From 5ea773c50568088b30e15728b65480d0335fe14e Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Mon, 22 Jul 2019 13:15:08 +0100
Subject: [PATCH] Cache get_version_string.

The version of a module isn't going to change over the lifetime of the
process (assuming no funky hot reloading is going on, which it isn't),
so let's just cache the result to avoid spawning lots of git
subprocesses.

Fixes #5672.
---
 synapse/util/versionstring.py | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/synapse/util/versionstring.py b/synapse/util/versionstring.py
index a4d9a462f..fa404b9d7 100644
--- a/synapse/util/versionstring.py
+++ b/synapse/util/versionstring.py
@@ -22,6 +22,23 @@ logger = logging.getLogger(__name__)
 
 
 def get_version_string(module):
+    """Given a module calculate a git-aware version string for it.
+
+    If called on a module not in a git checkout will return `__verison__`.
+
+    Args:
+        module (module)
+
+    Returns:
+        str
+    """
+
+    cached_version = getattr(module, "_synapse_version_string_cache", None)
+    if cached_version:
+        return cached_version
+
+    version_string = module.__version__
+
     try:
         null = open(os.devnull, "w")
         cwd = os.path.dirname(os.path.abspath(module.__file__))
@@ -80,8 +97,10 @@ def get_version_string(module):
                 s for s in (git_branch, git_tag, git_commit, git_dirty) if s
             )
 
-            return "%s (%s)" % (module.__version__, git_version)
+            version_string = "%s (%s)" % (module.__version__, git_version)
     except Exception as e:
         logger.info("Failed to check for git repository: %s", e)
 
-    return module.__version__
+    module._synapse_version_string_cache = version_string
+
+    return version_string