Merge #315: Retry missing font fallbacks after a font rescan

ROADMAP.md

@@ -31,6 +31,10 @@
 
 DesignCraft aims at full Adobe InDesign parity — and to be better: faster, open (documented JSON format + IDML), scriptable by agents (MCP), and available on the web.
 
+## Recent improvements (2026-10-08)
+
+- QA1: Font rescans retry missing glyphs and fallback families after new fonts arrive; a controlled Linux regression covers older in-flight misses after a rescan.
+
 ## Status (2026-10-01)
 
 **Working today**

crates/fonts/src/fontdb.rs

@@ -357,17 +357,28 @@ pub struct FontDb {
     /// System fallback state: enabled, and characters no system font covers.
     #[cfg(not(target_arch = "wasm32"))]
     sys: Mutex<SysFallback>,
+    /// Pause one fallback after copying catalog paths, with no database locks held.
+    #[cfg(all(test, target_os = "linux"))]
+    fallback_snapshot_hook: Mutex<Option<FallbackSnapshotHook>>,
 }
 
 #[cfg(not(target_arch = "wasm32"))]
 #[derive(Default)]
 struct SysFallback {
     enabled: bool,
+    /// Rescan generation: an older in-flight lookup must not restore a cleared miss.
+    generation: u64,
     misses: std::collections::HashSet<char>,
     /// CJK chain families already looked up in the catalog (loaded, or not installed).
     chain_tried: std::collections::HashSet<&'static str>,
 }
 
+#[cfg(all(test, target_os = "linux"))]
+struct FallbackSnapshotHook {
+    reached: std::sync::mpsc::SyncSender<()>,
+    resume: Arc<std::sync::Barrier>,
+}
+
 /// Families tried (when installed) for characters the loaded fonts lack: CJK, symbols, emoji.
 #[cfg(not(target_arch = "wasm32"))]
 const SYSTEM_FALLBACKS: &[&str] = &[
@@ -838,6 +849,8 @@ impl FontDb {
             cataloged: std::sync::OnceLock::new(),
             #[cfg(not(target_arch = "wasm32"))]
             sys: Mutex::new(SysFallback { enabled: true, ..Default::default() }),
+            #[cfg(all(test, target_os = "linux"))]
+            fallback_snapshot_hook: Mutex::new(None),
         }
     }
 
@@ -964,7 +977,14 @@ impl FontDb {
         // The first scan, or a rescan once it is done (never both at once).
         let mut first = None;
         self.cataloged.get_or_init(|| first = Some(self.scan_font_dirs()));
-        first.unwrap_or_else(|| self.scan_font_dirs())
+        let count = first.unwrap_or_else(|| self.scan_font_dirs());
+        // Retry characters and CJK chain families that newly installed fonts can now cover.
+        // Reset outside scan_font_dirs: a chain lookup can hold sys during the initial scan.
+        let mut sys = self.sys.lock().unwrap_or_else(|e| e.into_inner());
+        sys.generation = sys.generation.wrapping_add(1);
+        sys.misses.clear();
+        sys.chain_tried.clear();
+        count
     }
 
     #[cfg(not(target_arch = "wasm32"))]
@@ -1172,12 +1192,13 @@ impl FontDb {
         if c.is_control() || c.is_whitespace() {
             return false;
         }
-        {
+        let generation = {
             let sys = self.sys.lock().unwrap_or_else(|e| e.into_inner());
             if !sys.enabled || sys.misses.contains(&c) {
                 return false;
             }
-        }
+            sys.generation
+        };
         let covered = |db: &FontDb| db.read_faces().iter().any(|f| f.covers(c));
         let cataloged: Vec<String> = self.read_catalog().iter().map(|e| e.family.clone()).collect();
         for fam in SYSTEM_FALLBACKS {
@@ -1191,6 +1212,14 @@ impl FontDb {
         let mut paths: Vec<std::path::PathBuf> = self.catalog.read().unwrap_or_else(|e| e.into_inner()).iter().map(|e| e.path.clone()).collect();
         paths.sort();
         paths.dedup();
+        #[cfg(all(test, target_os = "linux"))]
+        {
+            let hook = self.fallback_snapshot_hook.lock().unwrap_or_else(|e| e.into_inner()).take();
+            if let Some(hook) = hook {
+                let _ = hook.reached.send(());
+                hook.resume.wait();
+            }
+        }
         for p in paths {
             if std::fs::metadata(&p).map(|m| m.len() > 40 << 20).unwrap_or(true) {
                 continue;
@@ -1202,7 +1231,10 @@ impl FontDb {
                 return true;
             }
         }
-        self.sys.lock().unwrap_or_else(|e| e.into_inner()).misses.insert(c);
+        let mut sys = self.sys.lock().unwrap_or_else(|e| e.into_inner());
+        if sys.generation == generation {
+            sys.misses.insert(c);
+        }
         false
     }
 

crates/fonts/src/tests_sysfonts.rs

@@ -164,3 +164,82 @@ fn font_menus_group_installed_and_loaded_fonts_without_loading_them() {
     assert_eq!(infos.iter().filter(|i| i.family == "Groups Mincho").count(), 1);
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn rescanning_retries_missing_glyphs_and_absent_fallback_families() {
+    use crate::testing::font_with;
+    let dir = font_dir("rescan-fallback-ttf");
+    // Only synthetic TTF files in this folder are scanned. Keep the loaded Latin fallback
+    // alone so optional craft-fonts cannot provide the characters used by this fixture.
+    let db = FontDb::with_font_dirs(vec![dir.clone()]);
+    db.faces.write().unwrap().retain(|f| f.family == FALLBACK_FAMILY);
+    let c = '\u{1e900}';
+    assert!(db.fallback_for(c, 0, None).is_none());
+    // A loaded non-chain face supplies Hangul while all installed Korean chain faces are absent.
+    db.add_font(font_with("Earlier Korean Fallback", &['가']).unwrap());
+    let chain_family = "Noto Serif CJK KR";
+    assert!(!db.has_family(chain_family));
+    assert_eq!(db.fallback_for('가', 0, Some("ko")).unwrap().family, "Earlier Korean Fallback");
+    let family = "Installed After Glyph Miss";
+    std::fs::write(dir.join("Later.ttf"), font_with(family, &[c]).unwrap()).unwrap();
+    std::fs::write(dir.join("LaterKorean.ttf"), font_with(chain_family, &['가']).unwrap()).unwrap();
+    // Before an explicit rescan, both fallbacks still use the previous catalog.
+    assert!(db.fallback_for(c, 0, None).is_none());
+    assert_eq!(db.fallback_for('가', 0, Some("ko")).unwrap().family, "Earlier Korean Fallback");
+    assert_eq!(db.load_system_fonts(), 4);
+    assert!(db.has_family(family) && db.has_family(chain_family));
+    assert_eq!(db.fallback_for(c, 0, None).unwrap().family, family);
+    assert_eq!(db.fallback_for('가', 0, Some("ko")).unwrap().family, chain_family);
+    std::fs::remove_dir_all(dir).unwrap();
+}
+
+#[cfg(target_os = "linux")]
+#[test]
+fn a_rescan_does_not_cache_an_older_in_flight_glyph_miss() {
+    use crate::testing::font_with;
+    use std::sync::{Barrier, mpsc};
+    use std::time::Duration;
+
+    let dir = font_dir("rescan-in-flight-ttf");
+    let db = Arc::new(FontDb::with_font_dirs(vec![dir.clone()]));
+    // Optional craft-fonts must not supply the test glyph from an already loaded face.
+    db.faces.write().unwrap().retain(|f| f.family == FALLBACK_FAMILY);
+    let c = '\u{1e900}';
+    let family = "Installed During Glyph Lookup";
+    let font = font_with(family, &[c]).unwrap();
+    assert!(db.read_faces().iter().all(|f| !f.covers(c)));
+    assert_eq!(db.load_system_fonts(), 2);
+
+    let deadline = Duration::from_secs(10);
+    let (reached_tx, reached_rx) = mpsc::sync_channel(1);
+    let resume = Arc::new(Barrier::new(2));
+    *db.fallback_snapshot_hook.lock().unwrap() = Some(FallbackSnapshotHook { reached: reached_tx, resume: resume.clone() });
+    let (lookup_tx, lookup_rx) = mpsc::sync_channel(1);
+    let lookup_db = db.clone();
+    let lookup = std::thread::spawn(move || {
+        let face = lookup_db.fallback_for(c, 0, None).map(|f| f.family.clone());
+        let _ = lookup_tx.send(face);
+    });
+    reached_rx.recv_timeout(deadline).expect("the lookup must reach its old catalog snapshot");
+
+    // Finish the rescan while the old lookup is paused. A timeout also detects accidentally
+    // holding sys/catalog locks at the hook; release the lookup before reporting that failure.
+    let (scan_tx, scan_rx) = mpsc::sync_channel(1);
+    let scan_db = db.clone();
+    let scan = std::thread::spawn(move || {
+        let result = std::fs::write(dir.join("Later.ttf"), font).map(|()| scan_db.load_system_fonts());
+        let _ = scan_tx.send(result);
+    });
+    let scanned = scan_rx.recv_timeout(deadline);
+    resume.wait();
+    let old_face = lookup_rx.recv_timeout(deadline).expect("the old lookup must finish after release");
+    lookup.join().unwrap();
+    assert_eq!(scanned.expect("the rescan must finish before releasing the old lookup").unwrap(), 3);
+    scan.join().unwrap();
+    assert!(old_face.is_none(), "the paused lookup only searched the old paths");
+    assert!(db.has_family(family));
+    assert!(!db.is_loaded(family), "a rescan catalogs the font without loading it");
+    // The old miss must not hide the new catalog entry; no second rescan is performed.
+    assert_eq!(db.fallback_for(c, 0, None).unwrap().family, family);
+    std::fs::remove_dir_all(db.font_dirs.first().unwrap()).unwrap();
+}