[ELF] Write the output to lld::outs() when -o is - (#209064)

When the output file is "-", write the image to lld::outs() (the
stdoutOS
argument of lld::elf::link()) instead of committing the
FileOutputBuffer,
which writes to the process's stdout. This lets lld used as a library
capture
the output in a raw_ostream without an open syscall.

This reimplements the stale #72061 and adds a unittest.

lld/ELF/Writer.cpp

@@ -364,7 +364,14 @@ template <class ELFT> void Writer<ELFT>::run() {
     if (errCount(ctx))
       return;
 
-    if (!ctx.e.disableOutput) {
+    // With -o -, write to lld::outs() (the stdoutOS argument of
+    // link()) instead of committing the buffer, which would write to the
+    // process's stdout.
+    if (ctx.arg.outputFile == "-") {
+      ctx.e.outs() << StringRef(
+          reinterpret_cast<const char *>(buffer->getBufferStart()),
+          buffer->getBufferSize());
+    } else if (!ctx.e.disableOutput) {
       if (auto e = buffer->commit())
         Err(ctx) << "failed to write output '" << buffer->getPath()
                  << "': " << std::move(e);

lld/unittests/AsLibELF/CMakeLists.txt

@@ -3,6 +3,7 @@
 # target application executable.
 
 add_lld_unittests(LLDAsLibELFTests
+  OutputStream.cpp
   ROCm.cpp
   SomeDrivers.cpp
 )

lld/unittests/AsLibELF/OutputStream.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// This file is licensed under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_DEFAULT_LD_LLD_IS_MINGW
+
+#include "lld/Common/Driver.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gmock/gmock.h"
+
+LLD_HAS_DRIVER(elf)
+
+// With "-o -", the ELF driver writes the output image to the stdoutOS stream
+// passed to link(), so lld used as a library can capture it in a raw_ostream.
+TEST(AsLib, OutputToStream) {
+  llvm::SmallString<256> input(getenv("LLD_SRC_DIR"));
+  llvm::sys::path::append(input, "unittests", "AsLibELF", "Inputs",
+                          "kernel1.o");
+  std::vector<const char *> args{"ld.lld", "-shared", input.c_str(), "-o", "-"};
+
+  std::string buf;
+  llvm::raw_string_ostream os(buf);
+  lld::Result s =
+      lld::lldMain(args, os, llvm::errs(), {{lld::Gnu, &lld::elf::link}});
+  EXPECT_EQ(s.retCode, 0);
+  EXPECT_TRUE(s.canRunAgain);
+  EXPECT_TRUE(llvm::StringRef(buf).starts_with("\177ELF"));
+}
+#endif