[lldb] Introduce an always-on system log category/channel

Add an "always on" log category and channel. Unlike other, existing log
channels, it is not exposed to users. The channel is meant to be used
sparsely and deliberately for logging high-value information to the
system log.

We have a similar concept in the downstream Swift fork and this has
proven to be extremely valuable. This is especially true on macOS where
system log messages are automatically captured as part of a sysdiagnose.

This patch revives https://reviews.llvm.org/D135621 although the purpose
is slightly different (logging to the system log rather than to a ring
buffer dumped as part of the diagnostics). This patch addresses all the
remaining oustanding comments.

Author: JDevlieghere

lldb/include/lldb/Host/Host.h

@@ -31,6 +31,25 @@ class ProcessInstanceInfo;
 class ProcessInstanceInfoMatch;
 typedef std::vector<ProcessInstanceInfo> ProcessInstanceInfoList;
 
+// System log category and channel. This log channel is always enabled and
+// therefore is supposed to be used sparsely. Use this log channel to log
+// critical information that is expected to be relevant to the majority of bug
+// reports.
+enum class SystemLog : Log::MaskType {
+  System = Log::ChannelFlag<0>,
+  LLVM_MARK_AS_BITMASK_ENUM(System)
+};
+
+LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
+
+class LogChannelSystem {
+public:
+  static void Initialize();
+  static void Terminate();
+};
+
+template <> Log::Channel &LogChannelFor<SystemLog>();
+
 // Exit Type for inferior processes
 struct WaitStatus {
   enum Type : uint8_t {

lldb/include/lldb/Utility/Log.h

@@ -272,6 +272,12 @@ class Log final {
   void VAFormatf(llvm::StringRef file, llvm::StringRef function,
                  const char *format, va_list args);
 
+  void Enable(const std::shared_ptr<LogHandler> &handler_sp,
+              uint32_t options = 0,
+              MaskType flags = std::numeric_limits<Log::MaskType>::max());
+
+  void Disable(MaskType flags = 0);
+
 private:
   Channel &m_channel;
 
@@ -297,11 +303,6 @@ class Log final {
     return m_handler;
   }
 
-  void Enable(const std::shared_ptr<LogHandler> &handler_sp, uint32_t options,
-              MaskType flags);
-
-  void Disable(MaskType flags);
-
   bool Dump(llvm::raw_ostream &stream);
 
   typedef llvm::StringMap<Log> ChannelMap;

lldb/source/API/SystemInitializerFull.cpp

@@ -20,6 +20,8 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/TargetSelect.h"
 
+#include "lldb/Version/Version.h"
+
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wglobal-constructors"
 #include "llvm/ExecutionEngine/MCJIT.h"
@@ -83,6 +85,9 @@ llvm::Error SystemInitializerFull::Initialize() {
   // Use the Debugger's LLDBAssert callback.
   SetLLDBAssertCallback(Debugger::AssertCallback);
 
+  // Log the LLDB version to the system log.
+  LLDB_LOG(GetLog(SystemLog::System), "{0}", GetVersion());
+
   return llvm::Error::success();
 }
 

lldb/source/Host/common/Host.cpp

@@ -125,6 +125,22 @@ void Host::SystemLog(Severity severity, llvm::StringRef message) {
 #endif
 #endif
 
+static constexpr Log::Category g_categories[] = {
+    {{"system"}, {"system log"}, SystemLog::System}};
+
+static Log::Channel g_system_channel(g_categories, SystemLog::System);
+static Log g_system_log(g_system_channel);
+
+template <> Log::Channel &lldb_private::LogChannelFor<SystemLog>() {
+  return g_system_channel;
+}
+
+void LogChannelSystem::Initialize() {
+  g_system_log.Enable(std::make_shared<SystemLogHandler>());
+}
+
+void LogChannelSystem::Terminate() { g_system_log.Disable(); }
+
 #if !defined(__APPLE__) && !defined(_WIN32)
 static thread_result_t
 MonitorChildProcessThreadFunction(::pid_t pid,

lldb/source/Host/common/HostInfoBase.cpp

@@ -76,9 +76,11 @@ static HostInfoBase::SharedLibraryDirectoryHelper *g_shlib_dir_helper = nullptr;
 void HostInfoBase::Initialize(SharedLibraryDirectoryHelper *helper) {
   g_shlib_dir_helper = helper;
   g_fields = new HostInfoBaseFields();
+  LogChannelSystem::Initialize();
 }
 
 void HostInfoBase::Terminate() {
+  LogChannelSystem::Terminate();
   g_shlib_dir_helper = nullptr;
   delete g_fields;
   g_fields = nullptr;

lldb/test/Shell/Host/TestSytemLogChannel.test

@@ -0,0 +1,3 @@
+RUN: %lldb -o 'log list' -o 'log disable system' 2>&1 | FileCheck %s
+CHECK-NOT: Logging categories for 'system'
+CHECK: Invalid log channel 'system'