Refuse encoding unverified glyphs after a font decode allowance refusal

crates/fonts/src/pdf.rs

@@ -979,8 +979,14 @@ impl Metrics {
     }
 
     /// The bytes that show `text` in this font, or `None` if some character has no code or
-    /// no glyph in it (the caller then substitutes another font).
+    /// no glyph in it, or the metrics were read past their allowance (the caller then substitutes
+    /// another font). Empty text needs no verified glyph and always encodes.
     pub fn encode(&self, text: &str) -> Option<Vec<u8>> {
+        // A refused read cannot establish the mappings or glyphs needed to write new text.
+        // Empty text needs neither and remains usable for erasing existing text.
+        if self.past_allowance && !text.is_empty() {
+            return None;
+        }
         // The lowest code showing each wanted character. Explicit entries and the encoding are
         // checked one by one; each range is asked for the wanted characters it can show.
         let wanted: HashSet<char> = text.chars().collect();

crates/fonts/src/pdf/tests.rs

@@ -1173,3 +1173,26 @@ fn low_codes_resolve_to_the_covering_range() {
     assert_eq!(covered, [5, 6, 7, 100, 101]);
     assert_eq!((u.covering(0xFFFF).is_some(), u.covering(0x1_0001).is_some(), u.covering(0x1_0002).is_none()), (true, true, true));
 }
+
+#[test]
+fn refused_subset_reads_do_not_encode_unverified_glyphs() {
+    let mut doc = Document::new_empty();
+    let (mut f, sizes) = subset_type0(&mut doc, b"");
+    f.set(b"Encoding".to_vec(), Object::name("Identity-H"));
+    f.set(b"ToUnicode".to_vec(), Object::name("Identity-H"));
+    if let Some(Object::Array(descendants)) = f.get_mut(b"DescendantFonts")
+        && let Some(Object::Dict(descendant)) = descendants.first_mut()
+    {
+        descendant.set(b"DW".to_vec(), Object::Int(1000));
+    }
+    let whole = Metrics::from_dict(&doc, &f);
+    assert!(!whole.past_allowance && !whole.outlined_cids.is_empty());
+    assert!(whole.encode("\u{ffff}").is_none());
+    // Refuse the program, then the CID map after reading the complete program.
+    for allowance in [0, sizes[1]] {
+        let partial = Metrics::from_dict_within(&doc, &f, &mut DecodeBudget::new(allowance, 0));
+        assert!(partial.past_allowance);
+        assert_eq!(partial.encode(""), Some(Vec::new()));
+        assert!(partial.encode("\u{ffff}").is_none(), "allowance {allowance} must not turn /DW into proof of a glyph");
+    }
+}