fix: handle FDWatcher exception on closed sockets

Adding checks to handle exception thrown while constructing FDWatcher if the socket handle is closed.
Also checking the curl return code while adding handle to multi, and not proceeding if that failed.

Should fix #253

src/Curl/Multi.jl

@@ -43,14 +43,18 @@ end
 
 function add_handle(multi::Multi, easy::Easy)
     connect_semaphore_acquire(easy)
-    lock(multi.lock) do
+    added = lock(multi.lock) do
         if isempty(multi.easies)
             preserve_handle(multi)
         end
         push!(multi.easies, easy)
         init!(multi)
         @check curl_multi_add_handle(multi.handle, easy.handle)
     end
+    if added != 0
+        connect_semaphore_release(easy)
+    end
+    return added
 end
 
 const MULTIS_LOCK = Base.ReentrantLock()
@@ -179,7 +183,22 @@ function socket_callback(
         if action in (CURL_POLL_IN, CURL_POLL_OUT, CURL_POLL_INOUT)
             readable = action in (CURL_POLL_IN,  CURL_POLL_INOUT)
             writable = action in (CURL_POLL_OUT, CURL_POLL_INOUT)
-            watcher = FDWatcher(OS_HANDLE(sock), readable, writable)
+            watcher = try
+                FDWatcher(OS_HANDLE(sock), readable, writable)
+            catch watcher_ex
+                if watcher_ex isa Base.IOError
+                    task = @async begin
+                        lock(multi.lock) do
+                            @check curl_multi_socket_action(multi.handle, sock, CURL_CSELECT_ERR)
+                            check_multi_info(multi)
+                        end
+                    end
+                    @isdefined(errormonitor) && errormonitor(task)
+                    nothing
+                else
+                    rethrow()
+                end
+            end
             preserve_handle(watcher)
             watcher_p = pointer_from_objref(watcher)
             @check curl_multi_assign(multi.handle, sock, watcher_p)

src/Downloads.jl

@@ -387,7 +387,11 @@ function request(
                 easy_hook(downloader, easy, info)
 
                 # do the request
-                add_handle(downloader.multi, easy)
+                add_handle_error = add_handle(downloader.multi, easy)
+                if add_handle_error != 0
+                    no_response = Response(nothing, "", 0, "", [])
+                    throw(RequestError(url, add_handle_error, "", no_response))
+                end
                 try # ensure handle is removed
                     @sync begin
                         @async for buf in easy.output