workshop/{Operator,Workplace} use SOCK_SEQPACKET for return_{pidfd,cgroup}

Closing the other side of a SOCK_DGRAM does not finish waiting
recvmsg() calls.

Author: MaxKellermann

debian/changelog

@@ -1,6 +1,7 @@
 cm4all-workshop (6.9) unstable; urgency=low
 
   * spawn: fix assertion failure with large payloads
+  * spawn: fix hanging main process after spawner failure
   * translation: add packet REAL_UID_GID
   * switch to C++23
   * require Meson 1.2

src/workshop/Operator.cxx

@@ -289,7 +289,7 @@ DoSpawn(SpawnService &service, AllocatorPtr alloc,
 		p.stderr_fd = p.stdout_fd = stderr_w;
 
 	UniqueSocketDescriptor return_pidfd;
-	std::tie(return_pidfd, p.return_pidfd) = CreateSocketPair(SOCK_DGRAM);
+	std::tie(return_pidfd, p.return_pidfd) = CreateSocketPair(SOCK_SEQPACKET);
 
 	for (const char *arg : response.args) {
 		if (p.args.size() >= 4096)

src/workshop/Workplace.cxx

@@ -162,7 +162,7 @@ WorkshopWorkplace::Start(EventLoop &event_loop, const WorkshopJob &job,
 		if (client->SupportsCgroups()) {
 			cgroup.name = job.plan_name.c_str();
 
-			std::tie(return_cgroup, p.return_cgroup) = CreateSocketPair(SOCK_DGRAM);
+			std::tie(return_cgroup, p.return_cgroup) = CreateSocketPair(SOCK_SEQPACKET);
 		}
 	}
 
@@ -218,6 +218,13 @@ WorkshopWorkplace::Start(EventLoop &event_loop, const WorkshopJob &job,
 	       "') started");
 
 	if (return_cgroup.IsDefined()) {
+		/* close the other side of the socketpair if it's
+		   still open to avoid blocking the following receive
+		   call if the spawner has closed the socket without
+		   sending something */
+		if (p.return_cgroup.IsDefined())
+			p.return_cgroup.Close();
+
 		try {
 			o->SetCgroup(EasyReceiveMessageWithOneFD(return_cgroup));
 		} catch (...) {