bug#32986: 27.0.50; unexpected delay in while-no-input + accept-process-output

[João Távora]

Hello maintainers,

I've asked this in Emacs devel already:
https://lists.gnu.org/archive/html/emacs-devel/2018-10/msg00037.html. Someone
suggested I report it as a bug.  It happens Windows (machine of bug
report), but also on my Ubuntu virtual server.

I would expect while-no-input to break out of accept-process-output very
quickly after user keyboard input.  These expectations are met except
for some situations.  Here's some test code:

   (defmacro joaot/time (&rest body)
     `(let ((start (current-time)))
        (prog1
            (progn ,@body)
          (let ((msg (format "Took %s seconds and returned "
                             (format-time-string
                              "%S.%3N"
                              (time-subtract (current-time) start)))))
            (if current-prefix-arg
                (insert "; " msg)
              (message msg))))))

Now, after each of these, I'm pressing C-u C-x C-e SPC as fast as I can.
The second result is unexpected, all the others are fine. Moreover, it
varies from 0.7s to as much as 5s.  

    (joaot/time
     (while-no-input
       (while t (accept-process-output nil 0.05)))); Expected, took 00.201 
seconds and returned t 
     
    (joaot/time
     (while-no-input
       (while t (accept-process-output nil 30)))); Took 02.694 seconds and 
returned t 
     
    (joaot/time
     (while (sit-for 30))); Took 00.126 seconds and returned nil 
     
    (joaot/time
     (while (sit-for 0.1))) ; Took 00.093 seconds and returned nil

I tried quickly pluggin GDB in at the right time, but I don't know if
I'm using the right GDB (using msys's which is pretty slow) and I
probably should be using an unoptimized Emacs.  Anyway, as a start this
is what "bt full" look like:

   (gdb) bt full
   #0  0x00007ffc97b0d8c1 in ntdll!DbgBreakPoint () from 
/c/WINDOWS/SYSTEM32/ntdll.dll
   No symbol table info available.
   #1  0x00007ffc97b39a0b in ntdll!DbgUiRemoteBreakin () from 
/c/WINDOWS/SYSTEM32/ntdll.dll
   No symbol table info available.
   #2  0x00007ffc952b3034 in KERNEL32!BaseThreadInitThunk ()
      from /c/WINDOWS/System32/KERNEL32.DLL
   No symbol table info available.
   #3  0x00007ffc97ae1461 in ntdll!RtlUserThreadStart () from 
/c/WINDOWS/SYSTEM32/ntdll.dll
   No symbol table info available.
   #4  0x0000000000000000 in ?? ()
   No symbol table info available.
   Backtrace stopped: previous frame inner to this frame (corrupt stack?)
   (gdb) xbacktrace
   Undefined command: "xbacktrace".  Try "help".

xbacktrace probably failed because of some Python error loading .gdbinit

In GNU Emacs 27.0.50 (build 1, x86_64-w64-mingw32)
 of 2018-10-02 built on GONDOMAR
Repository revision: dfbb207ff946792efebb31c0c59b8245c304544a
Windowing system distributor 'Microsoft Corp.', version 10.0.17134
System Description: Microsoft Windows 10 Pro (v10.0.1803.17134.286)

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