mozglue/misc/TimeStamp.cpp

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * Implementation of the OS-independent methods of the TimeStamp class
   9  */
  10
  11 #include "mozilla/TimeStamp.h"
  12 #include "mozilla/Uptime.h"
  13 #include <string.h>
  14 #include <stdlib.h>
  15
  16 namespace mozilla {
  17
  18 /**
  19  * Wrapper class used to initialize static data used by the TimeStamp class
  20  */
  21 struct TimeStampInitialization {
  22   /**
  23    * First timestamp taken when the class static initializers are run. This
  24    * timestamp is used to sanitize timestamps coming from different sources.
  25    */
  26   TimeStamp mFirstTimeStamp;
  27
  28   /**
  29    * Timestamp representing the time when the process was created. This field
  30    * is populated lazily the first time this information is required and is
  31    * replaced every time the process is restarted.
  32    */
  33   TimeStamp mProcessCreation;
  34
  35   TimeStampInitialization() {
  36     TimeStamp::Startup();
  37     mFirstTimeStamp = TimeStamp::Now();
  38     // On Windows < 10, initializing the uptime requires `mFirstTimeStamp` to be
  39     // valid.
  40     mozilla::InitializeUptime();
  41   }
  42
  43   ~TimeStampInitialization() { TimeStamp::Shutdown(); }
  44 };
  45
  46 static TimeStampInitialization sInitOnce;
  47
  48 MFBT_API TimeStamp TimeStamp::ProcessCreation() {
  49   if (sInitOnce.mProcessCreation.IsNull()) {
  50     char* mozAppRestart = getenv("MOZ_APP_RESTART");
  51     TimeStamp ts;
  52
  53     /* When calling PR_SetEnv() with an empty value the existing variable may
  54      * be unset or set to the empty string depending on the underlying platform
  55      * thus we have to check if the variable is present and not empty. */
  56     if (mozAppRestart && (strcmp(mozAppRestart, "") != 0)) {
  57       /* Firefox was restarted, use the first time-stamp we've taken as the new
  58        * process startup time. */
  59       ts = sInitOnce.mFirstTimeStamp;
  60     } else {
  61       TimeStamp now = Now();
  62       uint64_t uptime = ComputeProcessUptime();
  63
  64       ts = now - TimeDuration::FromMicroseconds(static_cast<double>(uptime));
  65
  66       if ((ts > sInitOnce.mFirstTimeStamp) || (uptime == 0)) {
  67         ts = sInitOnce.mFirstTimeStamp;
  68       }
  69     }
  70
  71     sInitOnce.mProcessCreation = ts;
  72   }
  73
  74   return sInitOnce.mProcessCreation;
  75 }
  76
  77 void TimeStamp::RecordProcessRestart() {
  78   sInitOnce.mProcessCreation = TimeStamp();
  79 }
  80
  81 MFBT_API TimeStamp TimeStamp::FirstTimeStamp() {
  82   return sInitOnce.mFirstTimeStamp;
  83 }
  84
  85 class TimeStampTests {
  86   // Check that nullity is set/not set correctly.
  87   static_assert(TimeStamp{TimeStampValue{0}}.IsNull());
  88   static_assert(!TimeStamp{TimeStampValue{1}}.IsNull());
  89
  90   // Check that some very basic comparisons work correctly.
  91   static constexpr uint64_t sMidTime = (uint64_t)1 << 63;
  92   static_assert(TimeStampValue{sMidTime + 5} > TimeStampValue{sMidTime - 5});
  93   static_assert(TimeStampValue{sMidTime + 5} >= TimeStampValue{sMidTime - 5});
  94   static_assert(TimeStampValue{sMidTime - 5} < TimeStampValue{sMidTime + 5});
  95   static_assert(TimeStampValue{sMidTime - 5} <= TimeStampValue{sMidTime + 5});
  96   static_assert(TimeStampValue{sMidTime} == TimeStampValue{sMidTime});
  97   static_assert(TimeStampValue{sMidTime} >= TimeStampValue{sMidTime});
  98   static_assert(TimeStampValue{sMidTime} <= TimeStampValue{sMidTime});
  99   static_assert(TimeStampValue{sMidTime - 5} != TimeStampValue{sMidTime + 5});
 100
 101   // Check that comparisons involving very large and very small TimeStampValue's
 102   // work correctly. This may seem excessive, but these asserts would have
 103   // failed in the past due to a comparison such as "a > b" being implemented as
 104   // "<cast to signed 64-bit value>(a - b) > 0". When a-b didn't fit into a
 105   // signed 64-bit value, this would have given an incorrect result.
 106   static_assert(TimeStampValue{UINT64_MAX} > TimeStampValue{1});
 107   static_assert(TimeStampValue{1} < TimeStampValue{UINT64_MAX});
 108
 109   // NOTE/TODO: It would be nice to add some additional tests here that involve
 110   // arithmetic between TimeStamps and TimeDurations (and verifying some of the
 111   // special behaviors in some cases such as not wrapping around below zero) but
 112   // that is not possible right now because those operators are not constexpr.
 113 };
 114
 115 }  // namespace mozilla