Fonts: the file search for an uncovered character stops at a budget between scans

crates/text/src/fontdb.rs

@@ -192,6 +192,17 @@ mod alias_tests {
         font
     }
 
+    #[cfg(not(target_arch = "wasm32"))]
+    #[test]
+    fn the_file_search_for_uncovered_characters_stops_at_its_budget() {
+        let db = FontDb::with_font_dirs(vec![]);
+        db.sys.lock().unwrap().file_searches = names::MAX_FILE_SEARCHES;
+        assert!(!db.system_fallback('\u{E000}'));
+        let sys = db.sys.lock().unwrap();
+        assert_eq!(sys.file_searches, names::MAX_FILE_SEARCHES);
+        assert!(!sys.misses.contains(&'\u{E000}'), "an unsearched character is not a remembered miss");
+    }
+
     #[test]
     fn a_collection_lists_at_most_max_faces() {
         let mut font = name_font(&[(3, 0x0409, 1, "Fam"), (3, 0x0409, 2, "Regular")]);

crates/text/src/fontdb/db.rs

@@ -456,6 +456,7 @@ impl FontDb {
             let mut sys = self.sys.lock().unwrap_or_else(|e| e.into_inner());
             sys.scans = sys.scans.wrapping_add(1);
             sys.misses.clear();
+            sys.file_searches = 0;
         }
         self.changed();
         n

crates/text/src/fontdb/lookup.rs

@@ -501,6 +501,13 @@ impl FontDb {
                 return true;
             }
         }
+        {
+            let mut sys = self.sys.lock().unwrap_or_else(|e| e.into_inner());
+            if sys.file_searches >= super::names::MAX_FILE_SEARCHES {
+                return false;
+            }
+            sys.file_searches += 1;
+        }
         let mut paths: Vec<PathBuf> = self.read_catalog().values().flat_map(|c| c.faces.iter().map(|cf| cf.path.clone())).collect();
         paths.sort();
         paths.dedup();

crates/text/src/fontdb/names.rs

@@ -7,8 +7,16 @@ pub(super) struct SysFallback {
     pub(super) misses: std::collections::HashSet<char>,
     /// Counts the scans of the font folders: a search that a scan overtook doesn't record its miss.
     pub(super) scans: u64,
+    /// Characters that went on to read every cataloged font file (and found nothing, or a face).
+    /// Each such search rereads the catalog, so only [`MAX_FILE_SEARCHES`] are made per scan.
+    pub(super) file_searches: u32,
 }
 
+/// The most characters searched for in the font files themselves between two scans of the font
+/// folders. Real text has a few uncovered characters; a document of thousands of distinct
+/// private-use code points would reread the whole catalog for each.
+pub(super) const MAX_FILE_SEARCHES: u32 = 1024;
+
 /// Fonts made for Thai, tried first (when installed) for Thai the font in use lacks: Windows
 /// (Leelawadee UI; Tahoma before Windows 8), macOS (Thonburi, Sukhumvit Set) and Linux (Noto,
 /// TLWG).