From ce45b3f71fe991a094c82234a6821a56d0310cf9 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@element.io>
Date: Wed, 14 Oct 2026 16:30:22 +0000
Subject: [PATCH] Pass RUST_SYNC through to Complement homeservers

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0194eNQP5weBtVMg8NKtc6VU
---
 docker/complement/conf/start_for_complement.sh | 8 +++++++-
 scripts-dev/complement.sh                      | 6 ++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/docker/complement/conf/start_for_complement.sh b/docker/complement/conf/start_for_complement.sh
index 0c6ac6e264..e6a50da305 100755
--- a/docker/complement/conf/start_for_complement.sh
+++ b/docker/complement/conf/start_for_complement.sh
@@ -6,7 +6,7 @@ set -e
 
 echo "Complement Synapse launcher"
 echo "  Args: $*"
-echo "  Env: SYNAPSE_COMPLEMENT_DATABASE=$SYNAPSE_COMPLEMENT_DATABASE SYNAPSE_COMPLEMENT_USE_WORKERS=$SYNAPSE_COMPLEMENT_USE_WORKERS SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=$SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR SYNAPSE_USE_RUST_DRIVER=$SYNAPSE_USE_RUST_DRIVER SYNAPSE_RUST_STATE_RES=$SYNAPSE_RUST_STATE_RES"
+echo "  Env: SYNAPSE_COMPLEMENT_DATABASE=$SYNAPSE_COMPLEMENT_DATABASE SYNAPSE_COMPLEMENT_USE_WORKERS=$SYNAPSE_COMPLEMENT_USE_WORKERS SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=$SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR SYNAPSE_USE_RUST_DRIVER=$SYNAPSE_USE_RUST_DRIVER SYNAPSE_RUST_STATE_RES=$SYNAPSE_RUST_STATE_RES SYNAPSE_RUST_SYNC=$SYNAPSE_RUST_SYNC"
 
 function log {
     d=$(printf '%(%Y-%m-%d %H:%M:%S)T,%.3s\n' ${EPOCHREALTIME/./ })
@@ -135,6 +135,12 @@ if [[ -n "$SYNAPSE_RUST_STATE_RES" ]]; then
   sed -i "s/^experimental_features:$/experimental_features:\n  rust_state_res: \"$SYNAPSE_RUST_STATE_RES\"/" /conf/shared.yaml.j2
 fi
 
+# Compute sync rooms in Rust (`experimental_features.rust_sync`), allowing the
+# two event caches a test homeserver can afford.
+if [[ -n "$SYNAPSE_RUST_SYNC" ]]; then
+  sed -i "s/^experimental_features:$/experimental_features:\n  rust_sync: \"$SYNAPSE_RUST_SYNC\"\n  rust_sync_allow_unshared_event_cache: true/" /conf/shared.yaml.j2
+fi
+
 # Add a directory for tests to add config overrides if they want
 mkdir --parents /conf/homeserver.d
 export _SYNAPSE_COMPLEMENT_EXTRA_CONFIG_DIR=/conf/homeserver.d
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index 6e2bf37e42..a13f480f22 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -370,6 +370,12 @@ main() {
     export PASS_SYNAPSE_RUST_STATE_RES="$RUST_STATE_RES"
   fi
 
+  if [[ -n "$RUST_SYNC" ]]; then
+    # Set `experimental_features.rust_sync` (off, shadow or on). The Rust sync
+    # handler needs RUST_DRIVER=1.
+    export PASS_SYNAPSE_RUST_SYNC="$RUST_SYNC"
+  fi
+
   if [[ -n "$UNIX_SOCKETS" ]]; then
     # Enable full on Unix socket mode for Synapse, Redis and Postgresql
     export PASS_SYNAPSE_USE_UNIX_SOCKET=1