After pr containerd#8617, create handler of containerd-shim-runc-v2 will
call handleStarted() to record the init process and handle its exit.
Init process wouldn't quit so early in normal circumstances. But if
this screnario occurs, handleStarted() will call
handleProcessExit(), which will cause deadlock because create() had
acquired s.mu, and handleProcessExit() will try to lock it again.

For some historical reasons, the use of s.mu is a bit confusing,
sometimes used to protect s.containers only, and sometimes used as a
mutex for some functions.

According to analysis of @corhere in containerd#9103, we can know that:

Locking s.mu for Create() handler is introduced in containerd#1452, which was a
solution for the missed early-exits problem: Create() holds the mutex to
block checkProcesses() from handling exit events until after Create()
has added the container process to the s.processes map. This locking
logic was copied into the v2 shim implementation. containerd#8617 solves the same
problem using a strategy that does not rely on mutual exclusion between
container create and exit-event handling.

As for Shutdown(), the implementation was added in containerd#3004. And in this
initial implementation, the mutex is locked to safely access
s.containers, it is not unlocked because nothing matters after
os.Exit(0). Then ae87730 changed Shutdown() to return instead of
exiting, followed by containerd#4988 to unlock upon return. If the intention is
to block containers from being created while the shim's task service is
shutting down, locking a mutex is a poor solution since it makes the
create requests hang instead of returning an error, and is ineffective
after Shutdown() returns as the mutex is unlocked.

If Create() or handleProcessExit() enters again after Shutdown()
unlocking s.mu and returning, shim service will panic by sending to
a closed channel.

So I remove the unnecessary lock guards in service, and rename mu to
containersMu to make it clear that it is used to protect s.containers
only.

Fix: containerd#9103
Signed-off-by: Chen Yiyang <cyyzero@qq.com>

After pr containerd#8617, create handler of containerd-shim-runc-v2 will
call handleStarted() to record the init process and handle its exit.
Init process wouldn't quit so early in normal circumstances. But if
this screnario occurs, handleStarted() will call
handleProcessExit(), which will cause deadlock because create() had
acquired s.mu, and handleProcessExit() will try to lock it again.

For some historical reasons, the use of s.mu is a bit confusing,
sometimes used to protect s.containers only, and sometimes used as a
mutex for some functions.

According to analysis of @corhere in containerd#9103, we can know that:

Locking s.mu for Create() handler is introduced in containerd#1452, which was a
solution for the missed early-exits problem: Create() holds the mutex to
block checkProcesses() from handling exit events until after Create()
has added the container process to the s.processes map. This locking
logic was copied into the v2 shim implementation. containerd#8617 solves the same
problem using a strategy that does not rely on mutual exclusion between
container create and exit-event handling.

As for Shutdown(), the implementation was added in containerd#3004. And in this
initial implementation, the mutex is locked to safely access
s.containers, it is not unlocked because nothing matters after
os.Exit(0). Then ae87730 changed Shutdown() to return instead of
exiting, followed by containerd#4988 to unlock upon return. If the intention is
to block containers from being created while the shim's task service is
shutting down, locking a mutex is a poor solution since it makes the
create requests hang instead of returning an error, and is ineffective
after Shutdown() returns as the mutex is unlocked.

If Create() or handleProcessExit() enters again after Shutdown()
unlocking s.mu and returning, shim service will panic by sending to
a closed channel.

So I remove the unnecessary lock guards in service, and rename mu to
containersMu to make it clear that it is used to protect s.containers
only.

Fix: containerd#9103
Signed-off-by: Chen Yiyang <cyyzero@qq.com>