fix: avoid resending terminal cells for metadata updates (#4137)

1 files changed, 127 insertions, 0 deletions

diff --git a/src/protocol/surface_reuse.rs b/src/protocol/surface_reuse.rs
new file mode 100644
index 00000000..bba85824
--- /dev/null
+++ b/src/protocol/surface_reuse.rs
@@ -0,0 +1,127 @@
+//! Optional endpoint encoding that retains unchanged terminal cells across projections.
+
+use super::{CellData, PaneSurfaceFrame, ServerMessage};
+use serde::{Deserialize, Serialize};
+
+pub(crate) const CAPABILITY: &str = "surface_reuse";
+pub(crate) const MESSAGE_KIND: &str = "endpoint.surface-reuse.v1";
+
+#[derive(Serialize, Deserialize)]
+struct SurfaceReuse<S> {
+    base_surface_revision: u64,
+    surface: S,
+}
+
+pub(crate) fn message(
+    base_surface_revision: u64,
+    surface: &mut PaneSurfaceFrame,
+) -> serde_json::Result<Option<ServerMessage>> {
+    let cells = std::mem::take(&mut surface.frame.cells);
+    let data = serde_json::to_string(&SurfaceReuse {
+        base_surface_revision,
+        surface: &*surface,
+    });
+    surface.frame.cells = cells;
+    let message = ServerMessage::EndpointControl {
+        kind: MESSAGE_KIND.into(),
+        data: data?,
+    };
+    // JSON can expand non-cell data (for example escaped hyperlink URLs). A
+    // failed compact encoding must fall back, not strand a newer snapshot.
+    let size = bincode::serde::encode_into_std_write(
+        &message,
+        &mut std::io::sink(),
+        bincode::config::standard(),
+    );
+    match size {
+        Ok(size) => Ok((size <= super::MAX_FRAME_SIZE).then_some(message)),
+        Err(error) => {
+            tracing::warn!(%error, "failed to size surface reuse");
+            Ok(None)
+        }
+    }
+}
+
+#[derive(Default)]
+struct CellBaseline {
+    boot_id: String,
+    projection_revision: u64,
+    surface_revision: u64,
+    width: u16,
+    height: u16,
+    cells: Vec<CellData>,
+}
+
+/// Connection-local decoding happens before activation and presentation filtering, so
+/// switching endpoints cannot discard a baseline needed by the next wire message.
+#[derive(Default)]
+pub(crate) struct Decoder {
+    baseline: Option<CellBaseline>,
+}
+
+impl Decoder {
+    pub(crate) fn decode(&mut self, message: ServerMessage) -> Result<ServerMessage, String> {
+        let message = match message {
+            ServerMessage::EndpointControl { kind, data } if kind == MESSAGE_KIND => {
+                let reuse: SurfaceReuse<PaneSurfaceFrame> = serde_json::from_str(&data)
+                    .map_err(|error| format!("invalid surface reuse: {error}"))?;
+                let Some(base) = &mut self.baseline else {
+                    return Err("surface reuse without a baseline".into());
+                };
+                let mut surface = reuse.surface;
+                if base.boot_id != surface.boot_id
+                    || base.surface_revision != reuse.base_surface_revision
+                    || surface.surface_revision != base.surface_revision.saturating_add(1)
+                    || base.width != surface.frame.width
+                    || base.height != surface.frame.height
+                    || !surface.frame.cells.is_empty()
+                {
+                    return Err("surface reuse does not match its baseline".into());
+                }
+                surface.frame.cells.clone_from(&base.cells);
+                base.projection_revision = surface.projection_revision;
+                base.surface_revision = surface.surface_revision;
+                return Ok(ServerMessage::PaneSurface(surface));
+            }
+            message => message,
+        };
+        match &message {
+            ServerMessage::PaneSurface(surface) => {
+                let base = self.baseline.get_or_insert_with(CellBaseline::default);
+                base.boot_id.clone_from(&surface.boot_id);
+                base.projection_revision = surface.projection_revision;
+                base.surface_revision = surface.surface_revision;
+                base.width = surface.frame.width;
+                base.height = surface.frame.height;
+                base.cells.clone_from(&surface.frame.cells);
+            }
+            ServerMessage::PaneSurfacePatch(patch) => {
+                if let Some(base) = &mut self.baseline {
+                    if patch.boot_id != base.boot_id
+                        || patch.projection_revision != base.projection_revision
+                        || patch.base_surface_revision != base.surface_revision
+                        || patch.surface_revision != base.surface_revision.saturating_add(1)
+                    {
+                        self.baseline = None;
+                    } else {
+                        for row in &patch.rows {
+                            let start =
+                                usize::from(row.y) * usize::from(base.width) + usize::from(row.x);
+                            let end = start.saturating_add(row.cells.len());
+                            if row.y >= base.height
+                                || usize::from(row.x) + row.cells.len() > usize::from(base.width)
+                                || end > base.cells.len()
+                            {
+                                return Err("surface patch exceeds the cell baseline".into());
+                            }
+                            base.cells[start..end].clone_from_slice(&row.cells);
+                        }
+                        base.surface_revision = patch.surface_revision;
+                    }
+                }
+            }
+            _ => {}
+        }
+        Ok(message)
+    }
+}