The previous implementation mixed up the management
of locking a piece of code (both sync and async) with the
management of writing and removing the lockFile that is
used as the flag for which process has locked the code.
This change splits these two concepts up. Apart from
avoiding the awkward base class it allows the `LockFile`
implementation to be replaced cleanly.
PR Close#35861
ngcc uses a lockfile to prevent two ngcc instances from executing at the
same time. Previously, if a lockfile was found the current process would
error and exit.
Now, when in async mode, the current process is able to wait for the previous
process to release the lockfile before continuing itself.
PR Close#35131
Previously, it was possible for multiple instance of ngcc to be running
at the same time, but this is not supported and can cause confusing and
flakey errors at build time.
Now, only one instance of ngcc can run at a time. If a second instance
tries to execute it fails with an appropriate error message.
See https://github.com/angular/angular/issues/32431#issuecomment-571825781
PR Close#34722
Pete Bacon Darwin