[Support] Treat Windows "nul" as the null device in writeToOutput

[wenju-he]

llvm-objcopy (via writeToOutput) only special-cased "/dev/null", so on Windows an output path of "nul" fell through to TempFile::create() + rename. Windows reserves "nul" as a device name, so the rename fails with "permission denied". This broke clang-offload-bundler's SYCL fat object step when the driver is invoked with -o nul in downstream test https://github.com/intel/llvm/blob/sycl/sycl/test/include_deps/header_reach.cpp

Assisted by: Claude

[github-actions]

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You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions cpp -- llvm/lib/Support/raw_ostream.cpp llvm/unittests/Support/raw_ostream_test.cpp --diff_from_common_commit

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View the diff from clang-format here.

diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
index ed059faec..c7cb9e340 100644
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -636,8 +636,9 @@ TEST(raw_ostreamTest, writeToNul) {
       writeToOutput("nul",
                     [&](raw_ostream &Out) -> Error {
                       std::error_code EC;
-                      sys::fs::directory_iterator It(
-                          RootTestDirectory.path(), EC), End;
+                      sys::fs::directory_iterator It(RootTestDirectory.path(),
+                                                     EC),
+                          End;
                       EXPECT_FALSE(EC);
                       EXPECT_EQ(It, End)
                           << "unexpected temp file on disk during Write(): "

[llvmorg-github-actions]

@llvm/pr-subscribers-llvm-support

Author: Wenju He (wenju-he)

Changes

llvm-objcopy (via writeToOutput) only special-cased "/dev/null", so on Windows an output path of "nul" fell through to TempFile::create() + rename. Windows reserves "nul" as a device name, so the rename fails with "permission denied". This broke clang-offload-bundler's SYCL fat object step when the driver is invoked with -o nul in downstream test https://github.com/intel/llvm/blob/sycl/sycl/test/include_deps/header_reach.cpp

Assisted by: Claude

---

Full diff: https://github.com/llvm/llvm-project/pull/208179.diff

2 Files Affected:

• (modified) llvm/lib/Support/raw_ostream.cpp (+9-1)
• (modified) llvm/unittests/Support/raw_ostream_test.cpp (+29)

diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index eebc4db5ac878..8bc00dc07d522 100644
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -993,12 +993,20 @@ void buffer_ostream::anchor() {}
 
 void buffer_unique_ostream::anchor() {}
 
+static bool isNullDeviceName(StringRef OutputFileName) {
+#ifdef _WIN32
+  if (OutputFileName.equals_insensitive("nul"))
+    return true;
+#endif
+  return OutputFileName == "/dev/null";
+}
+
 Error llvm::writeToOutput(StringRef OutputFileName,
                           std::function<Error(raw_ostream &)> Write) {
   if (OutputFileName == "-")
     return Write(outs());
 
-  if (OutputFileName == "/dev/null") {
+  if (isNullDeviceName(OutputFileName)) {
     raw_null_ostream Out;
     return Write(Out);
   }
diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
index ed04721816476..c7cb9e340dbcf 100644
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -623,6 +623,35 @@ TEST(raw_ostreamTest, writeToDevNull) {
   EXPECT_TRUE(DevNullIsUsed);
 }
 
+#ifdef _WIN32
+TEST(raw_ostreamTest, writeToNul) {
+  // raw_null_ostream never touches the filesystem, so no "nul.temp-stream-*"
+  // file should exist in the CWD while Write() runs.
+  llvm::unittest::TempDir RootTestDirectory("writeToNul", /*Unique=*/true);
+  SmallString<128> SavedCWD;
+  ASSERT_FALSE(sys::fs::current_path(SavedCWD));
+  ASSERT_FALSE(sys::fs::set_current_path(RootTestDirectory.path()));
+
+  EXPECT_THAT_ERROR(
+      writeToOutput("nul",
+                    [&](raw_ostream &Out) -> Error {
+                      std::error_code EC;
+                      sys::fs::directory_iterator It(RootTestDirectory.path(),
+                                                     EC),
+                          End;
+                      EXPECT_FALSE(EC);
+                      EXPECT_EQ(It, End)
+                          << "unexpected temp file on disk during Write(): "
+                          << It->path();
+                      Out << "HelloWorld";
+                      return Error::success();
+                    }),
+      Succeeded());
+
+  ASSERT_FALSE(sys::fs::set_current_path(SavedCWD));
+}
+#endif
+
 TEST(raw_ostreamTest, nullStreamZeroBufferSize) {
   raw_ostream &NullStream = nulls();
   EXPECT_EQ(NullStream.GetBufferSize(), 0u);