Preventive compatibility fixes for flushing.

In pre 8.6, `Pp` provided its own reimplementation of
`Pervasives.flush_all`, with different semantics.

Unfortunately, with the removal of `Pp.flush_all` in #179, a couple of
points were silently switched to the `Pervasives` version, which may
lead to some subtle printing differences.

As a preventive measure, we restore the same semantics for these parts
of the codebase.

Note that we don't re-introduce Coq's `flush_all` for several reasons:

- Consumers of the logging API should not mess with flushing and
  Formatters as this is backend dependent (i.e: when in IDEs).

  Use of `Format` should be fully encapsulated if we want some hope of
  IDEs taking full control.

- As used, the old semantics of `flush_all` were fragile.

3 files changed, 11 insertions, 3 deletions

diff --git a/checker/check_stat.ml b/checker/check_stat.ml
index a26c93a30..a63705adc 100644
--- a/checker/check_stat.ml
+++ b/checker/check_stat.ml
@@ -18,7 +18,7 @@ let print_memory_stat () =
   if !memory_stat then begin
     Format.printf "total heap size = %d kbytes\n" (CObj.heap_size_kb ());
     Format.print_newline();
-    flush_all()
+    Format.print_flush()
   end
 
 let output_context = ref false
diff --git a/lib/errors.ml b/lib/errors.ml
index 8982dde14..1459141d1 100644
--- a/lib/errors.ml
+++ b/lib/errors.ml
@@ -144,5 +144,5 @@ let handled e =
 let fatal_error info anomaly =
   let msg = info ++ fnl () in
   pp_with ~pp_tag:Ppstyle.pp_tag !Pp_control.err_ft msg;
-  flush_all ();
+  Format.pp_print_flush !Pp_control.err_ft ();
   exit (if anomaly then 129 else 1)
diff --git a/toplevel/coqloop.ml b/toplevel/coqloop.ml
index c7a639803..1fe30217d 100644
--- a/toplevel/coqloop.ml
+++ b/toplevel/coqloop.ml
@@ -345,13 +345,21 @@ let feed_emacs = function
   | _  -> ()
 *)
 
+(* Flush in a compatible order with 8.5 *)
+(* This mimics the semantics of the old Pp.flush_all *)
+let loop_flush_all () =
+  Pervasives.flush stderr;
+  Pervasives.flush stdout;
+  Format.pp_print_flush !Pp_control.std_ft ();
+  Format.pp_print_flush !Pp_control.err_ft ()
+
 let rec loop () =
   Sys.catch_break true;
   if !Flags.print_emacs then Vernacentries.qed_display_script := false;
   Flags.coqtop_ui := true;
   try
     reset_input_buffer stdin top_buffer;
-    while true do do_vernac(); flush_all() done
+    while true do do_vernac(); loop_flush_all () done
   with
     | Errors.Drop -> ()
     | Errors.Quit -> exit 0