mux: make action parser buffer size configurable and smaller

Previously, we'd use a 1MB buffer both to read the output from the
associated pty (blocking), and the same size buffer again to do the
non-blocking read on top of that.

For pathological cases (eg: cat 100MB+ files), we could build a
resulting `Vec<Action>` with over 1mm entries and it could take as much
as 100ms to apply those actions to the terminal model.

This meant that the output could stutter/lag and appear to be processed
more slowly.

This commit introduces a configuration value for the buffer size for the
second stage, and makes it 10KB in size.  This helps to constrain the
size of the Action vec and keeps the incremental processing costs down,
while still managing the same throughput.

config/src/lib.rs

@@ -885,6 +885,13 @@ pub struct Config {
     #[serde(default = "default_ratelimit_line_prefetches_per_second")]
     pub ratelimit_mux_line_prefetches_per_second: u32,
 
+    /// The buffer size used by parse_buffered_data in the mux module.
+    /// This should not be too large, otherwise the processing cost
+    /// of applying a batch of actions to the terminal will be too
+    /// high and the user experience will be laggy and less responsive.
+    #[serde(default = "default_mux_output_parser_buffer_size")]
+    pub mux_output_parser_buffer_size: usize,
+
     #[serde(default)]
     pub keys: Vec<Key>,
     #[serde(
@@ -1755,6 +1762,10 @@ impl Config {
     }
 }
 
+fn default_mux_output_parser_buffer_size() -> usize {
+    10 * 1024
+}
+
 fn default_ratelimit_line_prefetches_per_second() -> u32 {
     10
 }

mux/src/lib.rs

@@ -99,7 +99,7 @@ fn send_actions_to_mux(pane_id: PaneId, dead: &Arc<AtomicBool>, actions: Vec<Act
 }
 
 fn parse_buffered_data(pane_id: PaneId, dead: &Arc<AtomicBool>, mut rx: FileDescriptor) {
-    let mut buf = vec![0; BUFSIZE];
+    let mut buf = vec![0; configuration().mux_output_parser_buffer_size];
     let mut parser = termwiz::escape::parser::Parser::new();
     let mut actions = vec![];
     let mut hold = false;
@@ -149,6 +149,8 @@ fn parse_buffered_data(pane_id: PaneId, dead: &Arc<AtomicBool>, mut rx: FileDesc
                 if !actions.is_empty() && !hold {
                     send_actions_to_mux(pane_id, dead, std::mem::take(&mut actions));
                 }
+
+                buf.resize(configuration().mux_output_parser_buffer_size, 0);
             }
         }
     }