[clangd] Output log messages to stderr when not configured (e.g. in tests). NFC

llvm-svn: 322827

2 files changed, 12 insertions, 5 deletions

diff --git a/clang-tools-extra/clangd/Logger.cpp b/clang-tools-extra/clangd/Logger.cpp
index 2c0c6f9974a..40a90504de2 100644
--- a/clang-tools-extra/clangd/Logger.cpp
+++ b/clang-tools-extra/clangd/Logger.cpp
@@ -8,6 +8,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "Logger.h"
+#include "llvm/Support/raw_ostream.h"
+#include <mutex>
 
 namespace clang {
 namespace clangd {
@@ -24,9 +26,13 @@ LoggingSession::LoggingSession(clangd::Logger &Instance) {
 LoggingSession::~LoggingSession() { L = nullptr; }
 
 void log(const Context &Ctx, const llvm::Twine &Message) {
-  if (!L)
-    return;
-  L->log(Ctx, Message);
+  if (L)
+    L->log(Ctx, Message);
+  else {
+    static std::mutex Mu;
+    std::lock_guard<std::mutex> Guard(Mu);
+    llvm::errs() << Message << "\n";
+  }
 }
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/Logger.h b/clang-tools-extra/clangd/Logger.h
index c856162b4b5..b6d7a1b4caf 100644
--- a/clang-tools-extra/clangd/Logger.h
+++ b/clang-tools-extra/clangd/Logger.h
@@ -16,8 +16,9 @@
 namespace clang {
 namespace clangd {
 
-/// Main logging function. Logs messages to a global logger, which can be set up
-/// by LoggingSesssion.
+/// Main logging function.
+/// Logs messages to a global logger, which can be set up by LoggingSesssion.
+/// If no logger is registered, writes to llvm::errs().
 void log(const Context &Ctx, const llvm::Twine &Message);
 
 /// Interface to allow custom logging in clangd.