A client that leaves before its reply arrives, which used to kill the daemon

Eight hit-and-run connections against the robustness daemon, then a check
that the session is still there and still knows what it installed. Fails
with 'killed by SIGPIPE' against the parent of this commit.

test/test_dev.ml

@@ -3044,11 +3044,66 @@ let () =
           | None -> fail "(probe-one) produced no value")
        | Some r -> fail "(probe-one): %s %s" (status r) (message r)
        | None -> ());
+      (* ── An editor that leaves before its reply does ──────────────── *)
+      (* The failure this closes was a flake in test_emacs, and it read like
+         nothing it was: two break-loop checks failing, and then
+         "cannot reconnect: Connection refused" on a socket path that plainly
+         existed. A write into a socket whose reader has gone is SIGPIPE, and
+         SIGPIPE's default action is to kill the process — which in the merged
+         build is the program, the compiler and the listener at once, and
+         leaves the socket file behind for the next client to be refused on.
+         It is not a contrived shape: reconnecting tears the old connection
+         down, and a reply already on its way out lands in the gap.
+
+         [c] is closed first because the daemon serves one connection at a
+         time: a second one would sit in the backlog until this one ended, and
+         then there would be no reply in flight to lose. *)
+      (try Unix.close c with Unix.Unix_error _ -> ());
+      let hit_and_run () =
+        let s = Unix.socket Unix.PF_UNIX Unix.SOCK_STREAM 0 in
+        (try
+           Unix.connect s (Unix.ADDR_UNIX rsock);
+           Wire.send s "(:op \"describe\")"
+         with Unix.Unix_error _ -> ());
+        (try Unix.close s with Unix.Unix_error _ -> ())
+      in
+      (* Several, because the first write into a freshly closed socket can
+         still land in a buffer nobody will read; the second is the one that
+         is refused. *)
+      for _ = 1 to 8 do hit_and_run (); ignore (Unix.select [] [] [] 0.02) done;
+      let c =
+        match Unix.waitpid [ Unix.WNOHANG ] rpid with
+        | 0, _ -> Some (connect rsock)
+        | _, st ->
+          (* Named rather than numbered, because [WSIGNALED] carries OCaml's
+             own signal numbering and "signal -8" would send the next reader
+             to the wrong page. SIGPIPE is the answer this check expects to
+             see when it fails. *)
+          fail "an editor that left before its reply did ended the daemon (%s)"
+            (match st with
+             | Unix.WEXITED n -> Printf.sprintf "exit %d" n
+             | Unix.WSIGNALED n when n = Sys.sigpipe -> "killed by SIGPIPE"
+             | Unix.WSIGNALED n -> Printf.sprintf "signal %d" n
+             | Unix.WSTOPPED n -> Printf.sprintf "stopped on %d" n);
+          None
+        | exception Unix.Unix_error _ -> Some (connect rsock)
+      in
+      (match c with
+       | None -> ()
+       | Some c ->
+         (match Wire.parse (Wire.send c "(:op \"describe\")"; Wire.recv c) with
+          | r ->
+            if not (contains_sub (Form.to_string (Option.get (Wire.field r "fns")))
+                      "probe-one")
+            then fail "the session lost track of the program after a client left"
+          | exception e ->
+            fail "the session did not answer after a client left: %s"
+              (Printexc.to_string e));
          (try
             ignore (Wire.send c "(:op \"close\")");
             ignore (Wire.recv c)
           with _ -> ());
-      (try Unix.close c with Unix.Unix_error _ -> ())
+         (try Unix.close c with Unix.Unix_error _ -> ()))
     end;
     (try Unix.kill rpid Sys.sigkill with Unix.Unix_error _ -> ());
     (try ignore (Unix.waitpid [] rpid) with Unix.Unix_error _ -> ());