BUG/MEDIUM: listener: duplicate inherited FDs if needed

Since commit 36d9097cf ("MINOR: fd: Add BUG_ON checks on fd_insert()"),
there is currently a test in fd_insert() to detect that we're not trying
to reinsert an FD that had already been inserted. This test catches the
following anomalies:

   frontend fail1
       bind fd@0
       bind fd@0

and:

   frontend fail2
       bind fd@0 shards 2

What happens is that clone_listener() is called on a listener already
having an FD, and when sock_{inet,unix}_bind_receiver() are called, the
same FD will be registered multiple times and rightfully crash in the
sanity check.

It wouldn't be correct to block shards though (e.g. they could be used
in a default-bind line). What looks like a safer and more future-proof
approach simply is to dup() the FD so that each listener has one copy.
This is also the only solution that might allow later to support more
than 64 threads on an inherited FD.

This needs to be backported as far as 2.4. Better wait for at least
one extra -dev version before backporting though, as the bug should
not be triggered often anyway.

src/sock_inet.c

@@ -312,6 +312,24 @@ int sock_inet_bind_receiver(struct receiver *rx, char **errmsg)
 		}
 	}
 
+	if (ext && fd < global.maxsock && fdtab[fd].owner) {
+		/* This FD was already bound so this means that it was already
+		 * known and registered before parsing, hence it's an inherited
+		 * FD. The only reason why it's already known here is that it
+		 * has been registered multiple times (multiple listeners on the
+		 * same, or a "shards" directive on the line). There cannot be
+		 * multiple listeners on one FD but at least we can create a
+		 * new one from the original one. We won't reconfigure it,
+		 * however, as this was already done for the first one.
+		 */
+		fd = dup(fd);
+		if (fd == -1) {
+			err |= ERR_RETRYABLE | ERR_ALERT;
+			memprintf(errmsg, "cannot dup() receiving socket (%s)", strerror(errno));
+			goto bind_return;
+		}
+	}
+
 	if (fd >= global.maxsock) {
 		err |= ERR_FATAL | ERR_ABORT | ERR_ALERT;
 		memprintf(errmsg, "not enough free sockets (raise '-n' parameter)");

src/sock_unix.c

@@ -226,6 +226,24 @@ int sock_unix_bind_receiver(struct receiver *rx, char **errmsg)
 	}
 
  fd_ready:
+	if (ext && fd < global.maxsock && fdtab[fd].owner) {
+		/* This FD was already bound so this means that it was already
+		 * known and registered before parsing, hence it's an inherited
+		 * FD. The only reason why it's already known here is that it
+		 * has been registered multiple times (multiple listeners on the
+		 * same, or a "shards" directive on the line). There cannot be
+		 * multiple listeners on one FD but at least we can create a
+		 * new one from the original one. We won't reconfigure it,
+		 * however, as this was already done for the first one.
+		 */
+		fd = dup(fd);
+		if (fd == -1) {
+			err |= ERR_RETRYABLE | ERR_ALERT;
+			memprintf(errmsg, "cannot dup() receiving socket (%s)", strerror(errno));
+			goto bind_return;
+		}
+	}
+
 	if (fd >= global.maxsock) {
 		err |= ERR_FATAL | ERR_ABORT | ERR_ALERT;
 		memprintf(errmsg, "not enough free sockets (raise '-n' parameter)");