[Support] Add WriteThroughMemoryBuffer.

This is like MemoryBuffer (read-only) and WritableMemoryBuffer
(writable private), but where the underlying file can be modified
after writing.  This is useful when you want to open a file, make
some targeted edits, and then write it back out.

Differential Revision: https://reviews.llvm.org/D44230

llvm-svn: 327057

1 files changed, 29 insertions, 0 deletions

diff --git a/llvm/unittests/Support/MemoryBufferTest.cpp b/llvm/unittests/Support/MemoryBufferTest.cpp
index 64a7bb676f9..bafdffe752f 100644
--- a/llvm/unittests/Support/MemoryBufferTest.cpp
+++ b/llvm/unittests/Support/MemoryBufferTest.cpp
@@ -260,4 +260,33 @@ TEST_F(MemoryBufferTest, writableSlice) {
   for (size_t i = 0; i < MB.getBufferSize(); i += 0x10)
     EXPECT_EQ("0123456789abcdef", MB.getBuffer().substr(i, 0x10)) << "i: " << i;
 }
+
+TEST_F(MemoryBufferTest, writeThroughFile) {
+  // Create a file initialized with some data
+  int FD;
+  SmallString<64> TestPath;
+  sys::fs::createTemporaryFile("MemoryBufferTest_WriteThrough", "temp", FD,
+                               TestPath);
+  FileRemover Cleanup(TestPath);
+  raw_fd_ostream OF(FD, true);
+  OF << "0123456789abcdef";
+  OF.close();
+  {
+    auto MBOrError = WriteThroughMemoryBuffer::getFile(TestPath);
+    ASSERT_FALSE(MBOrError.getError());
+    // Write some data.  It should be mapped readwrite, so that upon completion
+    // the original file contents are modified.
+    WriteThroughMemoryBuffer &MB = **MBOrError;
+    ASSERT_EQ(16, MB.getBufferSize());
+    char *Start = MB.getBufferStart();
+    ASSERT_EQ(MB.getBufferEnd(), MB.getBufferStart() + MB.getBufferSize());
+    ::memset(Start, 'x', MB.getBufferSize());
+  }
+
+  auto MBOrError = MemoryBuffer::getFile(TestPath);
+  ASSERT_FALSE(MBOrError.getError());
+  auto &MB = **MBOrError;
+  ASSERT_EQ(16, MB.getBufferSize());
+  EXPECT_EQ("xxxxxxxxxxxxxxxx", MB.getBuffer());
+}
 }