The robustness fixture says its failure is deliberate, and stops printing a linker's words

test/test_dev.ml

@@ -2829,7 +2829,22 @@ let () =
        outlive a sequence with several cold clang drivers in it, which is what
        programs/dev-robust.flan is for. *)
     let rsock = tmp "robust.sock" and rout = tmp "robust.out" in
+    (* This daemon gets a stderr of its own, which none of the others needs.
+       [Build.run] shells out with [Sys.command], so a clang driver's own words
+       go to whatever stderr it inherited — and the two failures below are
+       deliberate, so on a *passing* run the suite was printing
+       "cannot open output file ... Permission denied" and "linker command
+       failed" straight to the terminal. Four lines of what reads like a broken
+       toolchain, on every run, for years of handoffs. Noise that is always
+       there stops being read, and a real linker failure in this spot would
+       have been invisible behind it. So it goes to a file, and the file is
+       reprinted below only if a step in this block actually failed. *)
+    let rerr = tmp "robust.err" in
     let rcache = tmp "robust.cache" in
+    (* Counted from here, not from zero: [failures] is the whole file's, and a
+       failure in an earlier block is not a reason to reprint this daemon's
+       expected complaints. *)
+    let rfailures = !failures in
     let rec rm_rf path =
       match Sys.is_directory path with
       | true ->
@@ -2853,12 +2868,23 @@ let () =
          @ [ "FLAN_CACHE_DIR=" ^ rcache ])
     in
     let rfd = Unix.openfile rout [ Unix.O_WRONLY; Unix.O_CREAT; Unix.O_TRUNC ] 0o600 in
+    let refd = Unix.openfile rerr [ Unix.O_WRONLY; Unix.O_CREAT; Unix.O_TRUNC ] 0o600 in
+    (* Said out loud, because the alternative is a reader deciding for
+       themselves what a linker complaint in a test run means. Deliberately no
+       "error" or "FAIL" in the wording: the reader this line is for is
+       scanning for exactly those words. *)
+    print_endline
+      "dev: two builds below are made to fail on purpose — the robustness \
+       fixture holds its own cache directory read-only, so the linker cannot \
+       write its output there. The compiler's complaints go to a log and are \
+       reprinted only if a step actually goes wrong.";
     let rpid =
       Unix.create_process_env flan
         [| flan; "dev"; "programs/dev-robust.flan"; "-s"; rsock |]
-        renv Unix.stdin rfd Unix.stderr
+        renv Unix.stdin rfd refd
     in
     Unix.close rfd;
+    Unix.close refd;
     if not (listening ~pid:rpid rsock) then begin
       fail "the robustness daemon %s" !listen_why;
       (try Unix.kill rpid Sys.sigkill with Unix.Unix_error _ -> ())
@@ -3026,8 +3052,23 @@ let () =
     (try ignore (Unix.waitpid [] rpid) with Unix.Unix_error _ -> ());
     (try Unix.chmod rcache 0o700 with Unix.Unix_error _ -> ());
     rm_rf rcache;
+    (* Its contents rather than its name: [rerr] is under dune's per-run
+       TMPDIR, which is gone by the time anyone reads the run. *)
+    if !failures > rfailures then begin
+      print_endline
+        "dev: the robustness daemon's stderr follows. Two of the linker \
+         failures in it are the deliberate ones described above; anything \
+         else is not.";
+      match open_in_bin rerr with
+      | ic ->
+        let n = in_channel_length ic in
+        print_string (really_input_string ic n);
+        flush stdout;
+        close_in ic
+      | exception Sys_error e -> Printf.printf "  (unreadable: %s)\n" e
+    end;
     List.iter (fun f -> try Sys.remove f with Sys_error _ -> ())
-      [ rsock; rout ];
+      [ rsock; rout; rerr ];
 
     List.iter (fun f -> try Sys.remove f with Sys_error _ -> ())
       [ sock; out; bsock; bout ];