bug#54458: 27.2; erc-dcc-get: Re-entering top level after C stack overflow

["Mattias Engdegård" <mattiase@acm.org>]

[-- Attachment #1: Type: text/plain, Size: 923 bytes --]

28 mars 2022 kl. 14.08 skrev J.P. <jp@neverwas.me>:

> As you highlighted up thread, inhibited sends appeal to
> `wait_reading_process_output' to try and shake something loose. I agree
> such occasions seem likeliest to trigger "unexpected" filter nesting.

Although it seems reasonable to require that filter functions abstain from sending more data to the process, there may be another way: preventing re-entering process-send calls from recursing further.

Attached is a proof of concept: if process-send calls are invoked when another activation already exists, just enqueue the data and let the previous activation deal with the actual transmission. That nips the recursion in the buds.

The principle seems sound but if anyone can think of problems with the approach, please do tell.

Of course, one reason to make the change in ERC is that it would fix the problem in all Emacs versions, not just 29.


[-- Attachment #2: send-process-nesting.diff --]
[-- Type: application/octet-stream, Size: 1407 bytes --]

diff --git a/src/process.c b/src/process.c
index 993e1c5603..694307b43a 100644
--- a/src/process.c
+++ b/src/process.c
@@ -6393,6 +6393,14 @@ write_queue_pop (struct Lisp_Process *p, Lisp_Object *obj,
   return 1;
 }
 
+static int send_process_nesting = 0;
+
+static void
+unwind_send_process (void)
+{
+  send_process_nesting--;
+}
+
 /* Send some data to process PROC.
    BUF is the beginning of the data; LEN is the number of characters.
    OBJECT is the Lisp object that the data comes from.  If OBJECT is
@@ -6510,6 +6518,18 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
       buf = SSDATA (object);
     }
 
+  if (send_process_nesting > 0)
+    {
+      /* We are already inside a send_process activation higher up the
+	 call chain; let that take care of writing the queued data.  */
+      write_queue_push (p, object, buf, len, 0);
+      return;
+    }
+
+  specpdl_ref count = SPECPDL_INDEX ();
+  record_unwind_protect_void (unwind_send_process);
+  send_process_nesting++;
+
   /* If there is already data in the write_queue, put the new data
      in the back of queue.  Otherwise, ignore it.  */
   if (!NILP (p->write_queue))
@@ -6633,6 +6653,7 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
 	}
     }
   while (!NILP (p->write_queue));
+  unbind_to (count, Qnil);
 }
 
 DEFUN ("process-send-region", Fprocess_send_region, Sprocess_send_region,

[-- Attachment #3: Type: text/plain, Size: 2 bytes --]