multi_fdset must not report only the wakeup socket

[icing]

I did this

When a multi handle has no other sockets to monitor, calls like curl_multi_fdset() must not return the "wakeup" socket as the only one. This would let the application wait endlessly (or until someone calls curl_multi_wakeup().

reported-by: Bryan Henderson (libcurl dev list)

I expected the following

the returned fdset to be empty when there is nothing to monitor.

curl/libcurl version

curl 8.20.0

operating system

any

[bagder]

I figure it should only be returned while there is a name resolve in progress?

Eh, no. Not only.

[sjadhavar]

I also faced the issue where application wait endlessly as curl_multi_fdset() returned wakeup socket.

Below change in file lib/multi.c function: curl_multi_fdset() worked for me

do {
  struct Curl_easy *data = Curl_multi_get_easy(multi, mid);

  if(!data) {
    DEBUGASSERT(0);
    continue;
  }

+      /* Skip the internal admin easy when building fd_sets. So that wakeup socket from
+      * Curl_multi_pollset() is not exposed to legacy external-poll apps.
+       * curl_multi_poll()/curl_multi_wait() adds wakeup socket when needed.
+      */
+    if(data == multi->admin)
+        continue;

  Curl_multi_pollset(data, &ps);

[icing]

I propose to fix this via #22066

@sjadhavar that may get you intro trouble when using the threaded resolver.

[icing]

Bryan Henderson confirmed on the libcurl dev list that #22066 fixes the issue for him.