Skip /etc/hosts modification when hostname already resolves to loopback

Modern macOS and Linux systems with systemd-resolved natively resolve
*.localhost to 127.0.0.1/::1 per RFC 6761, making the /etc/hosts entry
redundant. This avoids an unnecessary sudo prompt on those systems.

Author: davegaeddert

plain-dev/plain/dev/core.py

@@ -254,6 +254,15 @@ def install_agent(self) -> None:
 
     def modify_hosts_file(self) -> None:
         """Modify the hosts file to map the custom domain to 127.0.0.1."""
+        # Check if the hostname already resolves to loopback (e.g., *.localhost on modern OS)
+        try:
+            results = socket.getaddrinfo(self.hostname, None)
+            addrs = {r[4][0] for r in results}
+            if addrs <= {"127.0.0.1", "::1"}:
+                return
+        except socket.gaierror:
+            pass  # Doesn't resolve; fall through to modify hosts file
+
         entry_identifier = "# Added by plain"
         hosts_entry = f"127.0.0.1 {self.hostname}  {entry_identifier}"