[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 +31,22 @@ class ProcessInstanceInfo;
 class ProcessInstanceInfoMatch;
 typedef std::vector<ProcessInstanceInfo> ProcessInstanceInfoList;
 
+// Always on system log category and channel.
+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/LLDBLog.h

@@ -49,6 +49,7 @@ enum class LLDBLog : Log::MaskType {
   Watchpoints = Log::ChannelFlag<30>,
   OnDemand = Log::ChannelFlag<31>,
   Source = Log::ChannelFlag<32>,
+  AlwaysOn = Log::ChannelFlag<32>,
   LLVM_MARK_AS_BITMASK_ENUM(OnDemand),
 };
 

lldb/include/lldb/Utility/Log.h

@@ -165,13 +165,14 @@ class Log final {
     friend class Log;
 
   public:
+    const bool internal = false;
     const llvm::ArrayRef<Category> categories;
     const MaskType default_flags;
 
     template <typename Cat>
     constexpr Channel(llvm::ArrayRef<Log::Category> categories,
-                      Cat default_flags)
-        : log_ptr(nullptr), categories(categories),
+                      Cat default_flags, bool internal = false)
+        : log_ptr(nullptr), internal(internal), categories(categories),
           default_flags(MaskType(default_flags)) {
       static_assert(
           std::is_same<Log::MaskType, std::underlying_type_t<Cat>>::value);

lldb/source/Host/common/Host.cpp

@@ -125,6 +125,25 @@ 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_channel(g_categories, SystemLog::System,
+                              /*internal=*/true);
+
+template <> Log::Channel &lldb_private::LogChannelFor<SystemLog>() {
+  return g_channel;
+}
+
+void LogChannelSystem::Initialize() {
+  Log::Register("system", g_channel);
+  const uint32_t log_options = LLDB_LOG_OPTION_PREPEND_THREAD_NAME;
+  Log::EnableLogChannel(std::make_shared<SystemLogHandler>(), log_options,
+                        "system", {"all"}, llvm::nulls());
+}
+
+void LogChannelSystem::Terminate() { Log::Unregister("system"); }
+
 #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

@@ -245,6 +245,11 @@ bool Log::DisableLogChannel(llvm::StringRef channel,
     error_stream << llvm::formatv("Invalid log channel '{0}'.\n", channel);
     return false;
   }
+  if (iter->second.m_channel.internal) {
+    error_stream << llvm::formatv(
+        "Cannot disable internal log channel '{0}'.\n", channel);
+    return false;
+  }
   MaskType flags = categories.empty()
                        ? std::numeric_limits<MaskType>::max()
                        : GetFlags(error_stream, *iter, categories);
@@ -296,8 +301,10 @@ void Log::ForEachChannelCategory(
 
 std::vector<llvm::StringRef> Log::ListChannels() {
   std::vector<llvm::StringRef> result;
-  for (const auto &channel : *g_channel_map)
-    result.push_back(channel.first());
+  for (const auto &entry : *g_channel_map) {
+    if (!entry.second.m_channel.internal)
+      result.push_back(entry.first());
+  }
   return result;
 }
 
@@ -307,8 +314,10 @@ void Log::ListAllLogChannels(llvm::raw_ostream &stream) {
     return;
   }
 
-  for (const auto &channel : *g_channel_map)
-    ListCategories(stream, channel);
+  for (const auto &entry : *g_channel_map) {
+    if (!entry.second.m_channel.internal)
+      ListCategories(stream, entry);
+  }
 }
 
 bool Log::GetVerbose() const {

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: Cannot disable internal log channel 'system'.