[PATCH] D56949: FileOutputBuffer: handle mmap(2) failure

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.
ruiu added reviewers: sbc100, thakis.
Herald added a subscriber: hiraditya.

If the underlying filesystem does not support mmap system call,
FileOutputBuffer may fail when it attempts to mmap an output temporary
file. This patch handles such situation.

Unfortunately, it looks like it is very hard to test this functionality
without a filesystem that doesn't support mmap using llvm-lit. I tested
this locally by passing an invalid parameter to mmap so that it fails and
falls back to the in-memory buffer. Maybe that's all what we can do.
I believe it is reasonable to submit this without a test.


https://reviews.llvm.org/D56949

Files:
  llvm/lib/Support/FileOutputBuffer.cpp


Index: llvm/lib/Support/FileOutputBuffer.cpp
===================================================================
--- llvm/lib/Support/FileOutputBuffer.cpp
+++ llvm/lib/Support/FileOutputBuffer.cpp
@@ -122,7 +122,7 @@
   return llvm::make_unique<InMemoryBuffer>(Path, MB, Mode, Size);
 }
 
-static Expected<std::unique_ptr<OnDiskBuffer>>
+static Expected<std::unique_ptr<FileOutputBuffer>>
 createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
   Expected<fs::TempFile> FileOrErr =
       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
@@ -146,10 +146,14 @@
   std::error_code EC;
   auto MappedFile = llvm::make_unique<fs::mapped_file_region>(
       File.FD, fs::mapped_file_region::readwrite, Size, 0, EC);
+
+  // mmap(2) can fail if the underlying filesystem does not support it.
+  // If that happens, we fall back to in-memory buffer as the last resort.
   if (EC) {
     consumeError(File.discard());
-    return errorCodeToError(EC);
+    return createInMemoryBuffer(Path, Size, Mode);
   }
+
   return llvm::make_unique<OnDiskBuffer>(Path, std::move(File),
                                          std::move(MappedFile));
 }


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