Include the desired version in a SaveBuffer RPC request

When handling this messages on the host, wait until the desired
version has been observed before performing the save.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/text/src/text.rs

@@ -21,7 +21,7 @@ use operation_queue::OperationQueue;
 pub use patch::Patch;
 pub use point::*;
 pub use point_utf16::*;
-use postage::{oneshot, prelude::*};
+use postage::{barrier, oneshot, prelude::*};
 #[cfg(any(test, feature = "test-support"))]
 pub use random_char_iter::*;
 use rope::TextDimension;
@@ -53,6 +53,7 @@ pub struct Buffer {
     pub lamport_clock: clock::Lamport,
     subscriptions: Topic,
     edit_id_resolvers: HashMap<clock::Local, Vec<oneshot::Sender<()>>>,
+    version_barriers: Vec<(clock::Global, barrier::Sender)>,
 }
 
 #[derive(Clone, Debug)]
@@ -574,6 +575,7 @@ impl Buffer {
             lamport_clock,
             subscriptions: Default::default(),
             edit_id_resolvers: Default::default(),
+            version_barriers: Default::default(),
         }
     }
 
@@ -835,6 +837,8 @@ impl Buffer {
                 }
             }
         }
+        self.version_barriers
+            .retain(|(version, _)| !self.snapshot.version().observed_all(version));
         Ok(())
     }
 
@@ -1305,6 +1309,16 @@ impl Buffer {
         }
     }
 
+    pub fn wait_for_version(&mut self, version: clock::Global) -> impl Future<Output = ()> {
+        let (tx, mut rx) = barrier::channel();
+        if !self.snapshot.version.observed_all(&version) {
+            self.version_barriers.push((version, tx));
+        }
+        async move {
+            rx.recv().await;
+        }
+    }
+
     fn resolve_edit(&mut self, edit_id: clock::Local) {
         for mut tx in self
             .edit_id_resolvers