bug#43389: 28.0.50; Emacs memory leaks using hard disk all time

[Trevor Bentley <trevor@trevorbentley.com>]

> You want visibility into what is USING that memory. 
> 
> With glibc-malloc-trace-utils you can try to do that with: 
> 
> LD_PRELOAD=libmtrace.so \ MTRACE_CTL_FILE=/home/user/app.mtr \ 
> MTRACE_CTL_BACKTRACE=1 \ ./app 
> 
> This will use libgcc's unwinder to get a copy of the malloc 
> caller address and then we'll have to decode that based on a 
> /proc/self/maps. 
> 
> Next steps: - Get a glibc-malloc-trace-utils trace of the 
> application ratcheting.  - Get a copy of /proc/$PID/maps for the 
> application (shorter version of smaps). 
> 

Oh, this is going to be a problem.  I guess it is producing one 
trace file per thread?

I ran it with libmtrace overnight.  Memory usage was very high, 
but it doesn't look like the same problem.  I hit 1550MB of RSS, 
but smaps reported only ~350MB of that was in the heap, which 
seemed reasonable for the ~150MB that emacs reported it was using. 
Does libmtrace add a lot of memory overhead?

However, libmtrace has made 4968 files totalling 26GB in that 
time.  Ouch.

It's going to be hard to tell when I hit the bug under libmtrace, 
questionable whether the report will even fit on my disk, and 
tricky to share however many tens of gigabytes of trace files it 
results in.

If it's one trace per thread, though, then we at least know that 
my emacs process in question is blazing through threads.  That 
could be relevant.

Other thing to note (for Eli): I wrapped garbage-collect like so:

---
(defun trev/garbage-collect (orig-fun &rest args) 
  (message "%s -- Starting garbage-collect." 
  (current-time-string)) (let ((time (current-time)) 
        (result (apply orig-fun args))) 
    (message "%s -- Finished garbage-collect in %.06f" 
    (current-time-string) (float-time (time-since time))) result)) 
(add-function :around (symbol-function 'garbage-collect) 
#'trev/garbage-collect)
---

This printed a start and stop message each time I evaluated 
garbage-collect manually.  It did not print any messages in 11 
hours of running unattended.  This is with an active network 
connection receiving messages fairly frequently, so there was 
plenty of consing going on.  Hard for me to judge if it should run 
any garbage collection in that time, but I would have expected so.

-Trevor