Don't allow pwrite to resize a stream.

The current implementations could exhibit some behavior differences:

raw_fd_ostream: Whatever the underlying fd does with seek+write. In a normal
file, the write position would be back to the old offset.

raw_svector_ostream: The write position is always the end of the stream, so
after pwrite the write position would be the new end. This matches what OS_X
(all BSD?) do with a pwrite in a O_APPEND fd.

Given that we don't need that feature and don't use O_APPEND a lot in LLVM,
just disallow it.

I am open to suggestions on renaming pwrite to something else, but this fixes
the issue for now.

Thanks to Yaron Keren for reporting it.

llvm-svn: 235303

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/unittests/Support/raw_pwrite_stream_test.cpp b/llvm/unittests/Support/raw_pwrite_stream_test.cpp
index bcbb29b86bd..2792f44b605 100644
--- a/llvm/unittests/Support/raw_pwrite_stream_test.cpp
+++ b/llvm/unittests/Support/raw_pwrite_stream_test.cpp
@@ -16,10 +16,18 @@ using namespace llvm;
 namespace {
 
 TEST(raw_pwrite_ostreamTest, TestSVector) {
-  SmallString<64> Buffer;
+  SmallVector<char, 0> Buffer;
   raw_svector_ostream OS(Buffer);
+  OS << "abcd";
   StringRef Test = "test";
   OS.pwrite(Test.data(), Test.size(), 0);
   EXPECT_EQ(Test, OS.str());
+
+#ifdef GTEST_HAS_DEATH_TEST
+#ifndef NDEBUG
+  EXPECT_DEATH(OS.pwrite("12345", 5, 0),
+               "We don't support extending the stream");
+#endif
+#endif
 }
 }