[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 outstanding 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,
+              std::optional<MaskType> flags = std::nullopt,
+              uint32_t options = 0);
+
+  void Disable(std::optional<MaskType> flags = std::nullopt);
+
 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

@@ -17,6 +17,7 @@
 #include "lldb/Interpreter/CommandInterpreter.h"
 #include "lldb/Target/ProcessTrace.h"
 #include "lldb/Utility/Timer.h"
+#include "lldb/Version/Version.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/TargetSelect.h"
 
@@ -83,6 +84,8 @@ llvm::Error SystemInitializerFull::Initialize() {
   // Use the Debugger's LLDBAssert callback.
   SetLLDBAssertCallback(Debugger::AssertCallback);
 
+  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/source/Utility/Log.cpp

@@ -93,22 +93,28 @@ Log::MaskType Log::GetFlags(llvm::raw_ostream &stream,
 }
 
 void Log::Enable(const std::shared_ptr<LogHandler> &handler_sp,
-                 uint32_t options, Log::MaskType flags) {
+                 std::optional<Log::MaskType> flags, uint32_t options) {
   llvm::sys::ScopedWriter lock(m_mutex);
 
-  MaskType mask = m_mask.fetch_or(flags, std::memory_order_relaxed);
-  if (mask | flags) {
+  if (!flags)
+    flags = m_channel.default_flags;
+
+  MaskType mask = m_mask.fetch_or(*flags, std::memory_order_relaxed);
+  if (mask | *flags) {
     m_options.store(options, std::memory_order_relaxed);
     m_handler = handler_sp;
     m_channel.log_ptr.store(this, std::memory_order_relaxed);
   }
 }
 
-void Log::Disable(Log::MaskType flags) {
+void Log::Disable(std::optional<Log::MaskType> flags) {
   llvm::sys::ScopedWriter lock(m_mutex);
 
-  MaskType mask = m_mask.fetch_and(~flags, std::memory_order_relaxed);
-  if (!(mask & ~flags)) {
+  if (!flags)
+    flags = std::numeric_limits<MaskType>::max();
+
+  MaskType mask = m_mask.fetch_and(~(*flags), std::memory_order_relaxed);
+  if (!(mask & ~(*flags))) {
     m_handler.reset();
     m_channel.log_ptr.store(nullptr, std::memory_order_relaxed);
   }
@@ -230,10 +236,11 @@ bool Log::EnableLogChannel(const std::shared_ptr<LogHandler> &log_handler_sp,
     error_stream << llvm::formatv("Invalid log channel '{0}'.\n", channel);
     return false;
   }
-  MaskType flags = categories.empty()
-                       ? iter->second.m_channel.default_flags
-                       : GetFlags(error_stream, *iter, categories);
-  iter->second.Enable(log_handler_sp, log_options, flags);
+
+  auto flags = categories.empty() ? std::optional<MaskType>{}
+                                  : GetFlags(error_stream, *iter, categories);
+
+  iter->second.Enable(log_handler_sp, flags, log_options);
   return true;
 }
 
@@ -245,9 +252,10 @@ bool Log::DisableLogChannel(llvm::StringRef channel,
     error_stream << llvm::formatv("Invalid log channel '{0}'.\n", channel);
     return false;
   }
-  MaskType flags = categories.empty()
-                       ? std::numeric_limits<MaskType>::max()
-                       : GetFlags(error_stream, *iter, categories);
+
+  auto flags = categories.empty() ? std::optional<MaskType>{}
+                                  : GetFlags(error_stream, *iter, categories);
+
   iter->second.Disable(flags);
   return true;
 }

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'