Reload the pyo3-log config when the Python logging config changes. (#14976)

Since pyo3-log is initialized very early in the Python start-up
it caches the state of the loggers before they're fully initialized
(and thus are essentially disabled). Whenever we reload the
logging configuration we now also tell pyo3-log to discard
any cached logging configuration it has; it will refetch the
current logging configuration from Python at the next point
it logs.

This fixes Rust log lines not appearing in the homeserver logs.

rust/src/lib.rs

@@ -1,7 +1,13 @@
+use lazy_static::lazy_static;
 use pyo3::prelude::*;
+use pyo3_log::ResetHandle;
 
 pub mod push;
 
+lazy_static! {
+    static ref LOGGING_HANDLE: ResetHandle = pyo3_log::init();
+}
+
 /// Returns the hash of all the rust source files at the time it was compiled.
 ///
 /// Used by python to detect if the rust library is outdated.
@@ -17,13 +23,20 @@ fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
     Ok((a + b).to_string())
 }
 
+/// Reset the cached logging configuration of pyo3-log to pick up any changes
+/// in the Python logging configuration.
+///
+#[pyfunction]
+fn reset_logging_config() {
+    LOGGING_HANDLE.reset();
+}
+
 /// The entry point for defining the Python module.
 #[pymodule]
 fn synapse_rust(py: Python<'_>, m: &PyModule) -> PyResult<()> {
-    pyo3_log::init();
-
     m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
     m.add_function(wrap_pyfunction!(get_rust_file_digest, m)?)?;
+    m.add_function(wrap_pyfunction!(reset_logging_config, m)?)?;
 
     push::register_module(py, m)?;