[lldb] Add SystemLogHandler for emitting log messages to the system log

Add a system log handler that emits log messages to the operating system
log. In addition to the log handler itself, this patch also introduces a
new Host::SystemLog helper function to abstract over writing to the
system log.

Differential revision: https://reviews.llvm.org/D128321

Author: JDevlieghere

lldb/include/lldb/Host/Host.h

@@ -13,6 +13,7 @@
 #include "lldb/Host/HostThread.h"
 #include "lldb/Utility/Environment.h"
 #include "lldb/Utility/FileSpec.h"
+#include "lldb/Utility/Log.h"
 #include "lldb/Utility/Timeout.h"
 #include "lldb/lldb-private-forward.h"
 #include "lldb/lldb-private.h"
@@ -86,6 +87,9 @@ class Host {
   StartMonitoringChildProcess(const MonitorChildProcessCallback &callback,
                               lldb::pid_t pid);
 
+  /// Emit the given message to the operating system log.
+  static void SystemLog(llvm::StringRef message);
+
   /// Get the process ID for the calling process.
   ///
   /// \return
@@ -252,6 +256,13 @@ class Host {
                                     ProcessInstanceInfoList &proc_infos);
 };
 
+/// Log handler that emits log messages to the operating system log.
+class SystemLogHandler : public LogHandler {
+public:
+  SystemLogHandler();
+  void Emit(llvm::StringRef message) override;
+};
+
 } // namespace lldb_private
 
 namespace llvm {

lldb/source/Host/common/Host.cpp

@@ -106,6 +106,10 @@ llvm::Expected<HostThread> Host::StartMonitoringChildProcess(
   });
 }
 
+#if !defined(__APPLE__)
+void Host::SystemLog(llvm::StringRef message) { llvm::errs() << message; }
+#endif
+
 #ifndef __linux__
 // Scoped class that will disable thread canceling when it is constructed, and
 // exception safely restore the previous value it when it goes out of scope.
@@ -627,3 +631,9 @@ uint32_t Host::FindProcesses(const ProcessInstanceInfoMatch &match_info,
 
   return result;
 }
+
+SystemLogHandler::SystemLogHandler() {}
+
+void SystemLogHandler::Emit(llvm::StringRef message) {
+  Host::SystemLog(message);
+}

lldb/source/Host/macosx/objcxx/Host.mm

@@ -82,6 +82,7 @@
 #include "../cfcpp/CFCString.h"
 
 #include <objc/objc-auto.h>
+#include <os/log.h>
 
 #include <CoreFoundation/CoreFoundation.h>
 #include <Foundation/Foundation.h>
@@ -98,6 +99,20 @@
 using namespace lldb;
 using namespace lldb_private;
 
+static os_log_t g_os_log;
+static std::once_flag g_os_log_once;
+
+void Host::SystemLog(llvm::StringRef message) {
+  if (__builtin_available(macos 10.12, iOS 10, tvOS 10, watchOS 3, *)) {
+    std::call_once(g_os_log_once, []() {
+      g_os_log = os_log_create("com.apple.dt.lldb", "lldb");
+    });
+    os_log(g_os_log, "%{public}s", message.str().c_str());
+  } else {
+    llvm::errs() << message;
+  }
+}
+
 bool Host::GetBundleDirectory(const FileSpec &file,
                               FileSpec &bundle_directory) {
 #if defined(__APPLE__)