let msg = createBigMessage(messageSize)
-    let address = server.localAddress()
-    let afut = server.accept()
-    let outTransp = await connect(address)
-    let inpTransp = await afut
-    let bytesSent = await outTransp.write(msg)
-    check bytesSent == messageSize
-    var rfut {.used.} = inpTransp.readExactly(addr buffer[0], messageSize)
-
-    proc waiterProc(udata: pointer) {.raises: [], gcsafe.} =
-      try:
-        waitFor(sleepAsync(0.milliseconds))
-      except CatchableError:
-        raiseAssert "Unexpected exception happened"
-    let timer {.used.} = setTimer(Moment.fromNow(0.seconds), waiterProc, nil)
-    await sleepAsync(100.milliseconds)
-
-    await inpTransp.closeWait()
-    await outTransp.closeWait()
-    await server.closeWait()
-    return true
-
   test "Issue #6":
     check waitFor(issue6()) == true
 
@@ -149,6 +115,3 @@ suite "Asynchronous issues test suite":
 
   test "Defer for asynchronous procedures test [Nim's issue #13899]":
     check waitFor(testDefer()) == true
-
-  test "IndexError crash test":
-    check waitFor(testIndexError()) == true

tests/testmacro.nim

@@ -555,3 +555,14 @@ suite "Exceptions tracking":
         await raiseException()
 
     waitFor(callCatchAll())
+
+  test "No poll re-entrancy allowed":
+    proc testReentrancy() {.async.} =
+      await sleepAsync(1.milliseconds)
+      poll()
+
+    let reenter = testReentrancy()
+    expect(Defect):
+      waitFor reenter
+
+    await reenter.cancelAndWait() # avoid pending future leaks