Estimate reduced JPEG warm reservations

crates/images/src/jpeg.rs

@@ -65,6 +65,36 @@ pub(crate) fn decode_cmyk(bytes: &[u8]) -> Option<(CmykRaster, Option<Vec<u8>>)>
 /// The largest reduction [`decode_reduced`] reads straight from a JPEG's 8×8 blocks: 1/8.
 pub(crate) const MAX_SHRINK: u8 = 3;
 
+/// Output size of a JPEG reduction without decoding pixels. `None` when the reduced decoder
+/// cannot handle the JPEG, including its full-size resource limits.
+pub(crate) fn reduced_size(bytes: &[u8], shrink: u8) -> Option<(u32, u32)> {
+    use jpeg_decoder::{CodingProcess, PixelFormat};
+    if !bytes.starts_with(&[0xFF, 0xD8]) || shrink == 0 || shrink > MAX_SHRINK {
+        return None;
+    }
+    let mut d = jpeg_decoder::Decoder::new(bytes);
+    d.set_max_decoding_buffer_size(MAX_BYTES);
+    d.read_info().ok()?;
+    let info = d.info()?;
+    let colour = match info.pixel_format {
+        PixelFormat::RGB24 => true,
+        PixelFormat::L8 => false,
+        PixelFormat::L16 | PixelFormat::CMYK32 => return None,
+    };
+    if crate::over_rgba_budget(u32::from(info.width), u32::from(info.height)) {
+        return None;
+    }
+    if info.coding_process == CodingProcess::DctProgressive {
+        let side = |n: u16| u64::from(n).next_multiple_of(32);
+        let components: u64 = if colour { 3 } else { 1 };
+        if side(info.width) * side(info.height) * components * 2 > MAX_BYTES as u64 {
+            return None;
+        }
+    }
+    let reduced = |n: u16| u32::from(n).div_ceil(1 << shrink);
+    Some((reduced(info.width), reduced(info.height)))
+}
+
 /// A YCbCr, RGB or grayscale JPEG decoded at 1/2^`shrink` of its size, each side rounded up, for
 /// `shrink` 1 to [`MAX_SHRINK`]: straight RGBA and whether the image has colour. `None` for CMYK,
 /// 12-bit and lossless JPEGs, other formats and anything that doesn't decode. `None` as well for a

crates/images/src/lib.rs

@@ -510,6 +510,12 @@ pub fn decode_rgba_reduced(bytes: &[u8], shrink: u8) -> Option<image::RgbaImage>
     to_display_rgba(px, Some(&icc), &DisplayColour::active())
 }
 
+/// Dimensions of a JPEG decoded at `1/2^shrink`, without decoding its pixels. `None` when the
+/// reduced decoder cannot handle the image.
+pub fn reduced_jpeg_size(bytes: &[u8], shrink: u8) -> Option<(u32, u32)> {
+    jpeg::reduced_size(bytes, shrink)
+}
+
 /// False when `bytes` is a WebP whose `ICCP` chunk declares more bytes than the file holds or than
 /// [`colour::MAX_ICC_BYTES`]. The decoder sizes its buffer from the declared length before it reads
 /// anything, so such a file must not reach `icc_profile()`; its pixels decode without it.

crates/render/src/images.rs

@@ -200,14 +200,11 @@ fn reduced_estimates(wants: &[Want]) -> Option<(usize, usize)> {
             return None;
         }
         let shrink = level.min(usize::from(designcraft_images::MAX_REDUCED_SHRINK));
+        let (decoded_w, decoded_h) = designcraft_images::reduced_jpeg_size(&w.bytes, shrink as u8)?;
         for cached in shrink..=level {
             levels[cached] = true;
         }
-        let denom = 1u32 << shrink;
-        let (w_px, h_px) = sizes[0];
-        // The reduced decoder rounds each side up before the mip is cropped to `sizes[shrink]`.
-        let decode_pixels = w_px.div_ceil(denom) as usize * h_px.div_ceil(denom) as usize;
-        peak = peak.max(decode_pixels.saturating_mul(8));
+        peak = peak.max(decoded_w as usize * decoded_h as usize * 8);
         if dims.as_ref().is_none_or(|d: &Vec<(u32, u32)>| d.len() < sizes.len()) {
             dims = Some(sizes);
         }