listener/tcp: better handling of min/max port env vars (#111)

* listener/tcp: better handling of min/max port env vars

* wrap errors to make the return value more indicative

* docs: add references to the port env vars

docs/internals.md

@@ -61,3 +61,16 @@ Where:
     is omitted (older versions), this is "netrpc" for Go net/rpc. This can
     also be "grpc". This is the protocol that the plugin wants to speak to
     the host process with.
+
+## Environment Variables
+
+When serving a plugin over TCP, the following environment variables can be
+specified to restrict the port that will be assigned to be from within a
+specific range. If not values are provided, the port will be randomly assigned
+by the operating system.
+
+ * `PLUGIN_MIN_PORT`: Specifies the minimum port value that will be assigned to
+ * the listener.
+
+ * `PLUGIN_MAX_PORT`: Specifies the maximum port value that will be assigned to
+ * the listener.

server.go

@@ -363,14 +363,34 @@ func serverListener() (net.Listener, error) {
 }
 
 func serverListener_tcp() (net.Listener, error) {
-	minPort, err := strconv.ParseInt(os.Getenv("PLUGIN_MIN_PORT"), 10, 32)
-	if err != nil {
-		return nil, err
+	envMinPort := os.Getenv("PLUGIN_MIN_PORT")
+	envMaxPort := os.Getenv("PLUGIN_MAX_PORT")
+
+	var minPort, maxPort int64
+	var err error
+
+	switch {
+	case len(envMinPort) == 0:
+		minPort = 0
+	default:
+		minPort, err = strconv.ParseInt(envMinPort, 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("Couldn't get value from PLUGIN_MIN_PORT: %v", err)
+		}
 	}
 
-	maxPort, err := strconv.ParseInt(os.Getenv("PLUGIN_MAX_PORT"), 10, 32)
-	if err != nil {
-		return nil, err
+	switch {
+	case len(envMaxPort) == 0:
+		maxPort = 0
+	default:
+		maxPort, err = strconv.ParseInt(envMaxPort, 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("Couldn't get value from PLUGIN_MAX_PORT: %v", err)
+		}
+	}
+
+	if minPort > maxPort {
+		return nil, fmt.Errorf("ENV_MIN_PORT value of %d is greater than PLUGIN_MAX_PORT value of %d", minPort, maxPort)
 	}
 
 	for port := minPort; port <= maxPort; port++ {