Use the SYNAPSE_VIZTRACER as the output path

mypy.ini

@@ -349,5 +349,8 @@ ignore_missing_imports = True
 [mypy-twisted.*]
 ignore_missing_imports = True
 
+[mypy-viztracer]
+ignore_missing_imports = True
+
 [mypy-zope]
 ignore_missing_imports = True

synapse/app/homeserver.py

@@ -446,10 +446,42 @@ def run(hs: HomeServer) -> None:
     )
 
 
+def setup_viztracer() -> None:
+    """
+    If installed and enabled by environment variable SYNAPSE_VIZTRACER=1,
+    install VizTracer's hooks so that Synapse can be attached to by VizTracer
+    at some point during runtime.
+    """
+
+    def eprint(msg: str) -> None:
+        print(msg, file=sys.stderr)
+
+    viztracer_output_path = os.environ.get("SYNAPSE_VIZTRACER", "")
+    if viztracer_output_path:
+        eprint(
+            f"SYNAPSE_VIZTRACER is set. Installing VizTracer hooks to output to {viztracer_output_path}."
+        )
+
+        try:
+            from viztracer import VizTracer
+
+            VizTracer(output_file=viztracer_output_path).install()
+        except ImportError:
+            eprint("VizTracer could not be imported: can't install hooks.")
+    else:
+        eprint(
+            "SYNAPSE_VIZTRACER not set (or is empty): won't install VizTracer hooks."
+        )
+
+
 def main() -> None:
     with LoggingContext("main"):
         # check base requirements
         check_requirements()
+
+        # Set up VizTracer for debugging (if enabled)
+        setup_viztracer()
+
         hs = setup(sys.argv[1:])
 
         # redirect stdio to the logs, if configured.

synapse/python_dependencies.py

@@ -116,6 +116,9 @@ CONDITIONAL_REQUIREMENTS = {
     "redis": ["txredisapi>=1.4.7", "hiredis"],
     # Required to use experimental `caches.track_memory_usage` config option.
     "cache_memory": ["pympler"],
+    # Required to install VizTracer hooks, which allows VizTracer to be attached
+    # at runtime.
+    "viztracer": ["viztracer"],
 }
 
 ALL_OPTIONAL_REQUIREMENTS: Set[str] = set()