From 7ecfe8b1a86d1f6a11dba283ee1468d516b0a6c2 Mon Sep 17 00:00:00 2001 From: Eric Eastwood Date: Fri, 12 Sep 2025 09:29:35 -0500 Subject: [PATCH] Better explain which context the task is run in when using `run_in_background(...)` or `run_as_background_process(...)` (#18906) Follow-up to https://github.com/element-hq/synapse/pull/18900 --- changelog.d/18906.misc | 1 + synapse/logging/context.py | 6 ++++-- synapse/metrics/background_process_metrics.py | 8 +++++--- 3 files changed, 10 insertions(+), 5 deletions(-) create mode 100644 changelog.d/18906.misc diff --git a/changelog.d/18906.misc b/changelog.d/18906.misc new file mode 100644 index 000000000..d7d8b47eb --- /dev/null +++ b/changelog.d/18906.misc @@ -0,0 +1 @@ +Better explain how we manage the logcontext in `run_in_background(...)` and `run_as_background_process(...)`. diff --git a/synapse/logging/context.py b/synapse/logging/context.py index 6eaa19d2f..aa4b98e7c 100644 --- a/synapse/logging/context.py +++ b/synapse/logging/context.py @@ -802,8 +802,9 @@ def run_in_background( deferred returned by the function completes. To explain how the log contexts work here: - - When this function is called, the current context is stored ("original"), we kick - off the background task, and we restore that original context before returning + - When `run_in_background` is called, the current context is stored ("original"), + we kick off the background task in the current context, and we restore that + original context before returning - When the background task finishes, we don't want to leak our context into the reactor which would erroneously get attached to the next operation picked up by the event loop. We add a callback to the deferred which will clear the logging @@ -828,6 +829,7 @@ def run_in_background( """ calling_context = current_context() try: + # (kick off the task in the current context) res = f(*args, **kwargs) except Exception: # the assumption here is that the caller doesn't want to be disturbed diff --git a/synapse/metrics/background_process_metrics.py b/synapse/metrics/background_process_metrics.py index c6ee21d42..633705b02 100644 --- a/synapse/metrics/background_process_metrics.py +++ b/synapse/metrics/background_process_metrics.py @@ -286,9 +286,11 @@ def run_as_background_process( ).dec() # To explain how the log contexts work here: - # - When this function is called, the current context is stored (using - # `PreserveLoggingContext`), we kick off the background task, and we restore the - # original context before returning (also part of `PreserveLoggingContext`). + # - When `run_as_background_process` is called, the current context is stored + # (using `PreserveLoggingContext`), we kick off the background task, and we + # restore the original context before returning (also part of + # `PreserveLoggingContext`). + # - The background task runs in its own new logcontext named after `desc` # - When the background task finishes, we don't want to leak our background context # into the reactor which would erroneously get attached to the next operation # picked up by the event loop. We use `PreserveLoggingContext` to set the