Add ENVOY_KILL_API env variable

This supports istio by allowing for a separate URL for the quitquitquit
command.

Fixes #6

Author: jackkleeman

README.md

@@ -20,6 +20,7 @@ When the application exits, as long as it does so with exit code 0, `envoy-prefl
 | Variable              | Purpose                                                                                                                                                                                                                                                                                                                                  |
 |-----------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `ENVOY_ADMIN_API`     | This is the path to envoy's administration interface, in the format `http://127.0.0.1:9010`. If provided, `envoy-preflight` will poll this url at `/server_info` waiting for envoy to report as `LIVE`. If provided and local (`127.0.0.1` or `localhost`), then envoy will be instructed to shut down if the application exits cleanly. |
+| `ENVOY_KILL_API`      | This is the endpoint of the POST command to kill envoy, which defaults to `$ENVOY_ADMIN_API/quitquitquit`, but you can provide any value in format `http://127.0.0.1:9010/quitquitquit`. This can be used to support istio by providing the pilot-agent port.                                                                            |
 | `NEVER_KILL_ENVOY`    | If provided and set to `true`, `envoy-preflight` will not instruct envoy to exit under any circumstances.                                                                                                                                                                                                                                |
 | `ALWAYS_KILL_ENVOY`   | If provided and set to `true`, `envoy-preflight` will instruct envoy to exit, even if the main application exits with a nonzero exit code.                                                                                                                                                                                               |
 | `START_WITHOUT_ENVOY` | If provided and set to `true`, `envoy-preflight` will not wait for envoy to be LIVE before starting the main application. However, it will still instruct envoy to exit.                                                                                                                                                                 |

main.go

@@ -24,6 +24,11 @@ func main() {
 		block(host)
 	}
 
+	killAPI, killOk := os.LookupEnv("ENVOY_KILL_API")
+	if !killOk {
+		killAPI = fmt.Sprintf("%s/quitquitquit", host)
+	}
+
 	if len(os.Args) < 2 {
 		return
 	}
@@ -72,9 +77,7 @@ func main() {
 		// We're configured never to kill envoy, do nothing
 	case os.Getenv("ALWAYS_KILL_ENVOY") == "true", exitCode == 0:
 		// Either we had a clean exit, or we are configured to kill envoy anyway
-		url := fmt.Sprintf("%s/quitquitquit", host)
-
-		_ = typhon.NewRequest(context.Background(), "POST", url, nil).Send().Response()
+		_ = typhon.NewRequest(context.Background(), "POST", killAPI, nil).Send().Response()
 	}
 
 	os.Exit(exitCode)