fix(desktop): persist the save when a ROM window ends on a diagnostic

A core, DMA, rendering, window, or audio error returned before the save was
written, so every completed in-game save since launch was lost. The window now
writes changed bytes on that path too. The diagnostic stays the session error;
a refused or failed write is reported as output. A failed state restore leaves
save memory undefined and is never written. Terminal runs keep the strict rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU

crates/desktop/src/cartridge.rs

@@ -329,7 +329,7 @@ pub fn execute(options: Options, writer: &mut impl Write) -> Result<(), Box<dyn
         .map(|path| save_file::SaveFile::open(path, &options.path, options.save_device))
         .transpose()?;
     if let Some(save) = save_file.as_ref() {
-        writeln!(writer, "Save file: {}; writes only on clean exit, with unique backups when replacing existing data.", if save.loaded() { "loaded" } else { "new erased image" })?;
+        writeln!(writer, "Save file: {}; writes on clean exit or a ROM window diagnostic, with unique backups when replacing existing data.", if save.loaded() { "loaded" } else { "new erased image" })?;
     } else if options.save_device != SaveDevice::None {
         writeln!(
             writer,
@@ -400,5 +400,36 @@ pub(crate) fn persist_save(
     Ok(())
 }
 
+/// Keep completed in-game saves when a window session ends on a diagnostic.
+/// The diagnostic stays the session's error, so a refused or failed write is only reported.
+/// A failed state restore leaves save memory undefined and is never written.
+pub(crate) fn persist_save_after_diagnostic(
+    save: &mut save_file::SaveFile,
+    memory: &gba_core::memory::Memory,
+    diagnostic: &(dyn Error + 'static),
+    writer: &mut impl Write,
+) -> io::Result<()> {
+    if matches!(
+        diagnostic.downcast_ref(),
+        Some(gba_core::snapshot::SnapshotError::RestoreFailed(_))
+    ) {
+        return writeln!(
+            writer,
+            "Save not written: machine state is undefined after a failed state restore."
+        );
+    }
+    match save.persist(memory) {
+        Ok(Some(saved)) => {
+            writeln!(writer, "Save persisted after diagnostic.")?;
+            if let Some(backup) = saved.backup {
+                writeln!(writer, "Previous save backup: {backup:?}")?;
+            }
+            Ok(())
+        }
+        Ok(None) => writeln!(writer, "Save unchanged; no file written."),
+        Err(error) => writeln!(writer, "Save not written after diagnostic: {error}"),
+    }
+}
+
 #[cfg(test)]
 mod tests;

crates/desktop/src/cartridge/save_file/tests.rs

@@ -199,3 +199,56 @@ fn reject_rom_aliases_symlinks_directories_and_readonly_images() {
     assert_eq!(fs::read(&f.save).unwrap(), image);
     fs::set_permissions(&f.save, fs::Permissions::from_mode(0o600)).unwrap();
 }
+
+fn after_diagnostic(
+    save: &mut SaveFile,
+    m: &Memory,
+    error: &(dyn std::error::Error + 'static),
+) -> String {
+    let mut output = Vec::new();
+    crate::cartridge::persist_save_after_diagnostic(save, m, error, &mut output).unwrap();
+    String::from_utf8(output).unwrap()
+}
+
+#[test]
+fn diagnostics_persist_completed_programs_and_report_refusals() {
+    let diagnostic = io::Error::other("unsupported hardware");
+    let f = Fixture::new();
+    let original = f.image(255);
+    let mut m = memory();
+    let mut save = f.open().unwrap();
+    save.initialize(&mut m).unwrap();
+    assert!(after_diagnostic(&mut save, &m, &diagnostic).contains("Save unchanged"));
+    program(&mut m, 0x12);
+    let text = after_diagnostic(&mut save, &m, &diagnostic);
+    assert!(text.contains("Save persisted after diagnostic."), "{text}");
+    assert!(text.contains("Previous save backup"), "{text}");
+    let written = fs::read(&f.save).unwrap();
+    assert_eq!(written[0], 0x12);
+    assert_eq!(written[1..], original[1..]);
+
+    // An incomplete command is reported, never written, and never replaces the diagnostic.
+    start_program(&mut m, 0x02);
+    let text = after_diagnostic(&mut save, &m, &diagnostic);
+    assert!(text.contains("Save not written after diagnostic"), "{text}");
+    assert!(text.contains("incomplete"), "{text}");
+    assert_eq!(fs::read(&f.save).unwrap(), written);
+}
+
+#[test]
+fn failed_state_restores_never_write_the_save() {
+    use gba_core::snapshot::SnapshotError;
+    let f = Fixture::new();
+    let original = f.image(255);
+    let mut m = memory();
+    let mut save = f.open().unwrap();
+    save.initialize(&mut m).unwrap();
+    program(&mut m, 0x12);
+    let error: Box<dyn std::error::Error> = Box::new(SnapshotError::RestoreFailed(Box::new(
+        SnapshotError::Truncated,
+    )));
+    let text = after_diagnostic(&mut save, &m, error.as_ref());
+    assert!(text.contains("failed state restore"), "{text}");
+    assert_eq!(fs::read(&f.save).unwrap(), original);
+    assert_eq!(fs::read_dir(&f.dir).unwrap().count(), 3); // ROM, image, owned lock only.
+}

crates/desktop/src/desktop/rom.rs

@@ -121,7 +121,7 @@ pub fn run(
     )?;
     writeln!(
         writer,
-        "Input releases and audio mutes on focus loss. Save files require --save-file and clean exit; state file: {state_path:?}. Startup frames are not suppressed."
+        "Input releases and audio mutes on focus loss. Save files require --save-file and persist on exit; state file: {state_path:?}. Startup frames are not suppressed."
     )?;
     writeln!(
         writer,
@@ -256,8 +256,19 @@ pub fn run(
     if let Some(output) = audio.as_ref() {
         output.report(writer)?;
     }
-    result?;
     writer.flush()?;
+    if let Err(diagnostic) = result {
+        // A player's completed in-game saves outlive the diagnostic that ended the session.
+        if let Some(save) = save_file {
+            crate::cartridge::persist_save_after_diagnostic(
+                save,
+                session.memory_mut(),
+                diagnostic.as_ref(),
+                writer,
+            )?;
+        }
+        return Err(diagnostic);
+    }
     if let Some(save) = save_file {
         crate::cartridge::persist_save(save, session.memory_mut(), writer)?;
     }

crates/desktop/src/main.rs

@@ -141,7 +141,7 @@ fn run() -> Result<(), Box<dyn Error>> {
                 --audio                  Enable macOS output at 1x (default: muted)\n\
                 --rtc                    Select cartridge RTC (UTC startup, host elapsed time)\n\
                 --save-type flash64|flash128  Select nominal Macronix Flash emulation\n\
-                --save-file PATH         Load raw Flash image; persist only on clean exit\n\
+                --save-file PATH         Load raw Flash image; persist on clean exit or window diagnostic\n\
                 --state-file PATH        ROM window save state; F5 saves, F9 loads (default: ROM path + .state)\n\
                 --test-suite PATH.json    Run bounded ROM assertions; write a JSON report\n\
                 --cpu-demo     Run the terminal-only ARM/Thumb instruction demo\n\