[Support] Allow FileOutputBuffer::create to create an empty file

[MaskRay]

Summary

Size==0 triggers assert(Size != 0) in mapped_file_region::init.
I plan to use an empty file in D79339 (llvm-objcopy --dump-section).

According to POSIX, "If len is zero, mmap() shall fail and no mapping
shall be established." Just specialize case Size=0 to use
createInMemoryBuffer.

[jhenderson]

LGTM.

llvm/lib/Support/FileOutputBuffer.cpp

FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
   // Handle "-" as stdout just like llvm::raw_ostream does.
   if (Path == "-")
     return createInMemoryBuffer("-", Size, /*Mode=*/0);
 
   unsigned Mode = fs::all_read | fs::all_write;
   if (Flags & F_executable)
     Mode |= fs::all_exe;
 
+  // If Size is zero, don't use mmap which will fail with EINVAL.
+  if (Size == 0)
+    return createInMemoryBuffer(Path, Size, Mode);
+
   fs::file_status Stat;
   fs::status(Path, Stat);
 
   // Usually, we want to create OnDiskBuffer to create a temporary file in
   // the same directory as the destination file and atomically replaces it
   // by rename(2).
   //
   // However, if the destination file is a special file, we don't want to

llvm/unittests/Support/FileOutputBufferTest.cpp

File5.append("/file5");
     ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
   }
 
   // Verify file is correct size.
   uint64_t File5Size;
   ASSERT_NO_ERROR(fs::file_size(Twine(File5), File5Size));
   ASSERT_EQ(File5Size, 8000ULL);
   ASSERT_NO_ERROR(fs::remove(File5.str()));
+
+  // TEST 6: Create an empty file.
+  SmallString<128> File6(TestDirectory);
+  File6.append("/file6");
+  {
+    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+        FileOutputBuffer::create(File6, 0);
+    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
+    ASSERT_NO_ERROR(errorToErrorCode((*BufferOrErr)->commit()));
+  }
+  uint64_t File6Size;
+  ASSERT_NO_ERROR(fs::file_size(Twine(File6), File6Size));
+  ASSERT_EQ(File6Size, 0ULL);
+  ASSERT_NO_ERROR(fs::remove(File6.str()));
+
   // Clean up.
   ASSERT_NO_ERROR(fs::remove(TestDirectory.str()));
 }
 } // anonymous namespace