|
The improved GC implementation uses refactored storage
classes instead of directly accessing "unknown" file paths.
The required storage class refactoring is quite substantial
and outlined in the following paragraphs.
The module `buildtool/file_system` was extended by:
- `ObjectCAS`: a plain CAS implementation for
reading/writing blobs and computing digests for a given
`ObjectType`. Depending on that type, files written to the
file system may have different properties (e.g., the x-bit
set) or the digest may be computed differently (e.g., tree
digests in non-compatible mode).
A new module `buildtool/storage` was introduced containing:
- `LocalCAS`: provides a common interface for the "logical
CAS", which internally combines three `ObjectCAS`s, one
for each `ObjectType` (file, executable, tree).
- `LocalAC`: implements the action cache, which needs the
`LocalCAS` for storing cache values.
- `TargetCache`: implements the high-level target cache,
which also needs the `LocalCAS` for storing cache values.
- `LocalStorage`: combines the storage classes `LocalCAS`,
`LocalAC`, and `TargetCache`. Those are initialized with
settings from `StorageConfig`, such as the build root base
path or number of generations for the garbage collector.
`LocalStorage` is templated with a Boolean parameter
`kDoGlobalUplink`, which indicates that, on every
read/write access, the garbage collector should be used
for uplinking across all generations (global).
- `GarbageCollector`: responsible for garbage collection and
the global uplinking across all generations. To do so, it
employs instances of `LocalStorage` with `kDoGlobalUplink`
set to false, in order to avoid endless recursion. The
actual (local) uplinking within two single generations is
performed by the corresponding storage class (e.g.,
`TargetCache` implements uplinking of target cache entries
between two target cache generations etc.). Thereby, the
actual knowledge how data should be uplinked is
implemented by the instance that is responsible for
creating the data in the first place.
|
