Internally, time is represent using the webrtc::Timestamp class. This represents time with a resolution of one microsecond, using a 64-bit integer, and provides converters to milliseconds or seconds as needed.
All timestamps need to be measured from the system monotonic time.
The epoch is not specified (because we can't always know if the system clock is correct), but whenever an absolute epoch is needed, the Unix time epoch (Jan 1, 1970 at 0:00 GMT) is used.
Conversion from/to other formats (for example milliseconds, NTP times, timestamp strings) should happen as close to the interface requiring that format as possible.
NOTE: There are parts of the codebase that don‘t use Timestamp, parts of the codebase that use the NTP epoch, and parts of the codebase that don’t use the monotonic clock. They need to be updated.
All execution happens on a TaskQueue instance. How a TaskQueue is implemented varies by platform, but they all have the webrtc::TaskQueueBase API.
This API offers primitives for posting tasks, with or without delay.
Some core parts use the rtc::Thread, which is a subclass of TaskQueueBase. This may contain a SocketServer for processing I/O, and is used for policing certain calling pattern between a few core threads (the NetworkThread cannot do Invoke on the Worker thread, for instance).
The preferred method for synchronization is to post tasks between threads, and to let each thread take care of its own variables (lock-free programming). All variables in classes intended to be used with multiple threads should therefore be annotated with RTC_GUARDED_BY(thread).
For classes used with only one thread, the recommended pattern is to let them own a webrtc::SequenceChecker (conventionally named sequence_checker_) and let all variables be RTC_GUARDED_BY(sequence_checker_).
Member variables marked const do not need to be guarded, since they never change. (But note that they may point to objects that can change!)
When posting tasks with callbacks, it is the duty of the caller to check that the object one is calling back into still exists when the callback is made. A helper for this task is the webrtc::ScopedTaskSafety flag, which can automatically drop callbacks in this situation, and associated classes.
When it is absolutely necessary to let one thread wait for another thread to do something, Thread::Invoke can be used. This function is DISCOURAGED, since it leads to performance issues, but is currently still widespread.
When it is absolutely necessary to access one variable from multiple threads, the webrtc::Mutex can be used. Such variables MUST be marked up with RTC_GUARDED_BY(mutex), to allow static analysis that lessens the chance of deadlocks or unintended consequences.
The following non-exhaustive list of synchronization primitives are in the (slow) process of being removed from the codebase.