1 files changed, 108 insertions, 0 deletions
diff --git a/synapse/util/logcontext.py b/synapse/util/logcontext.py
new file mode 100644
index 0000000000..13176b05ce
--- /dev/null
+++ b/synapse/util/logcontext.py
@@ -0,0 +1,108 @@
+import threading
+import logging
+
+
+class LoggingContext(object):
+    """Additional context for log formatting. Contexts are scoped within a
+    "with" block. Contexts inherit the state of their parent contexts.
+    Args:
+        name (str): Name for the context for debugging.
+    """
+
+    __slots__ = ["parent_context", "name", "__dict__"]
+
+    thread_local = threading.local()
+
+    class Sentinel(object):
+        """Sentinel to represent the root context"""
+
+        __slots__ = []
+
+        def copy_to(self, record):
+            pass
+
+    sentinel = Sentinel()
+
+    def __init__(self, name=None):
+        self.parent_context = None
+        self.name = name
+
+    def __str__(self):
+        return "%s@%x" % (self.name, id(self))
+
+    @classmethod
+    def current_context(cls):
+        """Get the current logging context from thread local storage"""
+        return getattr(cls.thread_local, "current_context", cls.sentinel)
+
+    def __enter__(self):
+        """Enters this logging context into thread local storage"""
+        if self.parent_context is not None:
+            raise Exception("Attempt to enter logging context multiple times")
+        self.parent_context = self.current_context()
+        self.thread_local.current_context = self
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """Restore the logging context in thread local storage to the state it
+        was before this context was entered.
+        Returns:
+            None to avoid suppressing any exeptions that were thrown.
+        """
+        if self.thread_local.current_context is not self:
+            logging.error(
+                "Current logging context %s is not the expected context %s",
+                self.thread_local.current_context,
+                self
+            )
+        self.thread_local.current_context = self.parent_context
+        self.parent_context = None
+
+    def __getattr__(self, name):
+        """Delegate member lookup to parent context"""
+        return getattr(self.parent_context, name)
+
+    def copy_to(self, record):
+        """Copy fields from this context and its parents to the record"""
+        if self.parent_context is not None:
+            self.parent_context.copy_to(record)
+        for key, value in self.__dict__.items():
+            setattr(record, key, value)
+
+
+class LoggingContextFilter(logging.Filter):
+    """Logging filter that adds values from the current logging context to each
+    record.
+    Args:
+        **defaults: Default values to avoid formatters complaining about
+            missing fields
+    """
+    def __init__(self, **defaults):
+        self.defaults = defaults
+
+    def filter(self, record):
+        """Add each fields from the logging contexts to the record.
+        Returns:
+            True to include the record in the log output.
+        """
+        context = LoggingContext.current_context()
+        for key, value in self.defaults.items():
+            setattr(record, key, value)
+        context.copy_to(record)
+        return True
+
+
+class PreserveLoggingContext(object):
+    """Captures the current logging context and restores it when the scope is
+    exited. Used to restore the context after a function using
+    @defer.inlineCallbacks is resumed by a callback from the reactor."""
+
+    __slots__ = ["current_context"]
+
+    def __enter__(self):
+        """Captures the current logging context"""
+        self.current_context = LoggingContext.current_context()
+
+    def __exit__(self, type, value, traceback):
+        """Restores the current logging context"""
+        LoggingContext.thread_local.current_context = self.current_context