[llvm] Add thread-local overrides for `llvm::errs()` and `llvm::outs()`. (PR #90374)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Chandler Carruth (chandlerc)

<details>
<summary>Changes</summary>

This allows library users of LLVM to intercept these streams and control where they write. While some components of LLVM already accept stream parameters to control this, not all do and so it seems useful to add this as a fallback. The motivating case is the verbose output in Clang's Driver library.

---
Full diff: https://github.com/llvm/llvm-project/pull/90374.diff


3 Files Affected:

- (modified) llvm/include/llvm/Support/raw_ostream.h (+24) 
- (modified) llvm/lib/Support/raw_ostream.cpp (+23) 
- (modified) llvm/unittests/Support/raw_fd_stream_test.cpp (+28) 


``````````diff
diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
index 696290a5d99cf5..f32400d9a1e0c5 100644
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -618,6 +618,30 @@ raw_fd_ostream &errs();
 /// This returns a reference to a raw_ostream which simply discards output.
 raw_ostream &nulls();
 
+/// Scoped thread-local override of `outs` and `errs`.
+///
+/// This can be used by libraries using LLVM that want to control where output
+/// is sent. While some systems explicitly accept a stream in their API, others
+/// directly use these and so we provide a scoped override as a fallback.
+///
+/// Note that these overrides rely on thread-local override, and so much be
+/// carefully instantiated on each thread where the override should be applied.
+class ScopedOutsAndErrsOverride {
+public:
+  // Install overrides for `outs` and `errs`. If the new stream is a null
+  // pointer it will cause that stream to use the system one as if no override
+  // is in place.
+  ScopedOutsAndErrsOverride(raw_fd_ostream *NewOuts, raw_fd_ostream *NewErrs);
+
+  // Restore any previous overrides when destroyed so that these overrides can
+  // be nested.
+  ~ScopedOutsAndErrsOverride();
+
+private:
+  raw_fd_ostream *PrevOuts;
+  raw_fd_ostream *PrevErrs;
+};
+
 //===----------------------------------------------------------------------===//
 // File Streams
 //===----------------------------------------------------------------------===//
diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index 8cb7b5ac68ea7e..1f8ac00cbe9881 100644
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -893,7 +893,14 @@ void raw_fd_ostream::anchor() {}
 //  outs(), errs(), nulls()
 //===----------------------------------------------------------------------===//
 
+static thread_local raw_fd_ostream *OutsOverride = nullptr;
+static thread_local raw_fd_ostream *ErrsOverride = nullptr;
+
 raw_fd_ostream &llvm::outs() {
+  if (auto *TLSOuts = OutsOverride; TLSOuts != nullptr) {
+    return *TLSOuts;
+  }
+  
   // Set buffer settings to model stdout behavior.
   std::error_code EC;
 #ifdef __MVS__
@@ -906,6 +913,10 @@ raw_fd_ostream &llvm::outs() {
 }
 
 raw_fd_ostream &llvm::errs() {
+  if (auto *TLSErrs = OutsOverride; TLSErrs != nullptr) {
+    return *TLSErrs;
+  }
+
   // Set standard error to be unbuffered and tied to outs() by default.
 #ifdef __MVS__
   std::error_code EC = enableAutoConversion(STDERR_FILENO);
@@ -921,6 +932,18 @@ raw_ostream &llvm::nulls() {
   return S;
 }
 
+ScopedOutsAndErrsOverride::ScopedOutsAndErrsOverride(raw_fd_ostream *NewOuts,
+                                                     raw_fd_ostream *NewErrs)
+    : PrevOuts(OutsOverride), PrevErrs(ErrsOverride) {
+  OutsOverride = NewOuts;
+  ErrsOverride = NewErrs;
+}
+
+ScopedOutsAndErrsOverride::~ScopedOutsAndErrsOverride() {
+  OutsOverride = PrevOuts;
+  ErrsOverride = PrevErrs;
+}
+
 //===----------------------------------------------------------------------===//
 // File Streams
 //===----------------------------------------------------------------------===//
diff --git a/llvm/unittests/Support/raw_fd_stream_test.cpp b/llvm/unittests/Support/raw_fd_stream_test.cpp
index 00d834da32101c..b7784f3b625ad4 100644
--- a/llvm/unittests/Support/raw_fd_stream_test.cpp
+++ b/llvm/unittests/Support/raw_fd_stream_test.cpp
@@ -64,4 +64,32 @@ TEST(raw_fd_streamTest, DynCast) {
   }
 }
 
+TEST(raw_fd_streamTest, OverrideOutsAndErrs) {
+  SmallString<64> Path;
+  int FD;
+  ASSERT_FALSE(sys::fs::createTemporaryFile("foo", "bar", FD, Path));
+  FileRemover Cleanup(Path);
+  std::error_code EC;
+  raw_fd_stream OS(Path, EC);
+  ASSERT_TRUE(!EC);
+
+  ScopedOutsAndErrsOverride Overrides(&OS, &OS);
+
+  // First test `outs`.
+  llvm::outs() << "outs";
+  llvm::outs().flush();
+  char Buffer[4];
+  OS.seek(0);
+  OS.read(Buffer, sizeof(Buffer));
+  EXPECT_EQ("outs", StringRef(Buffer, sizeof(Buffer)));
+
+  // Now test `errs`.
+  OS.seek(0);
+  llvm::errs() << "errs";
+  llvm::errs().flush();
+  OS.seek(0);
+  OS.read(Buffer, sizeof(Buffer));
+  EXPECT_EQ("errs", StringRef(Buffer, sizeof(Buffer)));
+}
+
 } // namespace

``````````

</details>


https://github.com/llvm/llvm-project/pull/90374