Desktop: a bare --control keeps the CADCRAFT_CONTROL_PORT port (fixes #844) (#848)

`--control` as the last argument read `args.next()` as None and reset the
port taken from CADCRAFT_CONTROL_PORT, without logging anything, so
`CADCRAFT_CONTROL_PORT=47321 cadcraft --control` started no control
channel. A bare `--control` now keeps the variable's port and logs a
warning either way; a port given on the command line still wins. The
precedence is documented in docs/control-protocol.md.

apps/cadcraft/src/main.rs

@@ -203,6 +203,19 @@ fn control_port_from(source: &str, value: Option<String>) -> Option<u16> {
     port
 }
 
+/// `--control` followed by `value`: a bare `--control` (last argument) keeps the port already
+/// read from `CADCRAFT_CONTROL_PORT` (`current`) instead of turning the control channel off.
+fn control_flag(current: Option<u16>, value: Option<String>) -> Option<u16> {
+    if value.is_some() {
+        return control_port_from("--control", value);
+    }
+    match current {
+        Some(port) => log::warn!("--control: no port given; using CADCRAFT_CONTROL_PORT ({port})"),
+        None => log::warn!("--control: no port given; the control channel is off"),
+    }
+    current
+}
+
 fn main() -> eframe::Result {
     // First, so every start-up record is captured; see `logging`.
     let logger = logging::install();
@@ -214,7 +227,7 @@ fn main() -> eframe::Result {
     let mut args = std::env::args().skip(1);
     while let Some(a) = args.next() {
         match a.as_str() {
-            "--control" => control_port = control_port_from("--control", args.next()),
+            "--control" => control_port = control_flag(control_port, args.next()),
             "--sample" => sample = true,
             "--version" | "-V" => {
                 println!("{}", version_string());
@@ -347,7 +360,7 @@ fn main() -> eframe::Result {
 
 #[cfg(test)]
 mod tests {
-    use super::{config_dir_from, control_port_from, filters_for_save, follow_desktop};
+    use super::{config_dir_from, control_flag, control_port_from, filters_for_save, follow_desktop};
     use std::ffi::OsString;
     use std::path::PathBuf;
 
@@ -381,6 +394,16 @@ mod tests {
         }
     }
 
+    #[test]
+    fn control_without_a_port_keeps_the_environment_port() {
+        // `CADCRAFT_CONTROL_PORT=47321 cadcraft --control` (#844)
+        assert_eq!(control_flag(Some(47321), None), Some(47321));
+        assert_eq!(control_flag(None, None), None);
+        // A port given on the command line still wins over the variable.
+        assert_eq!(control_flag(Some(47321), Some("7979".into())), Some(7979));
+        assert_eq!(control_flag(Some(47321), Some("abc".into())), None);
+    }
+
     #[test]
     fn the_override_variable_wins_and_empty_values_are_unset() {
         let vars = [("CADCRAFT_CONFIG_DIR", "/tmp/cc"), ("HOME", "/home/u"), ("XDG_CONFIG_HOME", "/x"), ("APPDATA", "C:\\a")];

docs/control-protocol.md

@@ -1,7 +1,8 @@
 # CADCraft control protocol
 
 Start the desktop app with `--control PORT` (or `CADCRAFT_CONTROL_PORT=PORT`). It listens on
-`127.0.0.1:PORT` for JSON lines: one request per line, one reply per line.
+`127.0.0.1:PORT` for JSON lines: one request per line, one reply per line. A port given with
+`--control` wins over the variable; a bare `--control` keeps the variable's port.
 
 ```json
 {"id": 1, "method": "cmdline.input", "params": {"text": "circle 0,0 5"}}