7f2330a968
When profiling ngcc it is notable that a large amount of time is spent dealing with an exception that is thrown (and handled internally by fs) when checking the existence of a file. We check file existence a lot in both finding entry-points and when TS is compiling code. This commit adds a simple cached `FileSystem`, which wraps a real `FileSystem` delegate. This will reduce the number of calls through to `fs.exists()` and `fs.readFile()` on the delegate. Initial benchmarks indicate that the cache is miss to hit ratio for `exists()` is about 2:1, which means that we save about 1/3 of the calls to `fs.existsSync()`. Note that this implements a "non-expiring" cache, so it is not suitable for a long lived `FileSystem`, where files may be modified externally. The cache will be updated if a file is changed or moved via calls to `FileSystem` methods but it will not be aware of changes to the files system from outside the `FileSystem` service. For ngcc we must create a new `FileSystem` service for each run of `mainNgcc` and ensure that all file operations (including TS compilation) use the `FileSystem` service. This ensures that it is very unlikely that a file will change externally during `mainNgcc` processing. PR Close #30525
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