images: a WMF-only DOS EPS is not read as a TIFF preview

The DOS EPS header is 30 bytes: WMF offsets at 12/16, TIFF at 20/24,
checksum at 28-29 (EPSF 3.0, section 5.2). Reading the TIFF preview from
the WMF slot stopped standard previews from loading; the offsets and the
fixture are back where the spec puts them, and a WMF-only file now has a
regression test.

crates/images/src/eps.rs

@@ -14,13 +14,10 @@ fn u32le(b: &[u8], at: usize) -> Option<usize> {
 }
 
 /// The PostScript section and the TIFF preview (DOS EPS).
-///
-/// The binary header is 22 bytes: the magic (0xC5D0, 0xD3C6), `psStart`, `psLength`,
-/// `tiffStart`, `tiffLength`, then a two-byte checksum.
 fn sections(bytes: &[u8]) -> (&[u8], Option<&[u8]>) {
     if bytes.starts_with(&[0xC5, 0xD0, 0xD3, 0xC6]) {
         let ps = u32le(bytes, 4).zip(u32le(bytes, 8)).and_then(|(o, l)| bytes.get(o..o.checked_add(l)?)).unwrap_or(&[]);
-        let tiff = u32le(bytes, 12).zip(u32le(bytes, 16)).filter(|(o, l)| *o > 0 && *l > 0).and_then(|(o, l)| bytes.get(o..o.checked_add(l)?));
+        let tiff = u32le(bytes, 20).zip(u32le(bytes, 24)).filter(|(o, l)| *o > 0 && *l > 0).and_then(|(o, l)| bytes.get(o..o.checked_add(l)?));
         return (ps, tiff);
     }
     (bytes, None)
@@ -227,16 +224,15 @@ mod tests {
         let (png, size) = eps_proxy(eps).unwrap();
         assert_eq!(size, (100.5, 50.25));
         assert_eq!(crate::pixel_size(&png), Some((201, 101)));
-        // DOS EPS with a TIFF preview: the preview is the proxy. The binary header is 22 bytes:
-        // the magic, psStart, psLength, tiffStart, tiffLength, then a two-byte checksum.
+        // DOS EPS with a TIFF preview: the preview is the proxy.
         let mut tiff = Vec::new();
         image::DynamicImage::ImageRgba8(image::RgbaImage::from_pixel(10, 5, image::Rgba([255, 0, 0, 255])))
             .write_to(&mut std::io::Cursor::new(&mut tiff), image::ImageFormat::Tiff)
             .unwrap();
         let mut dos = vec![0xC5, 0xD0, 0xD3, 0xC6];
-        let ps_off = 22usize;
+        let ps_off = 30usize;
         let tiff_off = ps_off + eps.len();
-        for v in [ps_off, eps.len(), tiff_off, tiff.len()] {
+        for v in [ps_off, eps.len(), 0, 0, tiff_off, tiff.len()] {
             dos.extend((v as u32).to_le_bytes());
         }
         dos.extend([0xFF, 0xFF]);
@@ -249,6 +245,29 @@ mod tests {
         assert!(!is_eps(b"%!PS-Adobe-3.0\nnot encapsulated"));
     }
 
+    #[test]
+    fn dos_eps_wmf_only_is_not_read_as_tiff() {
+        let eps = b"%!PS-Adobe-3.0 EPSF-3.0\n%%BoundingBox: 0 0 100 50\nnewpath\n%%EOF\n";
+        let wmf = b"WMFDATA!";
+        let mut dos = vec![0xC5, 0xD0, 0xD3, 0xC6];
+        let ps_off = 30usize;
+        let wmf_off = ps_off + eps.len();
+        for v in [ps_off, eps.len(), wmf_off, wmf.len(), 0, 0] {
+            dos.extend((v as u32).to_le_bytes());
+        }
+        dos.extend([0xFF, 0xFF]);
+        dos.extend_from_slice(eps);
+        dos.extend_from_slice(wmf);
+        assert!(is_eps(&dos));
+        // The WMF bytes are not a TIFF preview.
+        assert!(sections(&dos).1.is_none());
+        // The PostScript section still parses, and the proxy is the no-preview placeholder.
+        assert_eq!(bounding_box(&dos), Some([0.0, 0.0, 100.0, 50.0]));
+        let (png, size) = eps_proxy(&dos).unwrap();
+        assert_eq!(size, (100.0, 50.0));
+        assert_eq!(crate::pixel_size(&png), Some((200, 100)));
+    }
+
     #[test]
     fn eps_bounding_box_rejects_nonfinite_sizes() {
         let bb = |line: &str| bounding_box(format!("%!PS-Adobe-3.0 EPSF-3.0\n{line}\n").as_bytes());