Update LoggingTransaction.call_after and call_on_exception docstr…

…ings (#12315)

Document the behaviour of `LoggingTransaction.call_after` and
`LoggingTransaction.call_on_exception` when transactions are retried.

Signed-off-by: Sean Quah <seanq@element.io>

changelog.d/12315.doc

@@ -0,0 +1 @@
+Document the behaviour of `LoggingTransaction.call_after` and `LoggingTransaction.call_on_exception` methods when transactions are retried.

synapse/storage/database.py

@@ -241,9 +241,17 @@ def __init__(
         self.exception_callbacks = exception_callbacks
 
     def call_after(self, callback: Callable[..., object], *args: Any, **kwargs: Any):
-        """Call the given callback on the main twisted thread after the
-        transaction has finished. Used to invalidate the caches on the
-        correct thread.
+        """Call the given callback on the main twisted thread after the transaction has
+        finished.
+
+        Mostly used to invalidate the caches on the correct thread.
+
+        Note that transactions may be retried a few times if they encounter database
+        errors such as serialization failures. Callbacks given to `call_after`
+        will accumulate across transaction attempts and will _all_ be called once a
+        transaction attempt succeeds, regardless of whether previous transaction
+        attempts failed. Otherwise, if all transaction attempts fail, all
+        `call_on_exception` callbacks will be run instead.
         """
         # if self.after_callbacks is None, that means that whatever constructed the
         # LoggingTransaction isn't expecting there to be any callbacks; assert that
@@ -254,6 +262,15 @@ def call_after(self, callback: Callable[..., object], *args: Any, **kwargs: Any)
     def call_on_exception(
         self, callback: Callable[..., object], *args: Any, **kwargs: Any
     ):
+        """Call the given callback on the main twisted thread after the transaction has
+        failed.
+
+        Note that transactions may be retried a few times if they encounter database
+        errors such as serialization failures. Callbacks given to `call_on_exception`
+        will accumulate across transaction attempts and will _all_ be called once the
+        final transaction attempt fails. No `call_on_exception` callbacks will be run
+        if any transaction attempt succeeds.
+        """
         # if self.exception_callbacks is None, that means that whatever constructed the
         # LoggingTransaction isn't expecting there to be any callbacks; assert that
         # is not the case.