Put open::that inside spawn_blocking to launch the browser

`open::that` doews occasionally block. One example is when launching a
fresh instance of firefox on linux.

Another alternative is `open::that_detached`, but that reports a success
even if the browser exited with an error.

backend-local-server/src/bin/diffedit3-web.rs

@@ -20,8 +20,13 @@ pub struct LocalServerCli {
     /// Do not try to open the browser automatically
     ///
     /// See https://crates.io/crates/open for a brief description of how the
-    /// default browser is chosen. The `BROWSER` environment variable may be
-    /// considered by `xdg-open` and similar commands.
+    /// default browser is chosen. The `BROWSER` environment variable is not
+    /// respected, unfortunately.
+    // TODO(ilyagr): One way to respect the BROWSER environment variable might
+    // be to use the `webbrowser` crate to get the browser command. It'd be
+    // better to use something else to actually launch the browser, as
+    // `webbrowser::open` has limited error handlind, like
+    // `open::that_detached`.
     #[arg(long, short = 'N')]
     no_browser: bool,
     /// Make the server print debugging information

backend-local-server/src/local_server.rs

@@ -143,14 +143,19 @@ pub async fn run_server(
     // using `port` is to find out what port number the OS picked if `cli.port==0`.
     let socket_addr = acceptor_to_socket_address(&acceptor)?;
     // Now that the acceptor exists, the browser should be able to connect IIUC.
-    let http_address = format!("http://{socket_addr}");
-    eprintln!("Listening at {http_address}.");
+    eprintln!("Listening at {socket_addr}.");
     if open_browser {
-        eprint!("Trying to launch a browser at {http_address}...");
-        match open::that(&http_address) {
-            Ok(_) => eprintln!(" Success!"),
-            Err(err) => eprintln!("\nFailed to launch a browser: {err}"),
-        }
+        let http_address = format!("http://{socket_addr}");
+        tokio::task::spawn_blocking(move || {
+            // Use `spawn_blocking` since `webbrowser::open` may block (for text-mode
+            // browsers. TODO: find out if it blocks when running a fresh instance of
+            // `firefox` on Linux.)
+            eprintln!("Trying to launch a browser at {http_address}...");
+            match open::that(&http_address) {
+                Ok(_) => eprintln!("Successfully launched browser."),
+                Err(err) => eprintln!("Failed to launch a browser: {err}"),
+            }
+        });
     }
 
     let mut result = Ok(());