fix: kitty graphics streams freeze on partial updates and leak host images (#948)

* fix: request a render on kitty graphics writes

pty output containing a kitty graphics sequence only scheduled the
deferred settle render. that render's wake-up notifies the render loop
only when the dirty flag was not already set, so under a continuous
stream of image updates the notify can be dropped and the pane keeps
showing an old frame until something else forces a full render.

request an immediate render like any other output. the dirty flag and
the minimum render interval still coalesce chunked writes, and the
settle render stays as a backstop.

refs #947

* fix: fingerprint kitty images exactly across retransmissions

the image fingerprint hashed only the first, middle, and last 4096
bytes of pixel data. a program that streams updates by retransmitting
one image id keeps the length and dimensions constant, so a frame whose
changed pixels all fall outside those three windows gets the same
fingerprint as the frame before it. the encoder then treats the image
as already uploaded, emits no graphics bytes, and the frame is dropped
as identical to the previous render. the pane stays stuck on the last
uploaded image until a resize resets the graphics surface.

hash the full payload instead. to keep that off the render hot path,
cache the fingerprint per image id and recompute it only when the
image's transmit time changes. libghostty-vt already refreshes the
transmit time on every transmission; vendor patch 0002 exposes it
through a new GHOSTTY_KITTY_IMAGE_DATA_TRANSMIT_TIME_NS accessor. a
static image costs one map lookup per render and a streaming image one
hash per transmitted frame.

refs #947

* fix: delete superseded host kitty images

host image ids derive from image content, so a pane image whose pixels
change maps to a new host id on every update. nothing deleted the host
image it replaced, and a streaming pane left one full-size image behind
in the host terminal per changed frame until the graphics surface was
reset. track which host image backs each pane image and delete the
replaced one once no source references it.

refs #947

* fix: prune stale kitty graphics source entries

a source entry whose pane closed or whose placement disappeared stayed
in the cache forever. through the shared-reference check it could keep
an old host image alive that nothing visible used anymore, and the
entries themselves accumulated without bound. prune sources against
the visible set at the start of every encode pass.

refs #947

* docs: link issue and upstream discussion for vendor patch 0002

the transmit time accessor now has a removal path: the ghostty
discussion proposing it upstream is linked in the patch ledger.

refs #947

---------

Co-authored-by: Can Celik <ogulcancelik@gmail.com>

1 files changed, 130 insertions, 41 deletions

diff --git a/src/ghostty/mod.rs b/src/ghostty/mod.rs
index e79c9a16..97b22f59 100644
--- a/src/ghostty/mod.rs
+++ b/src/ghostty/mod.rs
@@ -11,7 +11,7 @@
 pub mod bindings;
 
 use std::collections::hash_map::DefaultHasher;
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::ffi::c_void;
 use std::fmt;
 use std::hash::{Hash, Hasher};
@@ -21,7 +21,7 @@ use std::ops::RangeInclusive;
 use std::os::raw::c_char;
 use std::ptr;
 use std::slice;
-use std::sync::{Once, OnceLock};
+use std::sync::{Mutex, Once, OnceLock};
 
 pub use bindings as ffi;
 
@@ -170,10 +170,6 @@ const TERMINAL_DATA_COLOR_CURSOR: ffi::GhosttyTerminalData = 20;
 const KITTY_IMAGE_STORAGE_LIMIT_BYTES: u64 = 64 * 1024 * 1024;
 const APC_MAX_BYTES: usize = 16 * 1024 * 1024;
 const APC_MAX_BYTES_KITTY: usize = 16 * 1024 * 1024;
-// Kitty image fingerprints are used as a display cache key, not a
-// cryptographic identity. Sampling keeps redraws cheap for multi-megabyte
-// images while still distinguishing normal screenshots/photos/diagrams.
-const KITTY_FINGERPRINT_SAMPLE_BYTES: usize = 4096;
 pub(crate) const KITTY_UNICODE_PLACEHOLDER: u32 = 0x10EEEE;
 // The vendored C headers expose these placement fields, but the checked-in
 // generated bindings predate the names. Keep the explicit values aligned with
@@ -219,6 +215,14 @@ pub struct KittyImageDescriptor {
     pub data_fingerprint: u64,
 }
 
+#[derive(Debug, Clone, Copy)]
+struct KittyImageFingerprintEntry {
+    transmit_time_ns: u64,
+    data_ptr: usize,
+    data_len: usize,
+    fingerprint: u64,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct KittyPlacementRenderInfo {
     pub pixel_width: u32,
@@ -568,6 +572,7 @@ pub fn encode_focus(event: FocusEvent) -> Result<Vec<u8>, Error> {
 pub struct Terminal {
     raw: ffi::GhosttyTerminal_ptr,
     write_pty_callback: Option<Box<WritePtyCallbackState>>,
+    kitty_fingerprints: Mutex<HashMap<u32, KittyImageFingerprintEntry>>,
 }
 
 impl Terminal {
@@ -585,6 +590,7 @@ impl Terminal {
         Ok(Self {
             raw,
             write_pty_callback: None,
+            kitty_fingerprints: Mutex::new(HashMap::new()),
         })
     }
 
@@ -1139,9 +1145,69 @@ impl Terminal {
         }
         placements.extend(self.kitty_virtual_image_placements(graphics, &mut needs_data)?);
         placements.sort_by_key(|placement| placement.z);
+        self.prune_kitty_fingerprints(&placements);
         Ok(placements)
     }
 
+    /// Fingerprint for `image`, cached per image id and recomputed only when
+    /// the image's transmit time (or data identity) changes.
+    fn kitty_image_fingerprint_cached(
+        &self,
+        image: ffi::GhosttyKittyGraphicsImage,
+        image_id: u32,
+        data: (*const u8, usize),
+        image_width: u32,
+        image_height: u32,
+        format: KittyImageFormat,
+    ) -> u64 {
+        let (data_ptr, data_len) = data;
+        let Ok(transmit_time_ns) = kitty_image_u64(
+            image,
+            ffi::GhosttyKittyGraphicsImageData_GHOSTTY_KITTY_IMAGE_DATA_TRANSMIT_TIME_NS,
+        ) else {
+            return kitty_image_fingerprint(data_ptr, data_len, image_width, image_height, format);
+        };
+
+        if let Ok(cache) = self.kitty_fingerprints.lock() {
+            if let Some(entry) = cache.get(&image_id) {
+                if entry.transmit_time_ns == transmit_time_ns
+                    && entry.data_ptr == data_ptr as usize
+                    && entry.data_len == data_len
+                {
+                    return entry.fingerprint;
+                }
+            }
+        }
+
+        let fingerprint =
+            kitty_image_fingerprint(data_ptr, data_len, image_width, image_height, format);
+        if let Ok(mut cache) = self.kitty_fingerprints.lock() {
+            cache.insert(
+                image_id,
+                KittyImageFingerprintEntry {
+                    transmit_time_ns,
+                    data_ptr: data_ptr as usize,
+                    data_len,
+                    fingerprint,
+                },
+            );
+        }
+        fingerprint
+    }
+
+    fn prune_kitty_fingerprints(&self, placements: &[KittyImagePlacement]) {
+        if let Ok(mut cache) = self.kitty_fingerprints.lock() {
+            if cache.is_empty() {
+                return;
+            }
+            let live: HashSet<u32> = placements
+                .iter()
+                .map(|placement| placement.image_id)
+                .collect();
+            cache.retain(|image_id, _| live.contains(image_id));
+        }
+    }
+
     fn kitty_image_placement<F>(
         &self,
         graphics: ffi::GhosttyKittyGraphics,
@@ -1198,8 +1264,14 @@ impl Terminal {
             ffi::GhosttyKittyGraphicsPlacementData_GHOSTTY_KITTY_GRAPHICS_PLACEMENT_DATA_PLACEMENT_ID,
         )?;
         let (data_ptr, data_len) = kitty_image_data_ptr_len(image)?;
-        let data_fingerprint =
-            kitty_image_fingerprint(data_ptr, data_len, image_width, image_height, format);
+        let data_fingerprint = self.kitty_image_fingerprint_cached(
+            image,
+            image_id,
+            (data_ptr, data_len),
+            image_width,
+            image_height,
+            format,
+        );
         let descriptor = KittyImageDescriptor {
             image_id,
             placement_id,
@@ -1336,8 +1408,14 @@ impl Terminal {
             };
             let placement_id = run.synthetic_placement_id();
             let (data_ptr, data_len) = kitty_image_data_ptr_len(image)?;
-            let data_fingerprint =
-                kitty_image_fingerprint(data_ptr, data_len, image_width, image_height, format);
+            let data_fingerprint = self.kitty_image_fingerprint_cached(
+                image,
+                image_id,
+                (data_ptr, data_len),
+                image_width,
+                image_height,
+                format,
+            );
             let descriptor = KittyImageDescriptor {
                 image_id,
                 placement_id,
@@ -1750,6 +1828,18 @@ fn kitty_image_u32(
     Ok(out)
 }
 
+fn kitty_image_u64(
+    image: ffi::GhosttyKittyGraphicsImage,
+    data: ffi::GhosttyKittyGraphicsImageData,
+) -> Result<u64, Error> {
+    let mut out = 0u64;
+    unsafe {
+        ffi::ghostty_kitty_graphics_image_get(image, data, (&mut out as *mut u64).cast())
+            .into_result()?;
+    }
+    Ok(out)
+}
+
 fn kitty_image_format(image: ffi::GhosttyKittyGraphicsImage) -> Result<KittyImageFormat, Error> {
     let mut out = ffi::GhosttyKittyImageFormat_GHOSTTY_KITTY_IMAGE_FORMAT_RGBA;
     unsafe {
@@ -1812,6 +1902,8 @@ fn kitty_image_data_from_ptr(ptr_out: *const u8, len: usize) -> Vec<u8> {
     unsafe { slice::from_raw_parts(ptr_out, len) }.to_vec()
 }
 
+// Hashes the full payload. Callers cache the result per image id and only
+// recompute it when the image's transmit time changes.
 fn kitty_image_fingerprint(
     ptr_out: *const u8,
     len: usize,
@@ -1829,23 +1921,10 @@ fn kitty_image_fingerprint(
     }
 
     let data = unsafe { slice::from_raw_parts(ptr_out, len) };
-    hash_kitty_image_data_sample(&mut hasher, data);
+    data.hash(&mut hasher);
     hasher.finish()
 }
 
-fn hash_kitty_image_data_sample(hasher: &mut impl Hasher, data: &[u8]) {
-    let sample = KITTY_FINGERPRINT_SAMPLE_BYTES;
-    if data.len() <= sample * 3 {
-        data.hash(hasher);
-        return;
-    }
-
-    data[..sample].hash(hasher);
-    let middle_start = (data.len() / 2).saturating_sub(sample / 2);
-    data[middle_start..middle_start + sample].hash(hasher);
-    data[data.len() - sample..].hash(hasher);
-}
-
 fn grid_ref_hyperlink_uri(grid_ref: &ffi::GhosttyGridRef) -> Result<Option<String>, Error> {
     let mut required = 0usize;
     let result =
@@ -2735,29 +2814,39 @@ mod tests {
     }
 
     #[test]
-    fn kitty_image_fingerprint_samples_large_payloads() {
-        let mut data = vec![1u8; KITTY_FINGERPRINT_SAMPLE_BYTES * 4];
+    fn kitty_image_fingerprint_covers_full_payload() {
+        let mut data = vec![1u8; 4096 * 4];
         let original =
             kitty_image_fingerprint(data.as_ptr(), data.len(), 100, 50, KittyImageFormat::Png);
 
-        data[KITTY_FINGERPRINT_SAMPLE_BYTES / 2] = 2;
-        let changed_prefix =
+        data[4096 + 123] = 2;
+        let changed_outside_sampled_windows =
             kitty_image_fingerprint(data.as_ptr(), data.len(), 100, 50, KittyImageFormat::Png);
-        assert_ne!(original, changed_prefix);
+        assert_ne!(original, changed_outside_sampled_windows);
+    }
 
-        data[KITTY_FINGERPRINT_SAMPLE_BYTES / 2] = 1;
-        let middle = data.len() / 2;
-        data[middle] = 3;
-        let changed_middle =
-            kitty_image_fingerprint(data.as_ptr(), data.len(), 100, 50, KittyImageFormat::Png);
-        assert_ne!(original, changed_middle);
+    #[test]
+    fn kitty_image_fingerprint_refreshes_on_retransmission() {
+        let mut terminal = Terminal::new(10, 5, 0).unwrap();
+        terminal.write(b"\x1b_Ga=T,f=32,t=d,i=7,p=3,s=1,v=1,c=10,r=5,q=2;/wAA/w==\x1b\\");
+        let first = terminal
+            .kitty_image_placements_with_data_filter(|_| true)
+            .unwrap();
+        assert_eq!(first.len(), 1);
 
-        data[middle] = 1;
-        let last = data.len() - 1;
-        data[last] = 4;
-        let changed_suffix =
-            kitty_image_fingerprint(data.as_ptr(), data.len(), 100, 50, KittyImageFormat::Png);
-        assert_ne!(original, changed_suffix);
+        // Same id and size, different pixels.
+        terminal.write(b"\x1b_Ga=t,f=32,t=d,i=7,s=1,v=1,q=2;AAAAAA==\x1b\\");
+        let second = terminal
+            .kitty_image_placements_with_data_filter(|_| true)
+            .unwrap();
+        assert_eq!(second.len(), 1);
+        assert_ne!(first[0].data_fingerprint, second[0].data_fingerprint);
+
+        // No retransmission, so the fingerprint stays stable across renders.
+        let third = terminal
+            .kitty_image_placements_with_data_filter(|_| true)
+            .unwrap();
+        assert_eq!(second[0].data_fingerprint, third[0].data_fingerprint);
     }
 
     #[test]