af18c6f22e
On Linux, mmap doesn't always zero-fill slack bytes ([man page]), despite being required to do so by POSIX. If the final page of a file is in the page cache and the bytes past the end of the file get overwritten by some process, those bytes then remain non-zero until the page falls out of the cache or another process overwrites them. Stop trusting that mmap behaves properly and instead check whether the buffer was indeed properly terminated. If not, fall back to using `read` to read the file contents. This fixes an obscure clang crash bug that can occur if another program (such as an editor) mmap's a source file and writes past the end of the mmap'd region shortly before clang or clangd attempts to parse the file. [man page]: https://man7.org/linux/man-pages/man2/mmap.2.html#BUGS (cherry picked from commit 85cd3d98686c47d015dbcc17f1f7d0714b00e172)
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