Use up-to-date time in wait_reading_process_output

In “Avoid duplicate calls to current_timespec” (2015-07-05)
we started caching current_timespec results in NOW.
However, this was buggy: we updated NOW only when the timeout was
nonzero, but the timeout can be set temporarily to zero in several
places in wait_reading_process_output (such as when checking for
process status changes), which would cause us to never update NOW
and therefore never detect that a timeout happened.

Also, this caching was wrong even in principle: since we call
Lisp code from wait_reading_process_output, substantial amounts
of time can pass, and we can be left using an outdated NOW and
incorrectly not time out.  Also, nowadays we can use
monotonic_coarse_timespec which is fast, and which is better
anyway because it’s immune to manual clock changes.

Co-authored-by: Spencer Baugh <sbaugh@janestreet.com>
* src/process.c (wait_reading_process_output):
Stop caching the current realtime.
Instead, use the coarse monotonic clock without caching.

1 files changed, 6 insertions, 15 deletions

diff --git a/src/process.c b/src/process.c
index 2804409f51e..8ec420aaa2c 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5353,9 +5353,6 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 #endif
   specpdl_ref count = SPECPDL_INDEX ();
 
-  /* Close to the current time if known, an invalid timespec otherwise.  */
-  struct timespec now = invalid_timespec ();
-
   eassert (wait_proc == NULL
            || NILP (wait_proc->thread)
            || XTHREAD (wait_proc->thread) == current_thread);
@@ -5380,7 +5377,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
   else if (time_limit > 0 || nsecs > 0)
     {
       wait = TIMEOUT;
-      now = current_timespec ();
+      struct timespec now = monotonic_coarse_timespec ();
       end_time = timespec_add (now, make_timespec (time_limit, nsecs));
     }
   else
@@ -5467,8 +5464,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
       /* Exit if already run out.  */
       if (wait == TIMEOUT)
         {
-          if (!timespec_valid_p (now))
-            now = current_timespec ();
+          struct timespec now = monotonic_coarse_timespec ();
           if (timespec_cmp (end_time, now) <= 0)
             break;
           timeout = timespec_sub (end_time, now);
@@ -5721,8 +5717,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
               && timespec_valid_p (timer_delay)
               && timespec_cmp (timer_delay, timeout) < 0)
             {
-              if (!timespec_valid_p (now))
-                now = current_timespec ();
+              struct timespec now = monotonic_coarse_timespec ();
               struct timespec timeout_abs = timespec_add (now, timeout);
               if (!timespec_valid_p (got_output_end_time)
                   || timespec_cmp (timeout_abs, got_output_end_time) < 0)
@@ -5732,10 +5727,6 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
           else
             got_output_end_time = invalid_timespec ();
 
-          /* NOW can become inaccurate if time can pass during pselect.  */
-          if (timeout.tv_sec > 0 || timeout.tv_nsec > 0)
-            now = invalid_timespec ();
-
 #if defined HAVE_GETADDRINFO_A || defined HAVE_GNUTLS
           if (retry_for_async
               && (timeout.tv_sec > 0 || timeout.tv_nsec > ASYNC_RETRY_NSEC))
@@ -5873,7 +5864,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
             }
           if (timespec_cmp (cmp_time, huge_timespec) < 0)
             {
-              now = current_timespec ();
+              struct timespec now = monotonic_coarse_timespec ();
               if (timespec_cmp (cmp_time, now) <= 0)
                 break;
             }
@@ -8132,7 +8123,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
   else if (time_limit > 0 || nsecs > 0)
     {
       wait = TIMEOUT;
-      end_time = timespec_add (current_timespec (),
+      end_time = timespec_add (monotonic_coarse_timespec (),
                                make_timespec (time_limit, nsecs));
     }
   else
@@ -8164,7 +8155,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
       /* Exit if already run out.  */
       if (wait == TIMEOUT)
         {
-          struct timespec now = current_timespec ();
+          struct timespec now = monotonic_coarse_timespec ();
           if (timespec_cmp (end_time, now) <= 0)
             break;
           timeout = timespec_sub (end_time, now);