upstream: fix deadlock when destroying connections

When workers are enabled and a timeout occurs in a connection most of
cases a deadlock is held in the active worker:

  ==1654992== Thread #4: Attempt to re-lock a non-recursive lock I already hold
  ==1654992==    at 0x484BB44: ??? (in /usr/libexec/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==1654992==    by 0x197579: prepare_destroy_conn_safe (flb_upstream.c:435)
  ==1654992==    by 0x197887: create_conn (flb_upstream.c:533)
  ==1654992==    by 0x197DBB: flb_upstream_conn_get (flb_upstream.c:674)
  ==1654992==    by 0x2396D3: http_post (http.c:86)
  ==1654992==    by 0x23A5E5: cb_http_flush (http.c:338)
  ==1654992==    by 0x17FE6B: output_pre_cb_flush (flb_output.h:511)
  ==1654992==    by 0x503DAA: co_init (amd64.c:117)
  ==1654992==  Lock was previously acquired
  ==1654992==    at 0x484BC0F: ??? (in /usr/libexec/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==1654992==    by 0x19815F: flb_upstream_conn_timeouts (flb_upstream.c:780)
  ==1654992==    by 0x17FEFC: cb_thread_sched_timer (flb_output_thread.c:58)
  ==1654992==    by 0x193ED7: flb_sched_event_handler (flb_scheduler.c:422)
  ==1654992==    by 0x180672: output_thread (flb_output_thread.c:265)
  ==1654992==    by 0x199602: step_callback (flb_worker.c:44)
  ==1654992==    by 0x484E8AA: ??? (in /usr/libexec/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==1654992==    by 0x4E3F926: start_thread (pthread_create.c:435)
  ==1654992==    by 0x4ECF9E3: clone (clone.S:100)

The following patch fix the behavior on prepare_destroy_conn_safe by 'trying to acquire'
the mutex lock, if it fails to acquire it, it will asssume it's already locked and no
new lock is required.

Signed-off-by: Eduardo Silva <eduardo@calyptia.com>

Author: edsiper

src/flb_upstream.c

@@ -429,15 +429,19 @@ static int prepare_destroy_conn(struct flb_upstream_conn *u_conn)
 static inline int prepare_destroy_conn_safe(struct flb_upstream_conn *u_conn)
 {
     int ret;
+    int locked = FLB_FALSE;
     struct flb_upstream *u = u_conn->u;
 
     if (u->thread_safe == FLB_TRUE) {
-        pthread_mutex_lock(&u->mutex_lists);
+        ret = pthread_mutex_trylock(&u->mutex_lists);
+        if (ret == 0) {
+            locked = FLB_TRUE;
+        }
     }
 
     ret = prepare_destroy_conn(u_conn);
 
-    if (u->thread_safe == FLB_TRUE) {
+    if (u->thread_safe == FLB_TRUE && locked) {
         pthread_mutex_unlock(&u->mutex_lists);
     }