Fix crash when multiple raw_fd_ostreams to stdout are created.

If raw_fd_ostream is constructed with the path of "-", it claims
ownership of the stdout file descriptor. This means that it closes
stdout when it is destroyed. If there are multiple users of
raw_fd_ostream wrapped around stdout, then a crash can occur because
of operations on a closed stream.

An example of this would be running something like "clang -S -o - -MD
-MF - test.cpp". Alternatively, using outs() (which creates a local
version of raw_fd_stream to stdout) anywhere combined with such a
stream usage would cause the crash.

The fix duplicates the stdout file descriptor when used within
raw_fd_ostream, so that only that particular descriptor is closed when
the stream is destroyed.

Patch by James Henderson!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297624 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/raw_ostream.cpp

@@ -473,7 +473,7 @@ static int getFD(StringRef Filename, std::error_code &EC,
     // possible.
     if (!(Flags & sys::fs::F_Text))
       sys::ChangeStdoutToBinary();
-    return STDOUT_FILENO;
+    return dup(STDOUT_FILENO);
   }
 
   int FD;

test/Other/writing-to-stdout.ll

@@ -0,0 +1,14 @@
+; Often LLVM tools use "-" to indicate that output should be written to stdout
+; instead of a file. This behaviour is implemented by the raw_fd_ostream class.
+; This test verifies that when doing so multiple times we don't try to access a
+; closed STDOUT_FILENO. The exact options used in this test are unimportant, as
+; long as they write to stdout using raw_fd_ostream.
+; RUN: llc %s -o=- -pass-remarks-output=- -filetype=asm | FileCheck %s
+; foobar should appear as a function somewhere in the assembly file.
+; CHECK: foobar
+; !Analysis appears at the start of pass-remarks-output.
+; CHECK: !Analysis
+
+define void @foobar() {
+  ret void
+}

unittests/Support/raw_ostream_test.cpp

@@ -9,6 +9,7 @@
 
 #include "gtest/gtest.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -330,4 +331,11 @@ TEST(raw_ostreamTest, FormattedHexBytes) {
             "0007: 68 69 6a 6b 6c        |hijkl|",
             format_bytes_with_ascii_str(B.take_front(12), 0, 7, 1));
 }
+
+TEST(raw_fd_ostreamTest, multiple_raw_fd_ostream_to_stdout) {
+  std::error_code EC;
+
+  { raw_fd_ostream("-", EC, sys::fs::OpenFlags::F_None); }
+  { raw_fd_ostream("-", EC, sys::fs::OpenFlags::F_None); }
+}
 }