feat: add support for a config file (#612)

This is a port of two commits:

- GoogleCloudPlatform/cloud-sql-proxy#2106
- GoogleCloudPlatform/cloud-sql-proxy#2130

README.md

@@ -288,6 +288,30 @@ principal as the user you want to log in as, and start the Proxy like so:
 Note: when using the query-string syntax, the instance URI and query parameters
 must be wrapped in quotes.
 
+### Config file
+
+The Proxy supports a configuration file. Supported file types are TOML, JSON,
+and YAML. Load the file with the `--config-file` flag:
+
+```shell
+./alloydb-auth-proxy --config-file /path/to/config.[toml|json|yaml]
+```
+
+The configuration file format supports all flags. The key names should match
+the flag names. For example:
+
+``` toml
+# use instance-uri-0, instance-uri-1, etc.
+# for multiple instances
+instance-uri = "<INSTANCE_URI>"
+auto-iam-authn = true
+debug = true
+debug-logs = true
+```
+
+Run `./alloydb-auth-proxy --help` for more details.
+
+
 ## Running behind a Socks5 proxy
 
 The AlloyDB Auth Proxy includes support for sending requests through a SOCKS5

cmd/config_test.go

@@ -0,0 +1,140 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import "testing"
+
+func assert[T comparable](t *testing.T, want, got T) {
+	t.Helper()
+	if got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
+func TestNewCommandWithConfigFile(t *testing.T) {
+	tcs := []struct {
+		desc   string
+		args   []string
+		setup  func()
+		assert func(t *testing.T, c *Command)
+	}{
+		{
+			desc:  "toml config file",
+			args:  []string{"--config-file", "testdata/config-toml.toml"},
+			setup: func() {},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, 1, len(c.conf.Instances))
+				assert(t, true, c.conf.Debug)
+				assert(t, 5555, c.conf.Port)
+				assert(t, true, c.conf.DebugLogs)
+				assert(t, true, c.conf.AutoIAMAuthN)
+			},
+		},
+		{
+			desc:  "yaml config file",
+			args:  []string{"--config-file", "testdata/config-yaml.yaml"},
+			setup: func() {},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, 1, len(c.conf.Instances))
+				assert(t, true, c.conf.Debug)
+			},
+		},
+		{
+			desc:  "json config file",
+			args:  []string{"--config-file", "testdata/config-json.json"},
+			setup: func() {},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, 1, len(c.conf.Instances))
+				assert(t, true, c.conf.Debug)
+			},
+		},
+		{
+			desc:  "config file with two instances",
+			args:  []string{"--config-file", "testdata/two-instances.toml"},
+			setup: func() {},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, 2, len(c.conf.Instances))
+			},
+		},
+		{
+			desc: "argument takes precedence over environment variable",
+			args: []string{sampleURI},
+			setup: func() {
+				t.Setenv("ALLOYDB_PROXY_INSTANCE_URI", altURI)
+			},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, sampleURI, c.conf.Instances[0].Name)
+			},
+		},
+		{
+			desc: "environment variable takes precedence over config file",
+			args: []string{"--config-file", "testdata/config.json"},
+			setup: func() {
+				t.Setenv("ALLOYDB_PROXY_INSTANCE_URI", altURI)
+			},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, altURI, c.conf.Instances[0].Name)
+			},
+		},
+		{
+			desc: "CLI flag takes precedence over environment variable",
+			args: []string{sampleURI, "--debug"},
+			setup: func() {
+				t.Setenv("ALLOYDB_PROXY_DEBUG", "false")
+			},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, true, c.conf.Debug)
+			},
+		},
+		{
+			desc: "CLI flag takes precedence over config file",
+			args: []string{
+				sampleURI,
+				"--config-file", "testdata/config.toml",
+				"--debug=false",
+			},
+			setup: func() {},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, false, c.conf.Debug)
+			},
+		},
+		{
+			desc: "environment variable takes precedence over config file",
+			args: []string{
+				sampleURI,
+				"--config-file", "testdata/config.toml",
+			},
+			setup: func() {
+				t.Setenv("ALLOYDB_PROXY_DEBUG", "false")
+			},
+			assert: func(t *testing.T, c *Command) {
+				assert(t, false, c.conf.Debug)
+			},
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.desc, func(t *testing.T) {
+			tc.setup()
+
+			cmd, err := invokeProxyCommand(tc.args)
+			if err != nil {
+				t.Fatalf("want error = nil, got = %v", err)
+			}
+
+			tc.assert(t, cmd)
+		})
+	}
+}