feat: allow multiple -instances flags (GoogleCloudPlatform#1046)

This PR adds support for specifying the `instances` flag repeatedly, 
as a convenience in templating contexts.

Fixes GoogleCloudPlatform#1030

Co-authored-by: Eno Compton <enocom@google.com>

Author: tsawada

cmd/cloud_sql_proxy/cloud_sql_proxy.go

@@ -73,15 +73,7 @@ Unix socket-based connections.`)
 		`Open sockets for each Cloud SQL Instance in the projects specified
 (comma-separated list)`,
 	)
-	instances = flag.String("instances", "",
-		`Comma-separated list of fully qualified instances (project:region:name)
-to connect to. If the name has the suffix '=tcp:port', a TCP server is opened
-on the specified port on localhost to proxy to that instance. It is also possible
-to listen on a custom address by providing a host, e.g., '=tcp:0.0.0.0:port'. If
-no value is provided for 'tcp', one socket file per instance is opened in 'dir'.
-You may use INSTANCES environment variable for the same effect. Using both will
-use value from flag, Not compatible with -fuse.`,
-	)
+	instances   stringListValue // -instances flag is defined in runProxy()
 	instanceSrc = flag.String("instances_metadata", "", `If provided, it is treated as a path to a metadata value which
 is polled for a comma-separated list of instances to connect to. For example,
 to use the instance metadata value named 'cloud-sql-instances' you would
@@ -198,6 +190,8 @@ Connection:
 
         -instances=my-project:my-region:my-instance
 
+    For convenience, this flag may be specified multiple times.
+
     For connectivity over TCP, you must specify a tcp port as part of the
     instance string. For example, the following example opens a loopback TCP
     socket on port 3306, which will be proxied to connect to the instance
@@ -295,6 +289,17 @@ func userAgentFromVersionString() string {
 
 const accountErrorSuffix = `Please create a new VM with Cloud SQL access (scope) enabled under "Identity and API access". Alternatively, create a new "service account key" and specify it using the -credential_file parameter`
 
+type stringListValue []string
+
+func (i *stringListValue) String() string {
+	return strings.Join(*i, ",")
+}
+
+func (i *stringListValue) Set(s string) error {
+	*i = append(*i, stringList(s)...)
+	return nil
+}
+
 func checkFlags(onGCE bool) error {
 	if !onGCE {
 		if *instanceSrc != "" {
@@ -481,6 +486,17 @@ func gcloudProject() ([]string, error) {
 }
 
 func runProxy() int {
+	flag.Var(&instances, "instances",
+		`Comma-separated list of fully qualified instances (project:region:name)
+to connect to. If the name has the suffix '=tcp:port', a TCP server is opened
+on the specified port on localhost to proxy to that instance. It is also possible
+to listen on a custom address by providing a host, e.g., '=tcp:0.0.0.0:port'. If
+no value is provided for 'tcp', one socket file per instance is opened in 'dir'.
+For convenience, this flag may be specified multiple times.
+You may use the INSTANCES environment variable for the same effect. Using both will
+use the value from the flag, Not compatible with -fuse.`,
+	)
+
 	flag.Parse()
 
 	if *version {
@@ -527,14 +543,13 @@ func runProxy() int {
 
 	// TODO: needs a better place for consolidation
 	// if instances is blank and env var INSTANCES is supplied use it
-	if envInstances := os.Getenv("INSTANCES"); *instances == "" && envInstances != "" {
-		*instances = envInstances
+	if envInstances := os.Getenv("INSTANCES"); len(instances) == 0 && envInstances != "" {
+		instances.Set(envInstances)
 	}
 
-	instList := stringList(*instances)
 	projList := stringList(*projects)
 	// TODO: it'd be really great to consolidate flag verification in one place.
-	if len(instList) == 0 && *instanceSrc == "" && len(projList) == 0 && !*useFuse {
+	if len(instances) == 0 && *instanceSrc == "" && len(projList) == 0 && !*useFuse {
 		var err error
 		projList, err = gcloudProject()
 		if err == nil {
@@ -571,8 +586,8 @@ func runProxy() int {
 		logging.Errorf(err.Error())
 		return 1
 	}
-	instList = append(instList, ins...)
-	cfgs, err := CreateInstanceConfigs(*dir, *useFuse, instList, *instanceSrc, client, *skipInvalidInstanceConfigs)
+	instances = append(instances, ins...)
+	cfgs, err := CreateInstanceConfigs(*dir, *useFuse, instances, *instanceSrc, client, *skipInvalidInstanceConfigs)
 	if err != nil {
 		logging.Errorf(err.Error())
 		return 1