Added missing _mutex->unlock() to FileBase::lookup().

[korjaa]

Description

My modified Pelion Device Management Client has been getting stuck in the startup for a while now. After days of debugging, I finally was able to pinpoint the system to a mutex deadlock.

I don't thought understand how this has worked with the already released PDMC.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

@kjbracey-arm, related to the discussion on the hallway, this fix is related to previous modification done in here #7924

[korjaa]

I understood the ScopedLock would be a better way to fix this. If we go with the better fix, please feel free to close this PR.

[kjbracey]

I do recall seeing this error during development of #7924.

I corrected it by changing to ScopedLock<PlatformMutex>, but when that was rejected during review, I dropped the commit, forgetting that the commit was actually fixing this mutex bug, not just a style adjustment. Whoops.

You could switch to ScopedLock again, but this is okay.

I think we've not seen a problem because it's a recursive mutex, so wouldn't show any issues until you've got two threads doing file opening (with at least one using /default.)

[kjbracey]

Actually, the ScopedLock version is a bit ugly until #8001, and that was also deferred from #7924, so won't be in until 5.11. This one is fine.

[korjaa]

👍

[cmonr]

/morph build

[mbed-ci]

Build : FAILURE

Build number : 3380
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8451/

[cmonr]

IAR network license issues.
/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3387
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8451/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3019
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8451/

[mbed-ci]

Test : SUCCESS

Build number : 3189
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8451/3189