Logging: enable configuration via ENV

This restores the functionality we had previously to set JULIA_DEBUG_LOADING=1 to activate logging;
now the equivalent would be to set JULIA_DEBUG=loading. But with the new infrastructure, this is also
generalized to allow filtering on any key (module, root-module, or filename), or "all".
This sits between the global disable (which still provides a fast-path to skip everything),
and the default min-level for a logger (which will be overridden by the environment variable).

This also tries to move a bit more work into non-inlined helper functions,
rather than inlining that code directly, to slightly reduce the size of
the emitted code.

As a possible future improvement, we could allow specifying specific
levels (such as `loading=MaxLevel` or `all=Debug`), to provide even more
fine-grained control.

fix #25549

base/logging.jl

@@ -298,12 +298,11 @@ function logmsg_code(_module, file, line, level, message, exs...)
  level = $level
  std_level = convert(LogLevel, level)
  if std_level >= _min_enabled_level[]
- logstate = current_logstate()
- if std_level >= logstate.min_enabled_level
-  logger = logstate.logger
-  _module = $_module
+ group = $group
+ _module = $_module
+ logger = current_logger_for_env(std_level, group, _module)
+ if !(logger === nothing)
  id = $id
- group = $group
  # Second chance at an early bail-out, based on arbitrary
  # logger-specific logic.
  if shouldlog(logger, level, _module, group, id)
@@ -313,9 +312,6 @@ function logmsg_code(_module, file, line, level, message, exs...)
  msg = $(esc(message))
  handle_message(logger, level, msg, _module, group, id, file, line; $(kwargs...))
  catch err
- if !catch_exceptions(logger)
- rethrow(err)
- end
  logging_error(logger, level, _module, group, id, file, line, err)
  end
  end
@@ -327,7 +323,10 @@ end
 
 # Report an error in log message creation (or in the logger itself).
 @noinline function logging_error(logger, level, _module, group, id,
- filepath, line, err)
+ filepath, line, @nospecialize(err))
+ if !catch_exceptions(logger)
+ rethrow(err)
+ end
  try
  msg = "Exception while generating log record in module $_module at $filepath:$line"
  handle_message(logger, Error, msg, _module, :logevent_error, id, filepath, line; exception=(err,catch_backtrace()))
@@ -371,7 +370,16 @@ _global_logstate = LogState(NullLogger())
 
 function current_logstate()
  logstate = current_task().logstate
- (logstate != nothing ? logstate : _global_logstate)::LogState
+ return (logstate !== nothing ? logstate : _global_logstate)::LogState
+end
+
+# helper function to get the current logger, if enabled for the specified message type
+@noinline function current_logger_for_env(std_level::LogLevel, group, _module::Module)
+ logstate = current_logstate()
+ if std_level >= logstate.min_enabled_level || env_override_minlevel(group, _module)
+ return logstate.logger
+ end
+ return nothing
 end
 
 function with_logstate(f::Function, logstate)
@@ -400,6 +408,42 @@ function disable_logging(level::LogLevel)
  _min_enabled_level[] = level + 1
 end
 
+let _debug_groups = Symbol[],
+ _debug_str::String = ""
+global function env_override_minlevel(group, _module::Module)
+ debug = get(ENV, "JULIA_DEBUG", "")
+ if !(debug === _debug_str)
+ _debug_str = debug
+ empty!(_debug_groups)
+ for g in split(debug, ',')
+ isempty(g) && continue
+ if g == "all"
+ empty!(_debug_groups)
+ push!(_debug_groups, :all)
+ break
+ end
+ push!(_debug_groups, Symbol(g))
+ end
+ end
+ if isempty(_debug_groups)
+ return false
+ end
+ if _debug_groups[1] == :all
+ return true
+ end
+ if isa(group, Symbol) && group in _debug_groups
+ return true
+ end
+ if nameof(_module) in _debug_groups
+ return true
+ end
+ if nameof(Base.moduleroot(_module)) in _debug_groups
+ return true
+ end
+ return false
+end
+end
+
 
 """
  global_logger()
@@ -485,7 +529,7 @@ function handle_message(logger::SimpleLogger, level, message, _module, group, id
  for i in 2:length(msglines)
  println(iob, "│ ", msglines[i])
  end
- for (key,val) in pairs(kwargs)
+ for (key, val) in kwargs
  println(iob, "│ ", key, " = ", val)
  end
  println(iob, "└ @ ", _module, " ", filepath, ":", line)