emacs-29 93005cd9dc2: with-display-message: Workaround for bug#63253

[Stefan Monnier]

branch: emacs-29
commit 93005cd9dc2bab882e66ac7b81f593cd6c021e43
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    with-display-message: Workaround for bug#63253
    
    Running arbitrary ELisp code from an atimer is still dangerous,
    at least because the regexp engine is not-reentrant, so let's patch up
    the case we bumped into.  There are probably many other such holes :-(
    
    * src/alloc.c (garbage_collection_inhibited): Make it non-static.
    
    * src/xdisp.c (garbage_collection_inhibited): Declare it.
    (set_message, clear_message): Use it as a proxy for "we're in
    a dangerous context like within `probably_quit`".
---
 src/alloc.c |  2 +-
 src/xdisp.c | 12 ++++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index 05a19f0b7e9..7ff2cd3b100 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -367,7 +367,7 @@ static ptrdiff_t pure_bytes_used_non_lisp;
 
 /* If positive, garbage collection is inhibited.  Otherwise, zero.  */
 
-static intptr_t garbage_collection_inhibited;
+intptr_t garbage_collection_inhibited;
 
 /* The GC threshold in bytes, the last time it was calculated
    from gc-cons-threshold and gc-cons-percentage.  */
diff --git a/src/xdisp.c b/src/xdisp.c
index 43847544396..e960901d5dc 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -12810,6 +12810,8 @@ truncate_message_1 (void *a1, Lisp_Object a2)
   return false;
 }
 
+extern intptr_t garbage_collection_inhibited;
+
 /* Set the current message to STRING.  */
 
 static void
@@ -12819,7 +12821,11 @@ set_message (Lisp_Object string)
 
   eassert (STRINGP (string));
 
-  if (FUNCTIONP (Vset_message_function))
+  if (FUNCTIONP (Vset_message_function)
+      /* FIXME: (bug#63253) We should really make the regexp engine re-entrant,
+         but in the mean time, let's ignore `set-message-function` when
+         called from `probably_quit`.  */
+      && !garbage_collection_inhibited)
     {
       specpdl_ref count = SPECPDL_INDEX ();
       specbind (Qinhibit_quit, Qt);
@@ -12896,7 +12902,9 @@ clear_message (bool current_p, bool last_displayed_p)
 
   if (current_p)
     {
-      if (FUNCTIONP (Vclear_message_function))
+      if (FUNCTIONP (Vclear_message_function)
+          /* FIXME: (bug#63253) Same as for `set-message-function` above.  */
+          && !garbage_collection_inhibited)
         {
           specpdl_ref count = SPECPDL_INDEX ();
           specbind (Qinhibit_quit, Qt);