fix crash after reading from fd > 1024 (#267)

The socket selector holds a `seq` of per-descriptor data. When a reader
is registered, a pointer to a seq item is stored - when the `seq` grows,
this pointer becomes dangling and causes crashes like
status-im/nimbus-eth2#3521.

It turns out that there already exist two mechanisms for passing user
data around - this PR simply removes one of them, saving on memory usage
and removing the need to store pointers to the `seq` data that become
dangling on resize.

chronos/asyncloop.nim

@@ -299,7 +299,6 @@ when defined(windows):
     CompletionKey = ULONG_PTR
 
     CompletionData* = object
-      fd*: AsyncFD
       cb*: CallbackFunc
       errCode*: OSErrorCode
       bytesCount*: int32
@@ -500,17 +499,9 @@ elif unixPlatform:
   type
     AsyncFD* = distinct cint
 
-    CompletionData* = object
-      fd*: AsyncFD
-      udata*: pointer
-
-    PCompletionData* = ptr CompletionData
-
     SelectorData* = object
       reader*: AsyncCallback
-      rdata*: CompletionData
       writer*: AsyncCallback
-      wdata*: CompletionData
 
     PDispatcher* = ref object of PDispatcherBase
       selector: Selector[SelectorData]
@@ -555,8 +546,6 @@ elif unixPlatform:
     ## Register file descriptor ``fd`` in thread's dispatcher.
     let loop = getThreadDispatcher()
     var data: SelectorData
-    data.rdata.fd = fd
-    data.wdata.fd = fd
     loop.selector.registerHandle(int(fd), {}, data)
 
   proc unregister*(fd: AsyncFD) {.raises: [Defect, CatchableError].} =
@@ -574,9 +563,8 @@ elif unixPlatform:
     let loop = getThreadDispatcher()
     var newEvents = {Event.Read}
     withData(loop.selector, int(fd), adata) do:
-      let acb = AsyncCallback(function: cb, udata: addr adata.rdata)
+      let acb = AsyncCallback(function: cb, udata: udata)
       adata.reader = acb
-      adata.rdata = CompletionData(fd: fd, udata: udata)
       newEvents.incl(Event.Read)
       if not(isNil(adata.writer.function)):
         newEvents.incl(Event.Write)
@@ -592,7 +580,6 @@ elif unixPlatform:
     withData(loop.selector, int(fd), adata) do:
       # We need to clear `reader` data, because `selectors` don't do it
       adata.reader = default(AsyncCallback)
-      # adata.rdata = CompletionData()
       if not(isNil(adata.writer.function)):
         newEvents.incl(Event.Write)
     do:
@@ -606,9 +593,8 @@ elif unixPlatform:
     let loop = getThreadDispatcher()
     var newEvents = {Event.Write}
     withData(loop.selector, int(fd), adata) do:
-      let acb = AsyncCallback(function: cb, udata: addr adata.wdata)
+      let acb = AsyncCallback(function: cb, udata: udata)
       adata.writer = acb
-      adata.wdata = CompletionData(fd: fd, udata: udata)
       newEvents.incl(Event.Write)
       if not(isNil(adata.reader.function)):
         newEvents.incl(Event.Read)
@@ -624,7 +610,6 @@ elif unixPlatform:
     withData(loop.selector, int(fd), adata) do:
       # We need to clear `writer` data, because `selectors` don't do it
       adata.writer = default(AsyncCallback)
-      # adata.wdata = CompletionData()
       if not(isNil(adata.reader.function)):
         newEvents.incl(Event.Read)
     do:
@@ -692,9 +677,7 @@ elif unixPlatform:
       var data: SelectorData
       result = loop.selector.registerSignal(signal, data)
       withData(loop.selector, result, adata) do:
-        adata.reader = AsyncCallback(function: cb, udata: addr adata.rdata)
-        adata.rdata.fd = AsyncFD(result)
-        adata.rdata.udata = udata
+        adata.reader = AsyncCallback(function: cb, udata: udata)
       do:
         raise newException(ValueError, "File descriptor not registered.")
 

chronos/transports/datagram.nim

@@ -185,7 +185,7 @@ elif defined(windows):
       else:
         ## Initiation
         transp.state.incl(WritePending)
-        let fd = SocketHandle(ovl.data.fd)
+        let fd = SocketHandle(transp.fd)
         var vector = transp.queue.popFirst()
         transp.setWriterWSABuffer(vector)
         var ret: cint
@@ -258,7 +258,7 @@ elif defined(windows):
         ## Initiation
         if transp.state * {ReadEof, ReadClosed, ReadError} == {}:
           transp.state.incl(ReadPending)
-          let fd = SocketHandle(ovl.data.fd)
+          let fd = SocketHandle(transp.fd)
           transp.rflag = 0
           transp.ralen = SockLen(sizeof(Sockaddr_storage))
           let ret = WSARecvFrom(fd, addr transp.rwsabuf, DWORD(1),
@@ -406,9 +406,9 @@ elif defined(windows):
     result.udata = udata
     result.state = {WritePaused}
     result.future = newFuture[void]("datagram.transport")
-    result.rovl.data = CompletionData(fd: localSock, cb: readDatagramLoop,
+    result.rovl.data = CompletionData(cb: readDatagramLoop,
                                       udata: cast[pointer](result))
-    result.wovl.data = CompletionData(fd: localSock, cb: writeDatagramLoop,
+    result.wovl.data = CompletionData(cb: writeDatagramLoop,
                                       udata: cast[pointer](result))
     result.rwsabuf = TWSABuf(buf: cast[cstring](addr result.buffer[0]),
                              len: int32(len(result.buffer)))
@@ -426,9 +426,8 @@ else:
   proc readDatagramLoop(udata: pointer) {.raises: Defect.}=
     var raddr: TransportAddress
     doAssert(not isNil(udata))
-    var cdata = cast[ptr CompletionData](udata)
-    var transp = cast[DatagramTransport](cdata.udata)
-    let fd = SocketHandle(cdata.fd)
+    let transp = cast[DatagramTransport](udata)
+    let fd = SocketHandle(transp.fd)
     if int(fd) == 0:
       ## This situation can be happen, when there events present
       ## after transport was closed.
@@ -459,9 +458,8 @@ else:
   proc writeDatagramLoop(udata: pointer) =
     var res: int
     doAssert(not isNil(udata))
-    var cdata = cast[ptr CompletionData](udata)
-    var transp = cast[DatagramTransport](cdata.udata)
-    let fd = SocketHandle(cdata.fd)
+    var transp = cast[DatagramTransport](udata)
+    let fd = SocketHandle(transp.fd)
     if int(fd) == 0:
       ## This situation can be happen, when there events present
       ## after transport was closed.