Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+<!-- go/cmark -->
+<!--* freshness: {owner: 'hta' reviewed: '2021-05-31'} *-->
+
+# Basic concepts and primitives
+
+## Time
+
+Internally, time is represent using the [webrtc::Timestamp][1] 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.
+
+## Threads
+
+All execution happens on a TaskQueue instance. How a TaskQueue is implemented
+varies by platform, but they all have the [webrtc::TaskQueueBase][3] API.
+
+This API offers primitives for posting tasks, with or without delay.
+
+Some core parts use the [rtc::Thread][2], 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).
+
+## Reserved class suffixes
+
+C++ classes with names ending in the suffixes "Factory", "Builder" and "Manager" are supposed to behave
+in certain well known ways.
+
+For a particular class name Foo, the following classes, if they exist, should
+behave as follows:
+
+* FooFactory: Has a Create function that creates a Foo object and returns the
+  object or an owning reference to it (for instance std::unique_ptr or
+  rtc::scoped_refptr<Foo>). The Create function should NOT alter the factory
+  state; ideally, it is marked const. Ownership of the returned object is only
+  with the caller.
+
+* FooBuilder: Has a Build function that returns ownership of a Foo object (as
+  above). The Builder can only be used once, and resources given to the Builder
+  before the Build function is called are either released or owned by the Foo
+  object. The Create function may be reference-qualified (declared as ```Foo
+  Build() &&```), which means it is invoked as ```std::move(builder).Build()```,
+  and C++ will ensure that it is not used again.
+
+* FooManager: Has a Create function that returns an rtc::scoped_refptr<Foo> (if
+  shared ownership) or a Foo* (if the Manager retains sole ownership). If
+  Create() cannot fail, consider returning a Foo&. The Manager is responsible
+  for keeping track of the object; if the Create function returns a Foo*, the
+  Foo object is guaranteed to be destroyed when the FooManager is destroyed.
+
+If a Manager class manages multiple classes of objects, the Create functions
+should be appropriately named (the FooAndBarManager would have CreateFoo() and
+CreateBar() functions), and the class will have a suitable name for the group of
+objects it is managing.
+
+FooFactory is mainly useful for the case where preparation for producing Foo
+objects is complex. If Foo can be created with just an argument list, consider
+exposing its constructor instead; if Foo creation can fail, consider having
+a free function called CreateFoo instead of a factory.
+
+Note that classes with these names exist that do not follow these conventions.
+When they are detected, they need to be marked with TODO statements and bugs
+filed on them to get them into a conformant state.
+
+## Synchronization primitives
+
+### PostTask and thread-guarded variables
+
+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][5]
+flag, which can automatically drop callbacks in this situation, and
+associated classes.
+
+### Synchronization primitives to be used when needed
+
+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.
+
+### Synchronization primitives that are being removed
+The following non-exhaustive list of synchronization primitives are
+in the (slow) process of being removed from the codebase.
+
+* sigslot. Use [webrtc::CallbackList][4] instead, or, when there's only one
+  signal consumer, a single std::function.
+  
+* AsyncInvoker.
+
+* RecursiveCriticalSection. Try to use [webrtc::Mutex][6] instead, and don't recurse.
+
+## Enum-To-String functions
+If there is a need to convert an enum to a string representation, such as for
+enums exposed at the Javascript API interface, the recommended way is to write
+a function named AsString, declared "static constexpr" and returning an
+absl::string_view. The declaration should be right after the enum declaration,
+in the same scope; the implementation (which must be marked "inline") should
+be at the end of the same header file.
+
+If the enum is not defined within a class, the "static" keyword is not needed.
+
+[1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/units/timestamp.h;drc=b95d90b78a3491ef8e8aa0640dd521515ec881ca;l=29
+[2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/thread.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=194
+[3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/task_queue/task_queue_base.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=25
+[4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/callback_list.h;drc=54b91412de3f579a2d5ccdead6e04cc2cc5ca3a1;l=162
+[5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/task_utils/pending_task_safety_flag.h;drc=86ee89f73e4f4799b3ebcc0b5c65837c9601fe6d;l=117
+[6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/synchronization/mutex.h;drc=0d3c09a8fe5f12dfbc9f1bcd5790fda8830624ec;l=40