Improve TimestampDifferenceMilliseconds to cope with overflow sanely.

We'd like to use TimestampDifferenceMilliseconds with the stop_time
possibly being TIMESTAMP_INFINITY, but up to now it's disclaimed
responsibility for overflow cases.  Define it to clamp its output to
the range [0, INT_MAX], handling overflow correctly.  (INT_MAX rather
than LONG_MAX seems appropriate, because the function is already
described as being intended for calculating wait times for WaitLatch
et al, and that infrastructure only handles waits up to INT_MAX.
Also, this choice gets rid of cross-platform behavioral differences.)

Having done that, we can replace some ad-hoc code in walreceiver.c
with a simple call to TimestampDifferenceMilliseconds.

While at it, fix some buglets in existing callers of
TimestampDifferenceMilliseconds: basebackup_copy.c had not read the
memo about TimestampDifferenceMilliseconds never returning a negative
value, and postmaster.c had not read the memo about Min() and Max()
being macros with multiple-evaluation hazards.  Neither of these
quite seem worth back-patching.

Patch by me; thanks to Nathan Bossart for review.

Discussion: https://postgr.es/m/3126727.1674759248@sss.pgh.pa.us

diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 05470057f5b64e6125337810eef38cda53c01b78..2bb6c89f8c0cd5af607509e1ba6070b4e24aad3d 100644 (file)
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -215,7 +215,8 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
         * the system clock was set backward, so that such occurrences don't
         * have the effect of suppressing further progress messages.
         */
-       if (ms < 0 || ms >= PROGRESS_REPORT_MILLISECOND_THRESHOLD)
+       if (ms >= PROGRESS_REPORT_MILLISECOND_THRESHOLD ||
+           now < mysink->last_progress_report_time)
        {
            mysink->last_progress_report_time = now;
 

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 5b775cf7d0695ee966c003f87e730428d1f87254..62fba5fceedcf28dbf65a143293999cd6a2c8989 100644 (file)
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1670,11 +1670,12 @@ DetermineSleepTime(void)
 
    if (next_wakeup != 0)
    {
-       /* Ensure we don't exceed one minute, or go under 0. */
-       return Max(0,
-                  Min(60 * 1000,
-                      TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
-                                                      next_wakeup)));
+       int         ms;
+
+       /* result of TimestampDifferenceMilliseconds is in [0, INT_MAX] */
+       ms = (int) TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
+                                                  next_wakeup);
+       return Min(60 * 1000, ms);
    }
 
    return 60 * 1000;

diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index e95398db05a2097f56c20346e92834872777fd27..b0cfddd54815ff4bbbb8a632510970137885dee3 100644 (file)
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -445,7 +445,7 @@ WalReceiverMain(void)
                pgsocket    wait_fd = PGINVALID_SOCKET;
                int         rc;
                TimestampTz nextWakeup;
-               int         nap;
+               long        nap;
 
                /*
                 * Exit walreceiver if we're not in recovery. This should not
@@ -528,15 +528,9 @@ WalReceiverMain(void)
                for (int i = 0; i < NUM_WALRCV_WAKEUPS; ++i)
                    nextWakeup = Min(wakeup[i], nextWakeup);
 
-               /*
-                * Calculate the nap time.  WaitLatchOrSocket() doesn't accept
-                * timeouts longer than INT_MAX milliseconds, so we limit the
-                * result accordingly.  Also, we round up to the next
-                * millisecond to avoid waking up too early and spinning until
-                * one of the wakeup times.
-                */
+               /* Calculate the nap time, clamping as necessary. */
                now = GetCurrentTimestamp();
-               nap = (int) Min(INT_MAX, Max(0, (nextWakeup - now + 999) / 1000));
+               nap = TimestampDifferenceMilliseconds(now, nextWakeup);
 
                /*
                 * Ideally we would reuse a WaitEventSet object repeatedly

diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 928c330897383e9ffda6ca9f01b8aa9479ad1923..47e059a409b98c25647b975917e8c9d0c876687d 100644 (file)
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -1690,26 +1690,31 @@ TimestampDifference(TimestampTz start_time, TimestampTz stop_time,
  *
  * This is typically used to calculate a wait timeout for WaitLatch()
  * or a related function.  The choice of "long" as the result type
- * is to harmonize with that.  It is caller's responsibility that the
- * input timestamps not be so far apart as to risk overflow of "long"
- * (which'd happen at about 25 days on machines with 32-bit "long").
- *
- * Both inputs must be ordinary finite timestamps (in current usage,
- * they'll be results from GetCurrentTimestamp()).
+ * is to harmonize with that; furthermore, we clamp the result to at most
+ * INT_MAX milliseconds, because that's all that WaitLatch() allows.
  *
  * We expect start_time <= stop_time.  If not, we return zero,
  * since then we're already past the previously determined stop_time.
  *
+ * Subtracting finite and infinite timestamps works correctly, returning
+ * zero or INT_MAX as appropriate.
+ *
  * Note we round up any fractional millisecond, since waiting for just
  * less than the intended timeout is undesirable.
  */
 long
 TimestampDifferenceMilliseconds(TimestampTz start_time, TimestampTz stop_time)
 {
-   TimestampTz diff = stop_time - start_time;
+   TimestampTz diff;
 
-   if (diff <= 0)
+   /* Deal with zero or negative elapsed time quickly. */
+   if (start_time >= stop_time)
        return 0;
+   /* To not fail with timestamp infinities, we must detect overflow. */
+   if (pg_sub_s64_overflow(stop_time, start_time, &diff))
+       return (long) INT_MAX;
+   if (diff >= (INT_MAX * INT64CONST(1000) - 999))
+       return (long) INT_MAX;
    else
        return (long) ((diff + 999) / 1000);
 }