Install jemalloc in docker image (#8553)

Co-authored-by: Will Hunt <willh@matrix.org>
Co-authored-by: Erik Johnston <erik@matrix.org>

changelog.d/8553.docker

@@ -0,0 +1 @@
+Use jemalloc if available in docker.

docker/Dockerfile

@@ -69,6 +69,7 @@ RUN apt-get update && apt-get install -y \
     libpq5 \
     libwebp6 \
     xmlsec1 \
+    libjemalloc2 \
  && rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /install /usr/local

docker/README.md

@@ -204,3 +204,8 @@ healthcheck:
   timeout: 10s
   retries: 3
 ```
+
+## Using jemalloc
+
+Jemalloc is embedded in the image and will be used instead of the default allocator.
+You can read about jemalloc by reading the Synapse [README](../README.md)

docker/start.py

@@ -3,6 +3,7 @@
 import codecs
 import glob
 import os
+import platform
 import subprocess
 import sys
 
@@ -213,6 +214,13 @@ def main(args, environ):
     if "-m" not in args:
         args = ["-m", synapse_worker] + args
 
+    jemallocpath = "/usr/lib/%s-linux-gnu/libjemalloc.so.2" % (platform.machine(),)
+
+    if os.path.isfile(jemallocpath):
+        environ["LD_PRELOAD"] = jemallocpath
+    else:
+        log("Could not find %s, will not use" % (jemallocpath,))
+
     # if there are no config files passed to synapse, try adding the default file
     if not any(p.startswith("--config-path") or p.startswith("-c") for p in args):
         config_dir = environ.get("SYNAPSE_CONFIG_DIR", "/data")
@@ -248,9 +256,9 @@ running with 'migrate_config'. See the README for more details.
     args = ["python"] + args
     if ownership is not None:
         args = ["gosu", ownership] + args
-        os.execv("/usr/sbin/gosu", args)
+        os.execve("/usr/sbin/gosu", args, environ)
     else:
-        os.execv("/usr/local/bin/python", args)
+        os.execve("/usr/local/bin/python", args, environ)
 
 
 if __name__ == "__main__":