complement.sh: Permit skipping docker build (#13143)

Add a `-f` argument to `complement.sh` to skip the docker build

changelog.d/13143.misc

@@ -0,0 +1 @@
+Add support to `complement.sh` for skipping the docker build.

scripts-dev/complement.sh

@@ -14,9 +14,12 @@
 # By default Synapse is run in monolith mode. This can be overridden by
 # setting the WORKERS environment variable.
 #
-# A regular expression of test method names can be supplied as the first
-# argument to the script. Complement will then only run those tests. If
-# no regex is supplied, all tests are run. For example;
+# You can optionally give a "-f" argument (for "fast") before any to skip
+# rebuilding the docker images, if you just want to rerun the tests.
+#
+# Remaining commandline arguments are passed through to `go test`. For example,
+# you can supply a regular expression of test method names via the "-run"
+# argument:
 #
 # ./complement.sh -run "TestOutboundFederation(Profile|Send)"
 #
@@ -32,6 +35,37 @@ echo_if_github() {
   fi
 }
 
+# Helper to print out the usage instructions
+usage() {
+    cat >&2 <<EOF
+Usage: $0 [-f] <go test arguments>...
+Run the complement test suite on Synapse.
+
+  -f    Skip rebuilding the docker images, and just use the most recent
+        'complement-synapse:latest' image
+
+For help on arguments to 'go test', run 'go help testflag'.
+EOF
+}
+
+# parse our arguments
+skip_docker_build=""
+while [ $# -ge 1 ]; do
+    arg=$1
+    case "$arg" in
+        "-h")
+            usage
+            exit 1
+            ;;
+        "-f")
+            skip_docker_build=1
+            ;;
+        *)
+            # unknown arg: presumably an argument to gotest. break the loop.
+            break
+    esac
+    shift
+done
 
 # enable buildkit for the docker builds
 export DOCKER_BUILDKIT=1
@@ -49,6 +83,7 @@ if [[ -z "$COMPLEMENT_DIR" ]]; then
   echo "Checkout available at 'complement-${COMPLEMENT_REF}'"
 fi
 
+if [ -z "$skip_docker_build" ]; then
     # Build the base Synapse image from the local checkout
     echo_if_github "::group::Build Docker image: matrixdotorg/synapse"
     docker build -t matrixdotorg/synapse -f "docker/Dockerfile" .
@@ -64,6 +99,7 @@ echo_if_github "::group::Build Docker image: complement/Dockerfile"
     docker build -t complement-synapse \
            -f "docker/complement/Dockerfile" "docker/complement"
     echo_if_github "::endgroup::"
+fi
 
 export COMPLEMENT_BASE_IMAGE=complement-synapse