Implement a clock.

GoogleDataLogger/GoogleDataLogger/Classes/GDLClock.m

@@ -16,6 +16,152 @@
 
 #import "GDLClock.h"
 
-@implementation GDLClock
+#import <sys/sysctl.h>
+
+// Using a monotonic clock is necessary because CFAbsoluteTimeGetCurrent(), NSDate, and related all
+// are subject to drift. That it to say, multiple consecutive calls do not always result in a
+// time that is in the future. Clocks may be adjusted by the user, NTP, or any number of external
+// factors. This class attempts to determine the wall-clock time at the time of log by capturing
+// the kernel start and time since boot to determine a wallclock time in UTC.
+//
+// Timezone offsets at the time of a snapshot are also captured in order to provide local-time
+// details. Other classes in this library depend on comparing times at some time in the future to
+// a time captured in the past, and this class needs to provide a mechanism to do that.
+//
+// TL;DR: This class attempts to accomplish two things: 1. Provide accurate event times. 2. Provide
+// a monotonic clock mechanism to accurately check if some clock snapshot was before or after
+// by using a shared reference point (kernel boot time).
+//
+// Note: Much of the mach time stuff doesn't work properly in the simulator. So this class can be
+// difficult to unit test.
+
+/** Returns the kernel boottime property from sysctl.
+ *
+ * Inspired by https://stackoverflow.com/a/40497811
+ *
+ * @return The KERN_BOOTTIME property from sysctl, in nanoseconds.
+ */
+static int64_t KernelBootTimeInNanoseconds() {
+  // Caching the result is not possible because clock drift would not be accounted for.
+  struct timeval boottime;
+  int mib[2] = {CTL_KERN, KERN_BOOTTIME};
+  size_t size = sizeof(boottime);
+  int rc = sysctl(mib, 2, &boottime, &size, NULL, 0);
+  if (rc != 0) {
+    return 0;
+  }
+  return (int64_t)boottime.tv_sec * NSEC_PER_MSEC + (int64_t)boottime.tv_usec;
+}
+
+/** Returns value of gettimeofday, in nanoseconds.
+ *
+ * Inspired by https://stackoverflow.com/a/40497811
+ *
+ * @return The value of gettimeofday, in nanoseconds.
+ */
+static int64_t UptimeInNanoseconds() {
+  int64_t before_now;
+  int64_t after_now;
+  struct timeval now;
+
+  before_now = KernelBootTimeInNanoseconds();
+  // Addresses a race condition in which the system time has updated, but the boottime has not.
+  do {
+    gettimeofday(&now, NULL);
+    after_now = KernelBootTimeInNanoseconds();
+  } while (after_now != before_now);
+  return (int64_t)now.tv_sec * NSEC_PER_MSEC + (int64_t)now.tv_usec - before_now;
+}
+
+// TODO: Consider adding a 'trustedTime' property that can be populated by the response from a BE.
+@implementation GDLClock {
+  /** The kernel boot time when this clock was created. */
+  int64_t _kernelBootTime;
+
+  /** The device uptime when this clock was created. */
+  int64_t _uptime;
+}
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _kernelBootTime = KernelBootTimeInNanoseconds();
+    _uptime = UptimeInNanoseconds();
+    _timeMillis = CFAbsoluteTimeGetCurrent() * NSEC_PER_USEC;
+    CFTimeZoneRef timeZoneRef = CFTimeZoneCopySystem();
+    _timezoneOffsetSeconds = CFTimeZoneGetSecondsFromGMT(timeZoneRef, 0);
+    CFRelease(timeZoneRef);
+  }
+  return self;
+}
+
++ (GDLClock *)snapshot {
+  return [[GDLClock alloc] init];
+}
+
++ (instancetype)clockSnapshotInTheFuture:(uint64_t)millisInTheFuture {
+  GDLClock *snapshot = [self snapshot];
+  snapshot->_timeMillis += millisInTheFuture;
+  return snapshot;
+}
+
+- (BOOL)isAfter:(GDLClock *)otherClock {
+  // These clocks are trivially comparable when they share a kernel boot time.
+  if (_kernelBootTime == otherClock->_kernelBootTime) {
+    return _uptime > otherClock->_uptime;
+  } else {
+    int64_t kernelBootTimeDiff = otherClock->_kernelBootTime - _kernelBootTime;
+    // This isn't a great solution, but essentially, if the other clock's boot time is 'later', NO
+    // is returned. This can be altered by changing the system time and rebooting.
+    return kernelBootTimeDiff < 0 ? YES : NO;
+  }
+}
+
+- (NSUInteger)hash {
+  // These casts lose some precision, but it's probably fine.
+  return (NSUInteger)_kernelBootTime ^ (NSUInteger)_uptime ^ (NSUInteger)_timeMillis;
+}
+
+- (BOOL)isEqual:(id)object {
+  return [self hash] == [object hash];
+}
+
+#pragma mark - NSSecureCoding
+
+/** NSKeyedCoder key for timeMillis property. */
+static NSString *const kGDLClockTimeMillisKey = @"GDLClockTimeMillis";
+
+/** NSKeyedCoder key for timezoneOffsetMillis property. */
+static NSString *const kGDLClockTimezoneOffsetSeconds = @"GDLClockTimezoneOffsetSeconds";
+
+/** NSKeyedCoder key for _kernelBootTime ivar. */
+static NSString *const kGDLClockKernelBootTime = @"GDLClockKernelBootTime";
+
+/** NSKeyedCoder key for _uptime ivar. */
+static NSString *const kGDLClockUptime = @"GDLClockUptime";
+
++ (BOOL)supportsSecureCoding {
+  return YES;
+}
+
+- (instancetype)initWithCoder:(NSCoder *)aDecoder {
+  self = [super init];
+  if (self) {
+    // TODO: If the kernelBootTime is more recent, we need to change the kernel boot time and
+    // uptimeMillis ivars
+    _timeMillis = [aDecoder decodeInt64ForKey:kGDLClockTimeMillisKey];
+    _timezoneOffsetSeconds = [aDecoder decodeInt64ForKey:kGDLClockTimezoneOffsetSeconds];
+    _kernelBootTime = [aDecoder decodeInt64ForKey:kGDLClockKernelBootTime];
+    _uptime = [aDecoder decodeInt64ForKey:kGDLClockUptime];
+  }
+  return self;
+}
+
+- (void)encodeWithCoder:(NSCoder *)aCoder {
+  [aCoder encodeInt64:_timeMillis forKey:kGDLClockTimeMillisKey];
+  [aCoder encodeInt64:_timezoneOffsetSeconds forKey:kGDLClockTimezoneOffsetSeconds];
+  [aCoder encodeInt64:_kernelBootTime forKey:kGDLClockKernelBootTime];
+  [aCoder encodeInt64:_uptime forKey:kGDLClockUptime];
+}
 
 @end

GoogleDataLogger/GoogleDataLogger/Classes/GDLLogEvent.m

@@ -45,8 +45,10 @@ - (instancetype)copy {
 
 - (NSUInteger)hash {
   // This loses some precision, but it's probably fine.
-  NSUInteger timeHash = (NSUInteger)(_clockSnapshot.timeMillis ^ _clockSnapshot.uptimeMillis);
-  return [_logMapID hash] ^ _logTarget ^ [_extensionBytes hash] ^ _qosTier ^ timeHash;
+  NSUInteger logMapIDHash = [_logMapID hash];
+  NSUInteger timeHash = [_clockSnapshot hash];
+  NSUInteger extensionBytesHash = [_extensionBytes hash];
+  return logMapIDHash ^ _logTarget ^ extensionBytesHash ^ _qosTier ^ timeHash;
 }
 
 - (void)setExtension:(id<GDLLogProto>)extension {
@@ -73,14 +75,8 @@ - (void)setExtension:(id<GDLLogProto>)extension {
 /** NSCoding key for qosTier property. */
 static NSString *qosTierKey = @"_qosTier";
 
-/** NSCoding key for clockSnapshot.timeMillis property. */
-static NSString *clockSnapshotTimeMillisKey = @"_clockSnapshotTimeMillis";
-
-/** NSCoding key for clockSnapshot.uptimeMillis property. */
-static NSString *clockSnapshotUpTimeMillis = @"_clockSnapshotUpTimeMillis";
-
-/** NSCoding key for clockSnapshot.timezoneOffsetMillis property. */
-static NSString *clockSnapshotTimezoneOffsetMillis = @"_clockSnapshotTimezoneOffsetMillis";
+/** NSCoding key for clockSnapshot property. */
+static NSString *clockSnapshotKey = @"_clockSnapshot";
 
 + (BOOL)supportsSecureCoding {
   return YES;
@@ -93,10 +89,7 @@ - (id)initWithCoder:(NSCoder *)aDecoder {
   if (self) {
     _extensionBytes = [aDecoder decodeObjectOfClass:[NSData class] forKey:extensionBytesKey];
     _qosTier = [aDecoder decodeIntegerForKey:qosTierKey];
-    _clockSnapshot.timeMillis = [aDecoder decodeInt64ForKey:clockSnapshotTimeMillisKey];
-    _clockSnapshot.uptimeMillis = [aDecoder decodeInt64ForKey:clockSnapshotUpTimeMillis];
-    _clockSnapshot.timezoneOffsetMillis =
-        [aDecoder decodeInt64ForKey:clockSnapshotTimezoneOffsetMillis];
+    _clockSnapshot = [aDecoder decodeObjectOfClass:[GDLClock class] forKey:clockSnapshotKey];
   }
   return self;
 }
@@ -106,9 +99,7 @@ - (void)encodeWithCoder:(NSCoder *)aCoder {
   [aCoder encodeInteger:_logTarget forKey:logTargetKey];
   [aCoder encodeObject:_extensionBytes forKey:extensionBytesKey];
   [aCoder encodeInteger:_qosTier forKey:qosTierKey];
-  [aCoder encodeInt64:_clockSnapshot.timeMillis forKey:clockSnapshotTimeMillisKey];
-  [aCoder encodeInt64:_clockSnapshot.uptimeMillis forKey:clockSnapshotUpTimeMillis];
-  [aCoder encodeInt64:_clockSnapshot.timezoneOffsetMillis forKey:clockSnapshotTimezoneOffsetMillis];
+  [aCoder encodeObject:_clockSnapshot forKey:clockSnapshotKey];
 }
 
 @end

GoogleDataLogger/GoogleDataLogger/Classes/GDLLogger.m

@@ -19,6 +19,7 @@
 
 #import "GDLAssert.h"
 #import "GDLLogEvent.h"
+#import "GDLLogEvent_Private.h"
 #import "GDLLogWriter.h"
 
 @implementation GDLLogger
@@ -39,16 +40,20 @@ - (instancetype)initWithLogMapID:(NSString *)logMapID
 }
 
 - (void)logTelemetryEvent:(GDLLogEvent *)logEvent {
+  // TODO: Determine if logging an event before registration is allowed.
   GDLAssert(logEvent, @"You can't log a nil event");
   GDLLogEvent *copiedLog = [logEvent copy];
   copiedLog.qosTier = GDLLogQoSTelemetry;
+  copiedLog.clockSnapshot = [GDLClock snapshot];
   [self.logWriterInstance writeLog:copiedLog afterApplyingTransformers:_logTransformers];
 }
 
 - (void)logDataEvent:(GDLLogEvent *)logEvent {
+  // TODO: Determine if logging an event before registration is allowed.
   GDLAssert(logEvent, @"You can't log a nil event");
   GDLAssert(logEvent.qosTier != GDLLogQoSTelemetry, @"Use -logTelemetryEvent, please.");
   GDLLogEvent *copiedLog = [logEvent copy];
+  copiedLog.clockSnapshot = [GDLClock snapshot];
   [self.logWriterInstance writeLog:copiedLog afterApplyingTransformers:_logTransformers];
 }
 

GoogleDataLogger/GoogleDataLogger/Classes/Private/GDLLogEvent_Private.h

@@ -26,7 +26,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property(nonatomic) NSData *extensionBytes;
 
 /** The clock snapshot at the time of logging. */
-@property(nonatomic) GDLLogClockSnapshot clockSnapshot;
+@property(nonatomic) GDLClock *clockSnapshot;
 
 @end
 

GoogleDataLogger/GoogleDataLogger/Classes/Public/GDLClock.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/** This class manages the device clock and produces snapshots of the current time. */
+@interface GDLClock : NSObject <NSSecureCoding>
+
+/** The wallclock time, UTC, in milliseconds. */
+@property(nonatomic, readonly) int64_t timeMillis;
+
+/** The offset from UTC in seconds. */
+@property(nonatomic, readonly) int64_t timezoneOffsetSeconds;
+
+/** Creates a GDLClock object using the current time and offsets.
+ *
+ * @return A new GDLClock object representing the current time state.
+ */
++ (instancetype)snapshot;
+
+/** Creates a GDLClock object representing a time in the future, relative to now.
+ *
+ * @param millisInTheFuture The millis in the future from now this clock should represent.
+ * @return An instance representing a future time.
+ */
++ (instancetype)clockSnapshotInTheFuture:(uint64_t)millisInTheFuture;
+
+/** Compares one clock with another, returns YES if the caller is after the parameter.
+ *
+ * @return YES if the calling clock's time is after the given clock's time.
+ */
+- (BOOL)isAfter:(GDLClock *)otherClock;
+
+@end
+
+NS_ASSUME_NONNULL_END

GoogleDataLogger/GoogleDataLogger/Classes/Public/GoogleDataLogger.h

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#import "GDLClock.h"
 #import "GDLLogEvent.h"
 #import "GDLLogPrioritizer.h"
 #import "GDLLogProto.h"

GoogleDataLogger/Tests/Unit/GDLClockTest.m

@@ -29,4 +29,37 @@ - (void)testInit {
   XCTAssertNotNil([[GDLClockTest alloc] init]);
 }
 
+/** Tests taking a snapshot. */
+- (void)testSnapshot {
+  GDLClock *snapshot;
+  XCTAssertNoThrow(snapshot = [GDLClock snapshot]);
+  XCTAssertGreaterThan(snapshot.timeMillis, 0);
+}
+
+/** Tests that the hash of two snapshots right after each other isn't equal. */
+- (void)testHash {
+  GDLClock *snapshot1 = [GDLClock snapshot];
+  GDLClock *snapshot2 = [GDLClock snapshot];
+  XCTAssertNotEqual([snapshot1 hash], [snapshot2 hash]);
+}
+
+/** Tests that the class supports NSSecureEncoding. */
+- (void)testSupportsSecureEncoding {
+  XCTAssertTrue([GDLClock supportsSecureCoding]);
+}
+
+- (void)testEncoding {
+  GDLClock *clock = [GDLClock snapshot];
+  NSData *clockData = [NSKeyedArchiver archivedDataWithRootObject:clock];
+  GDLClock *unarchivedClock = [NSKeyedUnarchiver unarchiveObjectWithData:clockData];
+  XCTAssertEqual([clock hash], [unarchivedClock hash]);
+  XCTAssertEqualObjects(clock, unarchivedClock);
+}
+
+- (void)testClockSnapshotInTheFuture {
+  GDLClock *clock1 = [GDLClock snapshot];
+  GDLClock *clock2 = [GDLClock clockSnapshotInTheFuture:1];
+  XCTAssertTrue([clock2 isAfter:clock1]);
+}
+
 @end