# Chrome Shutdown

[TOC]

This documents shutdown steps on Windows, Mac and Linux.

On Android, the system can terminate the Chrome app at any point without running
any shutdown step.

TODO: Document ChromeOS shutdown.

## Step 1: Exiting the main loop

Shutdown starts when nothing keeps Chrome alive. Typically, this happens when
all browser windows are closed, but other things can [keep Chrome
alive](https://source.chromium.org/chromium/chromium/src/+/master:components/keep_alive_registry/keep_alive_types.h).

When nothing keeps Chrome alive, `BrowserProcessImpl::Unpin` asks the main
thread's message loop to quit as soon as it no longer has tasks ready to run
immediately.

```
base::RunLoop::QuitWhenIdle
…
BrowserProcessImpl::Unpin
BrowserProcessImpl::OnKeepAliveStateChanged
KeepAliveRegistry::OnKeepAliveStateChanged
KeepAliveRegistry::Unregister
ScopedKeepAlive::~ScopedKeepAlive
...
Browser::UnregisterKeepAlive
BrowserList::RemoveBrowser
Browser::~Browser
```

Following this request, `ChromeBrowserMainParts::MainMessageLoopRun` exits. Tasks
posted to the main thread without a delay prior to this point are guaranteed to
have run; tasks posted to the main thread after this point will never run.

## Step 2: Cleaning up, after main loop exit

`BrowserMainRunnerImpl::Shutdown` is called on the main thread. Within that
method, `BrowserMainLoop::ShutdownThreadsAndCleanUp` orchestrates the main
shutdown steps.

`ChromeBrowserMainParts::PostMainMessageLoopRun` is invoked. It invokes the
`PostMainMessageLoopRun` method of each `ChromeBrowserMainExtraParts` instance.
This is a good place to perform shutdown steps of a component that require the
IO thread, the `ThreadPool` or the `Profile` to still be available.

`ChromeBrowserMainParts::PostMainMessageLoopRun` also invokes
`BrowserProcessImpl::StartTearDown` which deletes many services owned by
`BrowserProcessImpl` (aka `g_browser_process`). One of these services is the
`ProfileManager`. Deleting the `ProfileManager` deletes `Profiles`. As part of
deleting a `Profile`, its `KeyedServices` are deleted, including:

* Sync Service
* History Service

## Step 3: Joining other threads

The IO thread is joined. No IPC or Mojo can be received after this.

`ThreadPool` shutdown starts. At this point, no new `SKIP_ON_SHUTDOWN` or
`CONTINUE_ON_SHUTDOWN` task can start running (they are deleted without
running). The main thread blocks until all `SKIP_ON_SHUTDOWN` tasks that started
running prior to `ThreadPool` shutdown start are complete, and all
`BLOCK_SHUTDOWN` tasks are complete (irrespective of whether they were posted
before or after `ThreadPool` shutdown start). When no more `SKIP_ON_SHUTDOWN` is
running and no more `BLOCK_SHUTDOWN` task is queued or running, the main thread
is unblocked and `ThreadPool` shutdown is considered complete. Note:
`CONTINUE_ON_SHUTDOWN` tasks that started before `ThreadPool` shutdown may still
be running.

At this point, new tasks posted to the IO thread or to the `ThreadPool` cannot
run. It is illegal to post a `BLOCK_SHUTDOWN` task to the `ThreadPool` (enforced
by a `DCHECK`).

## Step 4: Cleaning up, after joining other threads

`ChromeBrowserMainParts::PostDestroyThreads` is invoked. It invokes
`BrowserProcessImpl::PostDestroyThreads`. Since it is guaranteed that no
`SKIP_ON_SHUTDOWN` or `BLOCK_SHUTDOWN` task is running at this point, it is a
good place to delete objects accessed directly from these tasks.

Then, if a new Chrome executable, it is swapped with the current one
(Windows-only).

```
upgrade_util::SwapNewChromeExeIfPresent
browser_shutdown::ShutdownPostThreadsStop
ChromeBrowserMainParts::PostDestroyThreads
content::BrowserMainLoop::ShutdownThreadsAndCleanUp
content::BrowserMainLoop::ShutdownThreadsAndCleanUp
content::BrowserMainRunnerImpl::Shutdown
```