MCP: an agent's folders hold for file loads through the app and for a design's font files (#1112)

doc.open and the STEP/mesh imports run on the file loader's worker with the request's path scope, and a font named by file path in a design is checked against the scope before it is read.

crates/sketch/src/fonts.rs

@@ -24,6 +24,15 @@ const MAX_CACHED_BYTES: usize = 128 << 20;
 /// Longest font name or path accepted.
 const MAX_FONT_NAME: usize = 1024;
 
+/// Asked before a font file named by its path is read. A design names such files, and an
+/// agent's folders (`vfs::PathScope`) live above this crate; the first check set stays.
+static FILE_CHECK: OnceLock<fn(&Path) -> std::result::Result<(), String>> = OnceLock::new();
+
+/// Set the check [`font_data`] runs on a font file's path before reading it.
+pub fn set_font_file_check(check: fn(&Path) -> std::result::Result<(), String>) {
+    let _ = FILE_CHECK.set(check);
+}
+
 /// The bytes of a font file and the face to use in it (collections hold several faces).
 #[derive(Clone)]
 pub struct FontData {
@@ -59,6 +68,10 @@ pub fn font_data(name: Option<&str>) -> Result<FontData> {
     }
     if is_path(name) {
         let path = expand(name);
+        // Before the cache too: a file read once for the user is not the agent's to read.
+        if let Some(check) = FILE_CHECK.get() {
+            check(&path).map_err(|e| SketchError::Invalid(format!("font file `{name}`: {e}")))?;
+        }
         let bytes = read_font(&path).map_err(|e| SketchError::Invalid(format!("font file `{name}`: {e}")))?;
         let index = 0;
         ttf_parser::Face::parse(&bytes, index)
@@ -306,13 +319,6 @@ fn normalize(s: &str) -> String {
     s.chars().filter(|c| c.is_alphanumeric()).flat_map(char::to_lowercase).collect()
 }
 
-/// The file a font name refers to (after `~` and `%VAR%` expansion), or `None` for the built-in
-/// font or an installed family, so callers can vet the path before [`font_data`] reads it.
-pub fn font_file_path(name: &str) -> Option<PathBuf> {
-    let name = name.trim();
-    (is_path(name) && name.len() <= MAX_FONT_NAME).then(|| expand(name))
-}
-
 fn is_path(name: &str) -> bool {
     name.contains('/') || name.contains('\\') || has_font_extension(name)
 }

crates/sketch/src/lib.rs

@@ -43,7 +43,7 @@ pub use curves::{
 };
 pub use decompose::Budget;
 pub use dims::{DimFrame, DimLayout, chain_centres, default_text, dim_frame, dim_layout, encode_text};
-pub use fonts::{DEFAULT_FONT_FAMILY, FontData, families_in, find_style_in, font_data, font_data_styled, font_file_path, installed_families};
+pub use fonts::{DEFAULT_FONT_FAMILY, FontData, families_in, find_style_in, font_data, font_data_styled, installed_families, set_font_file_check};
 pub use freeform_offset::{FreeformOffset, OFFSET_TOLERANCE};
 pub use link::{Link, LinkGeom, LinkKind, LinkSource, MAX_LINK_ENTITIES, SKETCH_AXIS_HALF, sketch_axis_geom};
 pub use model::{