Merge pull request GoogleCloudPlatform#145 from ceocoder/use_INSTANCES_flag

cmd: use INSTANCES env var

Author: AthenaShi

README.md

@@ -20,6 +20,7 @@ cloud_sql_proxy takes a few arguments to configure:
   directory indicated by `-dir` is mounted.
 * `-instances="project1:region:instance1,project3:region:instance1"`: A comma-separated list
   of instances to open inside `-dir`. Also supports exposing a tcp port instead of using Unix Domain Sockets; see examples below.
+  Same list can be provided via INSTANCES environment variable, in case when both are provided - proxy will use command line flag.
 * `-instances_metadata=metadata_key`: Usable on [GCE](https://cloud.google.com/compute/docs/quickstart) only. The given [GCE metadata](https://cloud.google.com/compute/docs/metadata) key will be
   polled for a list of instances to open in `-dir`. The format for the value is the same as the 'instances' flag. A hanging-poll strategy is used, meaning that changes to
   the metadata value will be reflected in the `-dir` even while the proxy is

cmd/cloud_sql_proxy/cloud_sql_proxy.go

@@ -63,7 +63,8 @@ Unix socket-based connections.`)
 	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 to proxy to that instance. Otherwise, one socket file per
-instance is opened in 'dir'. Not compatible with -fuse`)
+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`)
 	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
@@ -138,6 +139,9 @@ Connection:
 
      When connecting over TCP, the -instances parameter is required.
 
+     Supplying INSTANCES environment variable achieves the same effect.  One can
+     use that to keep k8s manifest files constant across multiple environments
+
   -instances_metadata
      When running on GCE (Google Compute Engine) you can avoid the need to
      specify the list of instances on the command line by using the Metadata
@@ -388,6 +392,12 @@ func main() {
 		log.SetOutput(ioutil.Discard)
 	}
 
+	// 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
+	}
+
 	instList := stringList(*instances)
 	projList := stringList(*projects)
 	// TODO: it'd be really great to consolidate flag verification in one place.