Issue #131: maintain a thread-local logger stack (RC fix)

Originally `byexample` used a global stack of loggers to support
contextual logging.

This works fine when we work with subprocesses because each one will
have its own stack. But with threads, all of them will see the same
shared stack.

This RC will confuse the logging system: while one thread enter in
`'byexample.exec'` context it may end logging as `'byexample.init'` because
other thread entered in that context.

Using a thread-local logger stack fixes the RC. Now the
`init_thread_specific_log_system` function needs to be called at the begin
of each thread to initialize it.

byexample/jobs.py

@@ -3,7 +3,7 @@
 from queue import Queue
 
 import signal, contextlib
-from .log import clog, CHAT
+from .log import clog, CHAT, init_thread_specific_log_system
 from .init import init_worker
 
 
@@ -23,6 +23,7 @@ def worker(func, input, output, cfg):
 
         After receiving a None, close the <output> queue.
         '''
+    init_thread_specific_log_system()
     harvester, executor = init_worker(cfg)
     for item in iter(input.get, None):
         output.put(func(item, harvester, executor, cfg['dry']))

byexample/log.py

@@ -4,7 +4,7 @@
 
 from .common import colored, highlight_syntax, indent
 from .log_level import TRACE, DEBUG, CHAT, INFO, NOTE, WARNING, ERROR, CRITICAL
-import functools
+import functools, threading
 
 
 class XFormatter(Formatter):
@@ -184,7 +184,30 @@ def exception(self, msg, *args, **kwargs):
             )
 
 
-_logger_stack = []
+class _LoggerLocalStack(threading.local):
+    def __init__(self):
+        self.stack = []
+
+    def __len__(self):
+        return len(self.stack)
+
+    def append(self, item):
+        return self.stack.append(item)
+
+    def pop(self):
+        return self.stack.pop()
+
+    def __getitem__(self, ix):
+        return self.stack[ix]
+
+    def __setitem__(self, ix, v):
+        self.stack[ix] = v
+
+    def __delitem__(self, ix):
+        del self.stack[ix]
+
+
+_logger_stack = _LoggerLocalStack()
 
 
 def clog():
@@ -286,10 +309,12 @@ def init_log_system(level=NOTE, use_colors=False):
     rlog.xstream_handler = ch
     rlog.bg_queue_listener = ql
 
-    # Set up the global logger.
+    # Set up the global logger (for this thread).
     # Activate and deactivate sub loggers using log_context
     # decorator on the top level functions
-    _logger_stack.append(rlog)
+    #
+    # Other threads will have to call this function too
+    init_thread_specific_log_system()
 
     assert level is not None
     assert use_colors is not None
@@ -300,6 +325,13 @@ def init_log_system(level=NOTE, use_colors=False):
     rlog.bg_queue_listener.start()
 
 
+def init_thread_specific_log_system():
+    global _logger_stack
+
+    rlog = getLogger(name='byexample')  # root
+    _logger_stack.append(rlog)
+
+
 def configure_log_system(default_level=None, use_colors=None, concerns=None):
     rlog = getLogger(name='byexample')  # root
     if default_level is not None: