class ErrorCollector

Constructs a DescriptorPool that, when it can't find something among the

descriptors already in the pool, looks for it in the given

DescriptorDatabase.

Notes:

- If a DescriptorPool is constructed this way, its BuildFile*() methods

must not be called (they will assert-fail). The only way to populate

the pool with descriptors is to call the Find*By*() methods.

- The Find*By*() methods may block the calling thread if the

DescriptorDatabase blocks. This in turn means that parsing messages

may block if they need to look up extensions.

- The Find*By*() methods will use mutexes for thread-safety, thus making

them slower even when they don't have to fall back to the database.

In fact, even the Find*By*() methods of descriptor objects owned by

this pool will be slower, since they will have to obtain locks too.

- An ErrorCollector may optionally be given to collect validation errors

in files loaded from the database. If not given, errors will be printed

to ABSL_LOG(ERROR). Remember that files are built on-demand, so this

ErrorCollector may be called from any thread that calls one of the

Find*By*() methods.

- The DescriptorDatabase must not be mutated during the lifetime of

the DescriptorPool. Even if the client takes care to avoid data races,

changes to the content of the DescriptorDatabase may not be reflected

in subsequent lookups in the DescriptorPool.