diff --git a/changelog.d/19209.misc b/changelog.d/19209.misc
new file mode 100644
index 0000000000..e64ca85d1d
--- /dev/null
+++ b/changelog.d/19209.misc
@@ -0,0 +1 @@
+Refactor `scripts-dev/complement.sh` logic to avoid `exit` to facilitate being able to source it from other scripts (composable).
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index c4d678b142..adb5807325 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -72,153 +72,154 @@ For help on arguments to 'go test', run 'go help testflag'.
 EOF
 }
 
-# parse our arguments
-skip_docker_build=""
-skip_complement_run=""
-while [ $# -ge 1 ]; do
+main() {
+  # parse our arguments
+  skip_docker_build=""
+  skip_complement_run=""
+  while [ $# -ge 1 ]; do
     arg=$1
     case "$arg" in
-        "-h")
-            usage
-            exit 1
-            ;;
-        "-f"|"--fast")
-            skip_docker_build=1
-            ;;
-        "--build-only")
-            skip_complement_run=1
-            ;;
-        "-e"|"--editable")
-            use_editable_synapse=1
-            ;;
-        "--rebuild-editable")
-            rebuild_editable_synapse=1
-            ;;
-        *)
-            # unknown arg: presumably an argument to gotest. break the loop.
-            break
+      "-h")
+        usage
+        return 1
+        ;;
+      "-f"|"--fast")
+        skip_docker_build=1
+        ;;
+      "--build-only")
+        skip_complement_run=1
+        ;;
+      "-e"|"--editable")
+        use_editable_synapse=1
+        ;;
+      "--rebuild-editable")
+        rebuild_editable_synapse=1
+        ;;
+      *)
+        # unknown arg: presumably an argument to gotest. break the loop.
+        break
     esac
     shift
-done
+  done
 
-# enable buildkit for the docker builds
-export DOCKER_BUILDKIT=1
+  # enable buildkit for the docker builds
+  export DOCKER_BUILDKIT=1
 
-# Determine whether to use the docker or podman container runtime.
-if [ -n "$PODMAN" ]; then
-  export CONTAINER_RUNTIME=podman
-  export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/podman/podman.sock
-  export BUILDAH_FORMAT=docker
-  export COMPLEMENT_HOSTNAME_RUNNING_COMPLEMENT=host.containers.internal
-else
-  export CONTAINER_RUNTIME=docker
-fi
+  # Determine whether to use the docker or podman container runtime.
+  if [ -n "$PODMAN" ]; then
+    export CONTAINER_RUNTIME=podman
+    export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/podman/podman.sock
+    export BUILDAH_FORMAT=docker
+    export COMPLEMENT_HOSTNAME_RUNNING_COMPLEMENT=host.containers.internal
+  else
+    export CONTAINER_RUNTIME=docker
+  fi
 
-# Change to the repository root
-cd "$(dirname $0)/.."
+  # Change to the repository root
+  cd "$(dirname $0)/.."
 
-# Check for a user-specified Complement checkout
-if [[ -z "$COMPLEMENT_DIR" ]]; then
-  COMPLEMENT_REF=${COMPLEMENT_REF:-main}
-  echo "COMPLEMENT_DIR not set. Fetching Complement checkout from ${COMPLEMENT_REF}..."
-  wget -Nq https://github.com/matrix-org/complement/archive/${COMPLEMENT_REF}.tar.gz
-  tar -xzf ${COMPLEMENT_REF}.tar.gz
-  COMPLEMENT_DIR=complement-${COMPLEMENT_REF}
-  echo "Checkout available at 'complement-${COMPLEMENT_REF}'"
-fi
+  # Check for a user-specified Complement checkout
+  if [[ -z "$COMPLEMENT_DIR" ]]; then
+    COMPLEMENT_REF=${COMPLEMENT_REF:-main}
+    echo "COMPLEMENT_DIR not set. Fetching Complement checkout from ${COMPLEMENT_REF}..."
+    wget -Nq https://github.com/matrix-org/complement/archive/${COMPLEMENT_REF}.tar.gz
+    tar -xzf ${COMPLEMENT_REF}.tar.gz
+    COMPLEMENT_DIR=complement-${COMPLEMENT_REF}
+    echo "Checkout available at 'complement-${COMPLEMENT_REF}'"
+  fi
 
-if [ -n "$use_editable_synapse" ]; then
+  if [ -n "$use_editable_synapse" ]; then
     if [[ -e synapse/synapse_rust.abi3.so ]]; then
-        # In an editable install, back up the host's compiled Rust module to prevent
-        # inconvenience; the container will overwrite the module with its own copy.
-        mv -n synapse/synapse_rust.abi3.so synapse/synapse_rust.abi3.so~host
-        # And restore it on exit:
-        synapse_pkg=`realpath synapse`
-        trap "mv -f '$synapse_pkg/synapse_rust.abi3.so~host' '$synapse_pkg/synapse_rust.abi3.so'" EXIT
+      # In an editable install, back up the host's compiled Rust module to prevent
+      # inconvenience; the container will overwrite the module with its own copy.
+      mv -n synapse/synapse_rust.abi3.so synapse/synapse_rust.abi3.so~host
+      # And restore it on exit:
+      synapse_pkg=`realpath synapse`
+      trap "mv -f '$synapse_pkg/synapse_rust.abi3.so~host' '$synapse_pkg/synapse_rust.abi3.so'" EXIT
     fi
 
     editable_mount="$(realpath .):/editable-src:z"
     if [ -n "$rebuild_editable_synapse" ]; then
-        unset skip_docker_build
+      unset skip_docker_build
     elif $CONTAINER_RUNTIME inspect complement-synapse-editable &>/dev/null; then
-        # complement-synapse-editable already exists: see if we can still use it:
-        # - The Rust module must still be importable; it will fail to import if the Rust source has changed.
-        # - The Poetry lock file must be the same (otherwise we assume dependencies have changed)
+      # complement-synapse-editable already exists: see if we can still use it:
+      # - The Rust module must still be importable; it will fail to import if the Rust source has changed.
+      # - The Poetry lock file must be the same (otherwise we assume dependencies have changed)
 
-        # First set up the module in the right place for an editable installation.
-        $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'cp' complement-synapse-editable -- /synapse_rust.abi3.so.bak /editable-src/synapse/synapse_rust.abi3.so
+      # First set up the module in the right place for an editable installation.
+      $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'cp' complement-synapse-editable -- /synapse_rust.abi3.so.bak /editable-src/synapse/synapse_rust.abi3.so
 
-        if ($CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'python' complement-synapse-editable -c 'import synapse.synapse_rust' \
-            && $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'diff' complement-synapse-editable --brief /editable-src/poetry.lock /poetry.lock.bak); then
-            skip_docker_build=1
-        else
-            echo "Editable Synapse image is stale. Will rebuild."
-            unset skip_docker_build
-        fi
+      if ($CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'python' complement-synapse-editable -c 'import synapse.synapse_rust' \
+        && $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'diff' complement-synapse-editable --brief /editable-src/poetry.lock /poetry.lock.bak); then
+        skip_docker_build=1
+      else
+        echo "Editable Synapse image is stale. Will rebuild."
+        unset skip_docker_build
+      fi
     fi
-fi
+  fi
 
-if [ -z "$skip_docker_build" ]; then
+  if [ -z "$skip_docker_build" ]; then
     if [ -n "$use_editable_synapse" ]; then
 
-        # Build a special image designed for use in development with editable
-        # installs.
-        $CONTAINER_RUNTIME build -t synapse-editable \
-            -f "docker/editable.Dockerfile" .
+      # Build a special image designed for use in development with editable
+      # installs.
+      $CONTAINER_RUNTIME build -t synapse-editable \
+        -f "docker/editable.Dockerfile" .
 
-        $CONTAINER_RUNTIME build -t synapse-workers-editable \
-            --build-arg FROM=synapse-editable \
-            -f "docker/Dockerfile-workers" .
+      $CONTAINER_RUNTIME build -t synapse-workers-editable \
+        --build-arg FROM=synapse-editable \
+        -f "docker/Dockerfile-workers" .
 
-        $CONTAINER_RUNTIME build -t complement-synapse-editable \
-            --build-arg FROM=synapse-workers-editable \
-            -f "docker/complement/Dockerfile" "docker/complement"
+      $CONTAINER_RUNTIME build -t complement-synapse-editable \
+        --build-arg FROM=synapse-workers-editable \
+        -f "docker/complement/Dockerfile" "docker/complement"
 
-        # Prepare the Rust module
-        $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'cp' complement-synapse-editable -- /synapse_rust.abi3.so.bak /editable-src/synapse/synapse_rust.abi3.so
+      # Prepare the Rust module
+      $CONTAINER_RUNTIME run --rm -v $editable_mount --entrypoint 'cp' complement-synapse-editable -- /synapse_rust.abi3.so.bak /editable-src/synapse/synapse_rust.abi3.so
 
     else
 
-        # Build the base Synapse image from the local checkout
-        echo_if_github "::group::Build Docker image: matrixdotorg/synapse"
-        $CONTAINER_RUNTIME build -t matrixdotorg/synapse \
-        --build-arg TEST_ONLY_SKIP_DEP_HASH_VERIFICATION \
-        --build-arg TEST_ONLY_IGNORE_POETRY_LOCKFILE \
-        -f "docker/Dockerfile" .
-        echo_if_github "::endgroup::"
+      # Build the base Synapse image from the local checkout
+      echo_if_github "::group::Build Docker image: matrixdotorg/synapse"
+      $CONTAINER_RUNTIME build -t matrixdotorg/synapse \
+      --build-arg TEST_ONLY_SKIP_DEP_HASH_VERIFICATION \
+      --build-arg TEST_ONLY_IGNORE_POETRY_LOCKFILE \
+      -f "docker/Dockerfile" .
+      echo_if_github "::endgroup::"
 
-        # Build the workers docker image (from the base Synapse image we just built).
-        echo_if_github "::group::Build Docker image: matrixdotorg/synapse-workers"
-        $CONTAINER_RUNTIME build -t matrixdotorg/synapse-workers -f "docker/Dockerfile-workers" .
-        echo_if_github "::endgroup::"
+      # Build the workers docker image (from the base Synapse image we just built).
+      echo_if_github "::group::Build Docker image: matrixdotorg/synapse-workers"
+      $CONTAINER_RUNTIME build -t matrixdotorg/synapse-workers -f "docker/Dockerfile-workers" .
+      echo_if_github "::endgroup::"
 
-        # Build the unified Complement image (from the worker Synapse image we just built).
-        echo_if_github "::group::Build Docker image: complement/Dockerfile"
-        $CONTAINER_RUNTIME build -t complement-synapse \
-            `# This is the tag we end up pushing to the registry (see` \
-            `# .github/workflows/push_complement_image.yml) so let's just label it now` \
-            `# so people can reference it by the same name locally.` \
-            -t ghcr.io/element-hq/synapse/complement-synapse \
-            -f "docker/complement/Dockerfile" "docker/complement"
-        echo_if_github "::endgroup::"
+      # Build the unified Complement image (from the worker Synapse image we just built).
+      echo_if_github "::group::Build Docker image: complement/Dockerfile"
+      $CONTAINER_RUNTIME build -t complement-synapse \
+        `# This is the tag we end up pushing to the registry (see` \
+        `# .github/workflows/push_complement_image.yml) so let's just label it now` \
+        `# so people can reference it by the same name locally.` \
+        -t ghcr.io/element-hq/synapse/complement-synapse \
+        -f "docker/complement/Dockerfile" "docker/complement"
+      echo_if_github "::endgroup::"
 
     fi
-fi
+  fi
 
-if [ -n "$skip_complement_run" ]; then
-    echo "Skipping Complement run as requested."
-    exit
-fi
+  if [ -n "$skip_complement_run" ]; then
+      echo "Skipping Complement run as requested."
+      return 0
+  fi
 
-export COMPLEMENT_BASE_IMAGE=complement-synapse
-if [ -n "$use_editable_synapse" ]; then
+  export COMPLEMENT_BASE_IMAGE=complement-synapse
+  if [ -n "$use_editable_synapse" ]; then
     export COMPLEMENT_BASE_IMAGE=complement-synapse-editable
     export COMPLEMENT_HOST_MOUNTS="$editable_mount"
-fi
+  fi
 
-extra_test_args=()
+  extra_test_args=()
 
-test_packages=(
+  test_packages=(
     ./tests/csapi
     ./tests
     ./tests/msc3874
@@ -231,71 +232,80 @@ test_packages=(
     ./tests/msc4140
     ./tests/msc4155
     ./tests/msc4306
-)
+  )
 
-# Enable dirty runs, so tests will reuse the same container where possible.
-# This significantly speeds up tests, but increases the possibility of test pollution.
-export COMPLEMENT_ENABLE_DIRTY_RUNS=1
+  # Enable dirty runs, so tests will reuse the same container where possible.
+  # This significantly speeds up tests, but increases the possibility of test pollution.
+  export COMPLEMENT_ENABLE_DIRTY_RUNS=1
 
-# All environment variables starting with PASS_ will be shared.
-# (The prefix is stripped off before reaching the container.)
-export COMPLEMENT_SHARE_ENV_PREFIX=PASS_
+  # All environment variables starting with PASS_ will be shared.
+  # (The prefix is stripped off before reaching the container.)
+  export COMPLEMENT_SHARE_ENV_PREFIX=PASS_
 
-# It takes longer than 10m to run the whole suite.
-extra_test_args+=("-timeout=60m")
+  # It takes longer than 10m to run the whole suite.
+  extra_test_args+=("-timeout=60m")
 
-if [[ -n "$WORKERS" ]]; then
-  # Use workers.
-  export PASS_SYNAPSE_COMPLEMENT_USE_WORKERS=true
+  if [[ -n "$WORKERS" ]]; then
+    # Use workers.
+    export PASS_SYNAPSE_COMPLEMENT_USE_WORKERS=true
 
-  # Pass through the workers defined. If none, it will be an empty string
-  export PASS_SYNAPSE_WORKER_TYPES="$WORKER_TYPES"
+    # Pass through the workers defined. If none, it will be an empty string
+    export PASS_SYNAPSE_WORKER_TYPES="$WORKER_TYPES"
 
-  # Workers can only use Postgres as a database.
-  export PASS_SYNAPSE_COMPLEMENT_DATABASE=postgres
-
-  # And provide some more configuration to complement.
-
-  # It can take quite a while to spin up a worker-mode Synapse for the first
-  # time (the main problem is that we start 14 python processes for each test,
-  # and complement likes to do two of them in parallel).
-  export COMPLEMENT_SPAWN_HS_TIMEOUT_SECS=120
-else
-  export PASS_SYNAPSE_COMPLEMENT_USE_WORKERS=
-  if [[ -n "$POSTGRES" ]]; then
+    # Workers can only use Postgres as a database.
     export PASS_SYNAPSE_COMPLEMENT_DATABASE=postgres
+
+    # And provide some more configuration to complement.
+
+    # It can take quite a while to spin up a worker-mode Synapse for the first
+    # time (the main problem is that we start 14 python processes for each test,
+    # and complement likes to do two of them in parallel).
+    export COMPLEMENT_SPAWN_HS_TIMEOUT_SECS=120
   else
-    export PASS_SYNAPSE_COMPLEMENT_DATABASE=sqlite
+    export PASS_SYNAPSE_COMPLEMENT_USE_WORKERS=
+    if [[ -n "$POSTGRES" ]]; then
+      export PASS_SYNAPSE_COMPLEMENT_DATABASE=postgres
+    else
+      export PASS_SYNAPSE_COMPLEMENT_DATABASE=sqlite
+    fi
   fi
+
+  if [[ -n "$ASYNCIO_REACTOR" ]]; then
+    # Enable the Twisted asyncio reactor
+    export PASS_SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=true
+  fi
+
+  if [[ -n "$UNIX_SOCKETS" ]]; then
+    # Enable full on Unix socket mode for Synapse, Redis and Postgresql
+    export PASS_SYNAPSE_USE_UNIX_SOCKET=1
+  fi
+
+  if [[ -n "$SYNAPSE_TEST_LOG_LEVEL" ]]; then
+    # Set the log level to what is desired
+    export PASS_SYNAPSE_LOG_LEVEL="$SYNAPSE_TEST_LOG_LEVEL"
+
+    # Allow logging sensitive things (currently SQL queries & parameters).
+    # (This won't have any effect if we're not logging at DEBUG level overall.)
+    # Since this is just a test suite, this is fine and won't reveal anyone's
+    # personal information
+    export PASS_SYNAPSE_LOG_SENSITIVE=1
+  fi
+
+  # Log a few more useful things for a developer attempting to debug something
+  # particularly tricky.
+  export PASS_SYNAPSE_LOG_TESTING=1
+
+  # Run the tests!
+  echo "Images built; running complement with ${extra_test_args[@]} $@ ${test_packages[@]}"
+  cd "$COMPLEMENT_DIR"
+
+  go test -v -tags "synapse_blacklist" -count=1 "${extra_test_args[@]}" "$@" "${test_packages[@]}"
+}
+
+main "$@"
+# For any non-zero exit code (indicating some sort of error happened), we want to exit
+# with that code.
+exit_code=$?
+if [ $exit_code -ne 0 ]; then
+    exit $exit_code
 fi
-
-if [[ -n "$ASYNCIO_REACTOR" ]]; then
-  # Enable the Twisted asyncio reactor
-  export PASS_SYNAPSE_COMPLEMENT_USE_ASYNCIO_REACTOR=true
-fi
-
-if [[ -n "$UNIX_SOCKETS" ]]; then
-  # Enable full on Unix socket mode for Synapse, Redis and Postgresql
-  export PASS_SYNAPSE_USE_UNIX_SOCKET=1
-fi
-
-if [[ -n "$SYNAPSE_TEST_LOG_LEVEL" ]]; then
-  # Set the log level to what is desired
-  export PASS_SYNAPSE_LOG_LEVEL="$SYNAPSE_TEST_LOG_LEVEL"
-
-  # Allow logging sensitive things (currently SQL queries & parameters).
-  # (This won't have any effect if we're not logging at DEBUG level overall.)
-  # Since this is just a test suite, this is fine and won't reveal anyone's
-  # personal information
-  export PASS_SYNAPSE_LOG_SENSITIVE=1
-fi
-
-# Log a few more useful things for a developer attempting to debug something
-# particularly tricky.
-export PASS_SYNAPSE_LOG_TESTING=1
-
-# Run the tests!
-echo "Images built; running complement with ${extra_test_args[@]} $@ ${test_packages[@]}"
-cd "$COMPLEMENT_DIR"
-
-go test -v -tags "synapse_blacklist" -count=1 "${extra_test_args[@]}" "$@" "${test_packages[@]}"