1 files changed, 50 insertions, 0 deletions
diff --git a/llvm/unittests/Support/FileOutputBufferTest.cpp b/llvm/unittests/Support/FileOutputBufferTest.cpp
index e7f1fd765bd..16fe01dd0b0 100644
--- a/llvm/unittests/Support/FileOutputBufferTest.cpp
+++ b/llvm/unittests/Support/FileOutputBufferTest.cpp
@@ -11,6 +11,7 @@
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
@@ -121,4 +122,53 @@ TEST(FileOutputBuffer, Test) {
   // Clean up.
   ASSERT_NO_ERROR(fs::remove(TestDirectory.str()));
 }
+
+TEST(FileOutputBuffer, TestModify) {
+  // Create unique temporary directory for these tests
+  SmallString<128> TestDirectory;
+  {
+    ASSERT_NO_ERROR(
+      fs::createUniqueDirectory("FileOutputBuffer-modify", TestDirectory));
+  }
+
+  SmallString<128> File1(TestDirectory);
+  File1.append("/file");
+  // First write some data.
+  {
+    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+      FileOutputBuffer::create(File1, 10);
+    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
+    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
+    memcpy(Buffer->getBufferStart(), "AAAAAAAAAA", 10);
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+  }
+
+  // Then re-open the file for modify and change only some bytes.
+  {
+    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+        FileOutputBuffer::create(File1, size_t(-1), FileOutputBuffer::F_modify);
+    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
+    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
+    ASSERT_EQ(10, Buffer->getBufferSize());
+    uint8_t *Data = Buffer->getBufferStart();
+    Data[0] = 'X';
+    Data[9] = 'X';
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+  }
+
+  // Finally, re-open the file for read and verify that it has the modified
+  // contents.
+  {
+    ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr = MemoryBuffer::getFile(File1);
+    ASSERT_NO_ERROR(BufferOrErr.getError());
+    std::unique_ptr<MemoryBuffer> Buffer = std::move(*BufferOrErr);
+    ASSERT_EQ(10, Buffer->getBufferSize());
+    EXPECT_EQ(StringRef("XAAAAAAAAX"), Buffer->getBuffer());
+  }
+
+  // Clean up.
+  ASSERT_NO_ERROR(fs::remove(File1));
+  ASSERT_NO_ERROR(fs::remove(TestDirectory));
+}
+
 } // anonymous namespace