[llvm] Work around documented Linux mmap bug. (PR #152595)

[Eli Friedman via llvm-commits]

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@@ -501,8 +501,15 @@ getOpenFileImpl(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize,
     std::unique_ptr<MB> Result(
         new (NamedBufferAlloc(Filename)) MemoryBufferMMapFile<MB>(
             RequiresNullTerminator, FD, MapSize, Offset, EC));
-    if (!EC)
-      return std::move(Result);
+    if (!EC) {
+#ifdef __linux__
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efriedma-quic wrote:

How sure are we that Linux is the only OS which has this issue?  Would it make sense to just remove the ifdef?

There's also a race condition here: if another processes modifies the file while it's mapped, you get undefined behavior.  But we can't really do much about that.

https://github.com/llvm/llvm-project/pull/152595