// To orchestrate this between independant PHP processes on the same hosts,
// we must have a common sense of time so that we only have to maintain
// a single counter in a single lock file.
+ //
+ // Given that:
+ // * The system clock can be observed via time(), without milliseconds.
+ // * Some other clock can be observed via microtime(), which also offers
+ // millisecond precision.
+ // * microtime() drifts in-process further and further away from the system
+ // clock the longer the longer the process runs for.
+ // For example, on 2018-10-03 an HHVM 3.18 JobQueue process at WMF,
+ // that ran for 9 min 55 sec, drifted 7 seconds.
+ // The drift is immediate for processes running while the system clock changes.
+ // time() does not have this problem. See https://bugs.php.net/bug.php?id=42659.
+ //
+ // We have two choices:
+ //
+ // 1. Use microtime() with the following caveats:
+ // - The last stored time may be in the future, or our current time
+ // may be in the past, in which case we'll frequently enter the slow
+ // timeWaitUntil() method to try and "sync" the current process with
+ // the previous process. We mustn't block for long though, max 10ms?
+ // - For any drift above 10ms, we pretend that the clock went backwards,
+ // and treat it the same way as after an NTP sync, by incrementing clock
+ // sequence instead. Given this rolls over automatically and silently
+ // and is meant to be rare, this is essentially sacrifices a reasonable
+ // guarantee of uniqueness.
+ // - For long running processes (e.g. longer than a few seconds) the drift
+ // can easily be more than 2 seconds. Because we only have a single lock
+ // file and don't want to keep too many counters and deal with clearing
+ // those, we fatal the user and refuse to make an ID. (T94522)
+ // 2. Use time() and expand the counter by 1000x and use the first digits
+ // as if they are the millisecond fraction of the timestamp.
+ // Known caveats or perf impact: None.
+ //
+ // We choose the latter.
+ $msecCounterSize = $counterSize * 1000;
rewind( $handle );
// Format of lock file contents:
- // "<clk seq> <sec> <msec> <counter> <rand offset>"
+ // "<clk seq> <sec> <msec counter> <rand offset>"
$data = explode( ' ', fgets( $handle ) );
- // Did the clock get moved back significantly?
- $clockChanged = false;
-
- if ( count( $data ) === 5 ) {
+ if ( count( $data ) === 4 ) {
// The UID lock file was already initialized
$clkSeq = (int)$data[0] % $clockSeqSize;
- $prevTime = [ (int)$data[1], (int)$data[2] ];
+ $prevSec = (int)$data[1];
// Counter for UIDs with the same timestamp,
- $counter = 0;
- $randOffset = (int)$data[4] % $counterSize;
+ $msecCounter = 0;
+ $randOffset = (int)$data[3] % $counterSize;
// If the system clock moved backwards by an NTP sync,
// or if the last writer process had its clock drift ahead,
// Try to catch up if the gap is small, so that we can keep a single
// monotonic logic file.
- $time = $this->timeWaitUntil( $prevTime );
- if ( $time === false ) {
- // Timed out. Treat it as a new clock
- $clockChanged = true;
- $time = $this->millitime();
- } elseif ( $time === $prevTime ) {
+ $sec = $this->timeWaitUntil( $prevSec );
+ if ( $sec === false ) {
+ // Gap is too big. Looks like the clock got moved back significantly.
+ // Start a new clock sequence, and re-randomize the extra offset,
+ // which is useful for UIDs that do not include the clock sequence number.
+ $clkSeq = ( $clkSeq + 1 ) % $clockSeqSize;
+ $sec = time();
+ $randOffset = mt_rand( 0, $offsetSize - 1 );
+ trigger_error( "Clock was set back; sequence number incremented." );
+ } elseif ( $sec === $prevSec ) {
// Sanity check, only keep remainder if a previous writer wrote
// something here that we don't accept.
- $counter = (int)$data[3] % $counterSize;
+ $msecCounter = (int)$data[2] % $msecCounterSize;
// Bump the counter if the time has not changed yet
- if ( ++$counter >= $counterSize ) {
+ if ( ++$msecCounter >= $msecCounterSize ) {
// More IDs generated with the same time than counterSize can accomodate
flock( $handle, LOCK_UN );
throw new RuntimeException( "Counter overflow for timestamp value." );
} else {
// Initialize UID lock file information
$clkSeq = mt_rand( 0, $clockSeqSize - 1 );
- $time = $this->millitime();
- $counter = 0;
- $randOffset = mt_rand( 0, $offsetSize - 1 );
- }
- // microtime() and gettimeofday() can drift from time() at least on Windows.
- // The drift is immediate for processes running while the system clock changes.
- // time() does not have this problem. See https://bugs.php.net/bug.php?id=42659.
- $drift = time() - $time[0];
- if ( abs( $drift ) >= 2 ) {
- // We don't want processes using too high or low timestamps to avoid duplicate
- // UIDs and clock sequence number churn. This process should just be restarted.
- flock( $handle, LOCK_UN ); // abort
- throw new RuntimeException( "Process clock is outdated or drifted ({$drift}s)." );
- }
- // If microtime() is synced and a clock change was detected, then the clock went back
- if ( $clockChanged ) {
- // Bump the clock sequence number and also randomize the extra offset,
- // which is useful for UIDs that do not include the clock sequence number.
- $clkSeq = ( $clkSeq + 1 ) % $clockSeqSize;
+ $sec = time();
+ $msecCounter = 0;
$randOffset = mt_rand( 0, $offsetSize - 1 );
- trigger_error( "Clock was set back; sequence number incremented." );
}
// Update and release the UID lock file
ftruncate( $handle, 0 );
rewind( $handle );
- fwrite( $handle, "{$clkSeq} {$time[0]} {$time[1]} {$counter} {$randOffset}" );
+ fwrite( $handle, "{$clkSeq} {$sec} {$msecCounter} {$randOffset}" );
fflush( $handle );
flock( $handle, LOCK_UN );
+ // Split msecCounter back into msec and counter
+ $msec = (int)( $msecCounter / 1000 );
+ $counter = $msecCounter % 1000;
+
return [
- 'time' => $time,
+ 'time' => [ $sec, $msec ],
'counter' => $counter,
'clkSeq' => $clkSeq,
'offset' => $randOffset,
* Wait till the current timestamp reaches $time and return the current
* timestamp. This returns false if it would have to wait more than 10ms.
*
- * @param array $time Result of UIDGenerator::millitime()
- * @return array|bool UIDGenerator::millitime() result or false
+ * @param int $time Result of time()
+ * @return int|bool Timestamp or false
*/
- protected function timeWaitUntil( array $time ) {
+ protected function timeWaitUntil( $time ) {
+ $start = microtime( true );
do {
- $ct = $this->millitime();
- if ( $ct >= $time ) { // https://secure.php.net/manual/en/language.operators.comparison.php
- return $ct; // current timestamp is higher than $time
+ $ct = time();
+ // https://secure.php.net/manual/en/language.operators.comparison.php
+ if ( $ct >= $time ) {
+ // current time is higher than or equal to than $time
+ return $ct;
}
- } while ( ( ( $time[0] - $ct[0] ) * 1000 + ( $time[1] - $ct[1] ) ) <= 10 );
+ } while ( ( microtime( true ) - $start ) <= 0.010 ); // upto 10ms
return false;
}
/**
- * @param array $time Result of UIDGenerator::millitime()
+ * @param array $time Array of second and millisecond integers
* @return string 46 LSBs of "milliseconds since epoch" in binary (rolls over in 4201)
* @throws RuntimeException
*/
}
/**
- * @param array $time Result of UIDGenerator::millitime()
+ * @param array $time Array of second and millisecond integers
* @param int $delta Number of intervals to add on to the timestamp
* @return string 60 bits of "100ns intervals since 15 October 1582" (rolls over in 3400)
* @throws RuntimeException
return $id_bin;
}
- /**
- * @return array (current time in seconds, milliseconds since then)
- */
- protected function millitime() {
- list( $msec, $sec ) = explode( ' ', microtime() );
-
- return [ (int)$sec, (int)( $msec * 1000 ) ];
- }
-
/**
* Delete all cache files that have been created.
*
dépôts / lhc / web / wiklou.git / commitdiff