address review: drop media copied by a rolled-back diagram fallback or parent drawing

crates/pptx/src/read.rs

@@ -76,6 +76,15 @@ impl Imp<'_> {
         self.media_by_part.insert(key, id);
         Some(id)
     }
+    /// Drop the media copied since the list was `len` long, with their part-cache entries: used when
+    /// the content that read them is discarded, so they are neither kept nor reused uncharged.
+    pub fn forget_media_from(&mut self, len: usize) {
+        if self.media.len() > len {
+            let gone: std::collections::HashSet<MediaId> = self.media[len..].iter().map(|m| m.id).collect();
+            self.media.truncate(len);
+            self.media_by_part.retain(|_, id| !gone.contains(id));
+        }
+    }
     /// Media item for an external link.
     pub fn media_link(&mut self, url: &str) -> MediaId {
         if let Some(m) = self.media.iter().find(|m| m.link.as_deref() == Some(url)) {

crates/pptx/src/read/shapes.rs

@@ -471,7 +471,7 @@ fn frame(imp: &mut Imp, part: &Part, ctx: &mut IdCtx, e: &El, depth: usize) -> S
     };
     // Keep the parts it points to so a save writes it back whole (#203). A part with relationships
     // of its own, or an external link, can't be carried: the save then falls back to the preview.
-    let xml_before = imp.diagram_xml_bytes;
+    let (xml_before, media_before) = (imp.diagram_xml_bytes, imp.media.len());
     let parts = super::rel_refs(e)
         .into_iter()
         .map(|id| {
@@ -496,6 +496,7 @@ fn frame(imp: &mut Imp, part: &Part, ctx: &mut IdCtx, e: &El, depth: usize) -> S
     // A fallback that drops its parts does not keep their bytes charged.
     if parts.is_none() {
         imp.diagram_xml_bytes = xml_before;
+        imp.forget_media_from(media_before);
     }
     let parts = parts.unwrap_or_default();
     s.kind = ShapeKind::Opaque { xml: part.keep(e), preview, label, parts };
@@ -601,7 +602,7 @@ fn smartart(imp: &mut Imp, part: &Part, gd: &El, frame: &Shape, depth: usize) ->
     let tree = d.root.child("spTree")?;
     let mut sub = IdCtx { fresh: true, ..Default::default() };
     let before = imp.smartart_shapes;
-    let xml_before = imp.diagram_xml_bytes;
+    let (xml_before, media_before) = (imp.diagram_xml_bytes, imp.media.len());
     let children = sp_tree(imp, &dpart, &mut sub, tree, depth + 1);
     if children.is_empty() {
         return None;
@@ -612,6 +613,7 @@ fn smartart(imp: &mut Imp, part: &Part, gd: &El, frame: &Shape, depth: usize) ->
         // Over the presentation's remaining budget: a placeholder, and nothing charged, nested SmartArt included.
         imp.smartart_shapes = before;
         imp.diagram_xml_bytes = xml_before;
+        imp.forget_media_from(media_before);
         return None;
     }
     imp.smartart_shapes = total;