soundcraft-cli: print logged warnings and errors to stderr

The CLI installed no logger, so every log::warn! and log::error! was
dropped: a plugin that failed to load just vanished from
engine.clap_plugins with no reason. A small logger now writes warnings
and errors to standard error as `warning: <message>` and
`error: <message>`, like the CLI's own errors. The level is fixed:
RUST_LOG stays a desktop app variable.

Fixes #1304.

Cargo.lock

@@ -4374,6 +4374,7 @@ dependencies = [
  "clap",
  "clap_complete",
  "clap_mangen",
+ "log",
  "same-file",
  "serde",
  "serde_json",

apps/soundcraft-cli/Cargo.toml

@@ -25,6 +25,7 @@ soundcraft-dsp.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 thiserror.workspace = true
+log.workspace = true
 clap.workspace = true
 same-file.workspace = true
 

apps/soundcraft-cli/src/lib.rs

@@ -11,6 +11,7 @@ mod catalog;
 mod convert;
 mod error;
 mod info;
+pub mod logger;
 mod mcp;
 mod session;
 

apps/soundcraft-cli/src/logger.rs

@@ -0,0 +1,49 @@
+//! The command line's logger. Without one, every `log::warn!` and `log::error!` in the workspace
+//! was dropped: a plugin that failed to load just vanished from `engine.clap_plugins`, and nothing
+//! said why. Warnings and errors now go to standard error as `warning: <message>` and
+//! `error: <message>`, like the CLI's own errors; standard output keeps only the results.
+//! `RUST_LOG` stays a desktop app variable (docs/cli.md), so the level is fixed.
+
+use std::io::Write;
+
+struct StderrLogger;
+
+impl log::Log for StderrLogger {
+    fn enabled(&self, metadata: &log::Metadata<'_>) -> bool {
+        metadata.level() <= log::Level::Warn
+    }
+
+    fn log(&self, record: &log::Record<'_>) {
+        if self.enabled(record.metadata()) {
+            // A closed standard error is not worth failing a command over.
+            let _ = std::io::stderr().write_all(line(record.level(), &record.args().to_string()).as_bytes());
+        }
+    }
+
+    fn flush(&self) {}
+}
+
+/// One record as the CLI prints it.
+fn line(level: log::Level, message: &str) -> String {
+    let label = if level == log::Level::Error { "error" } else { "warning" };
+    format!("{label}: {}\n", message.strip_suffix('\n').unwrap_or(message))
+}
+
+/// Installs the logger. Once per process: `main` calls it, not `run`, which tests call many times.
+pub fn install() {
+    static LOGGER: StderrLogger = StderrLogger;
+    if log::set_logger(&LOGGER).is_ok() {
+        log::set_max_level(log::LevelFilter::Warn);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn records_print_like_the_cli_errors() {
+        assert_eq!(line(log::Level::Warn, "cannot load CLAP plugin x.clap: bad file\n"), "warning: cannot load CLAP plugin x.clap: bad file\n");
+        assert_eq!(line(log::Level::Error, "panic: boom"), "error: panic: boom\n");
+    }
+}

apps/soundcraft-cli/src/main.rs

@@ -3,6 +3,7 @@
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 
 fn main() {
+    soundcraft_cli::logger::install();
     // `args_os`, not `args`: a non-Unicode argument must reach clap, not panic before anything is printed.
     let code = soundcraft_cli::run(std::env::args_os().skip(1), &mut std::io::stdout(), &mut std::io::stderr());
     // The subcommand's engine and its plugin instances are gone by now. This is process teardown,

apps/soundcraft-cli/tests/it/behaviour.rs

@@ -88,6 +88,25 @@ fn assert_refused(got: &Invoke, port: u16) {
     assert!(got.stderr.starts_with(&prefix) && got.stderr.contains(" (os error ") && got.stderr.ends_with(")\n"), "{}", got.stderr);
 }
 
+// ---- logging ----
+
+/// Warnings SoundCraft logs (a plugin that fails to load, a backup it can't write) go to standard
+/// error. Without a logger they were dropped.
+#[test]
+fn logged_warnings_reach_standard_error() {
+    let dir = sandbox("log");
+    let first = invoke(&dir, &["run", "--save", "s.scraft"]);
+    assert_eq!(first.code, 0, "{}", first.stderr);
+    // A file where the backups folder should go: the next save that changes the session can't
+    // back the old one up.
+    std::fs::write(dir.join("Backups"), b"not a folder").unwrap();
+    let again = invoke(&dir, &["run", "--in", "s.scraft", "--cmd", r#"track.new={"count":1}"#, "--save", "s.scraft", "--in-place"]);
+    assert_eq!(again.code, 0, "{}", again.stderr);
+    assert!(again.stderr.contains("warning: no backup of"), "{}", again.stderr);
+    assert!(!again.stdout.contains("no backup"), "results only on standard output: {}", again.stdout);
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
 // ---- unknown flags ----
 
 #[test]

docs/cli.md

@@ -38,7 +38,7 @@ Results go to standard output, as JSON where a program is meant to read them. `i
 file), `describe`, `app` and the `--inspect` output of `run` and `script` pretty-print it; `--compact`
 prints the same JSON on one line. The result lines of `run` and `script` (`ID: JSON`) and the
 `--bounce` and `--save` results are always one line each. `commands`, `plugins`, `parity` and the
-`.scraft` summary of `info` are text. Errors go to standard error as `error: <message>`. `mcp` writes only the protocol
+`.scraft` summary of `info` are text. Errors go to standard error as `error: <message>`, and so do the warnings and errors SoundCraft and the plugin hosts log (`warning: <message>`, `error: <message>`), such as a plugin that fails to load. `mcp` writes only the protocol
 to standard output.
 
 | Exit code | Meaning |