[Backport] Fixes callbacks being dropped on Linux/macOS/BSD.

Fixes #15003.

This is a serious bug which occurs when data cannot be read/sent
immediately and there are a bunch of other read/write events
pending. What happens is that the new events are dropped which
results in the case of the reported bug resulted in some data not
being sent (!).

lib/pure/asyncdispatch.nim

@@ -1228,14 +1228,19 @@ else:
     let newLength = max(len(curList), InitCallbackListSize)
     var newList = newSeqOfCap[Callback](newLength)
 
+    var eventsExtinguished = false
     for cb in curList:
+      if eventsExtinguished:
+        newList.add(cb)
+        continue
       if not cb(fd):
         # Callback wants to be called again.
         newList.add(cb)
         # This callback has returned with EAGAIN, so we don't need to
         # call any other callbacks as they are all waiting for the same event
         # on the same fd.
-        break
+        # We do need to ensure they are called again though.
+        eventsExtinguished = true
 
     withData(selector, fd.int, fdData) do:
       # Descriptor is still present in the queue.

tests/async/tasynceagain.nim

@@ -0,0 +1,67 @@
+discard """
+  disabled: "windows"
+  exitcode: 0
+"""
+# AsyncSocketBug.nim
+# Jens Alfke (@snej) -- 16 July 2020
+# Demonstrates data loss by Nim's AsyncSocket.
+# Just run it, and it will raise an assertion failure within a minute.
+
+import asyncdispatch, asyncnet, strformat, strutils, sugar
+
+const FrameSize = 9999   # Exact size not important, but larger sizes fail quicker
+
+proc runServer() {.async.} =
+  # Server side:
+  var server = newAsyncSocket()
+  server.bindAddr(Port(9001))
+  server.listen()
+  let client = await server.accept()
+  echo "Server got client connection"
+  var lastN = 0
+  while true:
+    let frame = await client.recv(FrameSize)
+    assert frame.len == FrameSize
+    let n = frame[0..<6].parseInt()
+    echo "RCVD #", n, ":  ", frame[0..80], "..."
+    if n != lastN + 1:
+      echo &"******** ERROR: Server received #{n}, but last was #{lastN}!"
+    assert n == lastN + 1
+    lastN = n
+    await sleepAsync 100
+
+
+proc main() {.async.} =
+  asyncCheck runServer()
+
+  # Client side:
+  let socket = newAsyncSocket(buffered = false)
+  await socket.connect("localhost", Port(9001))
+  echo "Client socket connected"
+
+  var sentCount = 0
+  var completedCount = 0
+
+  while sentCount < 2000:
+    sentCount += 1
+    let n = sentCount
+
+    var message = &"{n:06} This is message #{n} of ∞. Please stay tuned for more. "
+    #echo ">>> ", message
+    while message.len < FrameSize:
+      message = message & message
+    let frame = message[0..<FrameSize]
+
+    capture n:
+      socket.send(frame).addCallback proc(f: Future[void]) =
+        # Callback when the send completes:
+        assert not f.failed
+        echo "SENT #", n
+        if n != completedCount + 1:
+          echo &"******** ERROR: Client completed #{n}, but last completed was #{completedCount}!"
+        # If this assert is enabled, it will trigger earlier than the server-side assert above:
+        assert n == completedCount + 1
+        completedCount = n
+    await sleepAsync 1
+
+waitFor main()