Make CMD optional (#23)

`CMD` is not required if the use-case of `linkerd-await` is only to wait for readiness
but do nothing afterwards.

In linkerd/linkerd2#5967 this is the case. `linkerd-await` is executed as a container
hook and the only thing that it needs to do is prevent the hook from finishing until
the proxy is ready. Once it is ready, it exits without running any additional commands.

Signed-off-by: Kevin Leimkuhler <[email protected]>

Author: kleimkuhler

src/main.rs

@@ -29,14 +29,15 @@ struct Opt {
     #[structopt(
         short = "S",
         long = "shutdown",
-        help = "Forks the program and triggers proxy shutdown on completion"
+        help = "Forks the program and triggers proxy shutdown on completion",
+        requires("CMD")
     )]
     shutdown: bool,
 
-    #[structopt(name = "CMD")]
-    cmd: String,
+    #[structopt(name = "CMD", help = "The command to run after linkerd is ready")]
+    cmd: Option<String>,
 
-    #[structopt(name = "ARGS")]
+    #[structopt(name = "ARGS", help = "Arguments to pass to CMD if specified")]
     args: Vec<String>,
 }
 
@@ -64,7 +65,10 @@ async fn main() {
             await_ready(authority.clone(), backoff).await;
 
             if shutdown {
-                // If shutdown is configured, fork the process and proxy SIGTERM.
+                let cmd = cmd.expect("Command must be specified with --shutdown");
+
+                // If shutdown is configured, fork the process and proxy
+                // SIGTERM.
                 let ex = fork_with_sigterm(cmd, args).await;
 
                 // Once the process completes, issue a shutdown request to the
@@ -85,9 +89,11 @@ async fn main() {
         }
     }
 
-    // If Linkerd shutdown is not configured, exec the process directly so that
-    // the we don't have to bother with signal proxying, etc.
-    exec(cmd, args);
+    if let Some(cmd) = cmd {
+        // If Linkerd shutdown is not configured, exec the process directly so
+        // that the we don't have to bother with signal proxying, etc.
+        exec(cmd, args);
+    }
 }
 
 fn linkerd_disabled_reason() -> Option<String> {