Handle errors when restoring from cache

[dbu]

If the cache file became garbled for some reason, this should not break anything.

Q	A
Bug fix?	yes
New feature?	no
Doc updated	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-
License	MIT

See also #82 where i try to prevent us from writing corrupt caches.

[goetas]

Hi! Thanks for the PR. Did this effectively happen?

[dbu]

yes, thats what prompted me to do this ;-)

Uncaught PHP Exception Symfony\Component\Debug\Exception\FatalThrowableError: "syntax error, unexpected ''C:37:"JMS\\Serializer\\Metada' (T_ENCAPSED_AND_WHITESPACE)" at /home/vcap/app/var/cache/prod/jms_serializer/Namespace-Class.cache.php. one of our nodes ended up having this, and because this cache is not created at cache warmup but only on demand, the deployment process did not notice.

a few days ago, we had the other one with Uncaught PHP Exception LogicException: "The cache must return instances of ClassMetadata, but got 1." at /home/vcap/app/vendor/jms/metadata/src/Metadata/MetadataFactory.php line 73 . i don't know how it managed to end up with a file that did not return anything, and the cache file is now gone... but its what happens when using include.

i assume that there have been problems with the filesystem or network at the time (we have to cope with rather unreliable infrastructure).

i am now doing another PR to have the FileCache validate it actually writes the file correctly too.