add docker compose style tests

Author: rajatvig

docker-compose.yml

@@ -0,0 +1,5 @@
+services:
+  tests:
+    image: buildkite/plugin-tester:latest
+    volumes:
+      - .:/plugin

tests/command.bats

@@ -0,0 +1,96 @@
+#!/usr/bin/env bats
+
+load "$BATS_PLUGIN_PATH/load.bash"
+
+# Uncomment the following line to debug stubbed commands
+# export COMMAND_STUB_DEBUG=/dev/tty
+
+setup() {
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_BUILDKITE_PLUGIN_TEST_MODE=true
+  # Set a minimal plugin config (only used by Go binary, not the bash hook's download logic)
+  export BUILDKITE_PLUGINS='[{"monorepo-diff": {}}]'
+  
+  # Create a mock binary in the current directory for download mode tests
+  cat > "$PWD/monorepo-diff-buildkite-plugin" << 'MOCKBIN'
+#!/bin/bash
+echo "Mock binary executed with args: $@"
+MOCKBIN
+  chmod +x "$PWD/monorepo-diff-buildkite-plugin"
+}
+
+teardown() {
+  # Clean up the mock binary
+  rm -f "$PWD/monorepo-diff-buildkite-plugin"
+}
+
+@test "download=false with binary in PATH succeeds" {
+  # Buildkite sets this env var based on plugin config
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD=false
+  export PATH="$PWD:$PATH"
+
+  run "$PWD/hooks/command"
+
+  assert_success
+  assert_output --partial "Mock binary executed"
+}
+
+@test "download=false without binary in PATH fails with error" {
+  # Buildkite sets this env var based on plugin config
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD=false
+  
+  # Remove mock binary from current directory and don't add to PATH
+  rm -f "$PWD/monorepo-diff-buildkite-plugin"
+  export PATH="/usr/bin:/bin"
+
+  run "$PWD/hooks/command"
+
+  assert_failure
+  assert_output --partial "Binary 'monorepo-diff-buildkite-plugin' not found in PATH"
+  assert_output --partial "Please install it or set download: true"
+}
+
+@test "download=true executes binary in test mode" {
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD=true
+
+  # In test mode, it skips download but still executes the mock binary we created
+  run "$PWD/hooks/command"
+
+  assert_success
+  assert_output --partial "Mock binary executed"
+}
+
+@test "download defaults to true when not set" {
+  # Don't set BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD - should default to true
+
+  # Should default to true, skip download in test mode, and execute mock binary
+  run "$PWD/hooks/command"
+
+  assert_success
+  assert_output --partial "Mock binary executed"
+}
+
+@test "download=false passes arguments to preinstalled binary" {
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD=false
+  export PATH="$PWD:$PATH"
+
+  run "$PWD/hooks/command" arg1 arg2
+
+  assert_success
+  assert_output --partial "Mock binary executed with args: arg1 arg2"
+}
+
+@test "download=true in test mode skips actual download" {
+  export BUILDKITE_PLUGIN_MONOREPO_DIFF_DOWNLOAD=true
+
+  # Stub curl to verify it's NOT called in test mode
+  stub curl \
+    ": echo 'ERROR: curl should not be called in test mode'; exit 1"
+
+  run "$PWD/hooks/command"
+
+  # Should succeed without calling curl
+  assert_success
+  
+  # Clean up stub (it should not have been called)
+  unstub curl || true
+}