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86423aa8b7
This is needed to fix the reason 0a2be46cfdb698fe (Modules: Invalidate out-of-date PCMs as they're discovered) and 5b44a4b07fc1d ([modules] Do not cache invalid state for modules that we attempted to load.) were reverted. These patches changed Clang to use `isVolatile` when loading modules. This had the side effect of not using mmap when loading modules, and thus greatly increased memory usage. The reason it wasn't using mmap is because `MemoryBuffer` plays some games with file size when you request null termination, and it has to disable these when `isVolatile` is set as the size may change by the time it's mmapped. Clang by default passes `RequiresNullTerminator = true`, and `shouldUseMmap` ignored if `RequiresNullTerminator` was even requested. This patch adds `RequiresNullTerminator` to the `FileManager` interface so Clang can use it when loading modules, and changes `shouldUseMmap` to only take volatility into account if `RequiresNullTerminator` is true. This is fine as both `mmap` and a `read` loop are vulnerable to modifying the file while reading, but are immune to the rename Clang does when replacing a module file. Differential Revision: https://reviews.llvm.org/D77772