fix(tvix/nix_compat): Fix nix-daemon handshake r/8880

Existing handshake behavior assumed that the server version is always
at least as new as the client. Meaning that the client's version was
always picked the handshake details as well as for further communication

This change removes that assumption and correctly uses
min(server_version, client_version).

Change-Id: Ia5dad4613dd5f69a0aeb6c9d86982f1f36fe1a4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12722
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

2 files changed, 66 insertions, 11 deletions

diff --git a/tvix/nix-compat/src/nix_daemon/worker_protocol.rs b/tvix/nix-compat/src/nix_daemon/worker_protocol.rs
index 7e3adc0db2ff..cc99bdb54fab 100644
--- a/tvix/nix-compat/src/nix_daemon/worker_protocol.rs
+++ b/tvix/nix-compat/src/nix_daemon/worker_protocol.rs
@@ -1,4 +1,5 @@
 use std::{
+    cmp::min,
     collections::HashMap,
     io::{Error, ErrorKind},
 };
@@ -209,7 +210,7 @@ pub async fn read_client_settings<R: AsyncReadExt + Unpin>(
 ///
 /// # Return
 ///
-/// The protocol version of the client.
+/// The protocol version to use for further comms, min(client_version, our_version).
 pub async fn server_handshake_client<'a, RW: 'a>(
     mut conn: &'a mut RW,
     nix_version: &str,
@@ -239,28 +240,29 @@ where
                 format!("The nix client version {} is too old", client_version),
             ));
         }
-        if client_version.minor() >= 14 {
+        let picked_version = min(PROTOCOL_VERSION, client_version);
+        if picked_version.minor() >= 14 {
             // Obsolete CPU affinity.
             let read_affinity = conn.read_u64_le().await?;
             if read_affinity != 0 {
                 let _cpu_affinity = conn.read_u64_le().await?;
             };
         }
-        if client_version.minor() >= 11 {
+        if picked_version.minor() >= 11 {
             // Obsolete reserveSpace
             let _reserve_space = conn.read_u64_le().await?;
         }
-        if client_version.minor() >= 33 {
+        if picked_version.minor() >= 33 {
             // Nix version. We're plain lying, we're not Nix, but eh…
             // Setting it to the 2.3 lineage. Not 100% sure this is a
             // good idea.
             wire::write_bytes(&mut conn, nix_version).await?;
             conn.flush().await?;
         }
-        if client_version.minor() >= 35 {
+        if picked_version.minor() >= 35 {
             write_worker_trust_level(&mut conn, trusted).await?;
         }
-        Ok(client_version)
+        Ok(picked_version)
     }
 }
 
@@ -330,11 +332,64 @@ mod tests {
             // Trusted (1 == client trusted
             .write(&[1, 0, 0, 0, 0, 0, 0, 0])
             .build();
-        let client_version = server_handshake_client(&mut test_conn, "2.18.2", Trust::Trusted)
+        let picked_version = server_handshake_client(&mut test_conn, "2.18.2", Trust::Trusted)
             .await
             .unwrap();
 
-        assert_eq!(client_version, PROTOCOL_VERSION)
+        assert_eq!(picked_version, PROTOCOL_VERSION)
+    }
+
+    #[tokio::test]
+    async fn test_init_hanshake_with_newer_client_should_use_older_version() {
+        let mut test_conn = tokio_test::io::Builder::new()
+            .read(&WORKER_MAGIC_1.to_le_bytes())
+            .write(&WORKER_MAGIC_2.to_le_bytes())
+            .write(&[37, 1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // Client is newer than us.
+            .read(&[38, 1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // cpu affinity
+            .read(&[0; 8])
+            // reservespace
+            .read(&[0; 8])
+            // version (size)
+            .write(&[0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // version (data == 2.18.2 + padding)
+            .write(&[50, 46, 49, 56, 46, 50, 0, 0])
+            // Trusted (1 == client trusted
+            .write(&[1, 0, 0, 0, 0, 0, 0, 0])
+            .build();
+        let picked_version = server_handshake_client(&mut test_conn, "2.18.2", Trust::Trusted)
+            .await
+            .unwrap();
+
+        assert_eq!(picked_version, PROTOCOL_VERSION)
+    }
+
+    #[tokio::test]
+    async fn test_init_hanshake_with_older_client_should_use_older_version() {
+        let mut test_conn = tokio_test::io::Builder::new()
+            .read(&WORKER_MAGIC_1.to_le_bytes())
+            .write(&WORKER_MAGIC_2.to_le_bytes())
+            .write(&[37, 1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // Client is newer than us.
+            .read(&[24, 1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // cpu affinity
+            .read(&[0; 8])
+            // reservespace
+            .read(&[0; 8])
+            // NOTE: we are not writing version and trust since the client is too old.
+            // version (size)
+            //.write(&[0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+            // version (data == 2.18.2 + padding)
+            //.write(&[50, 46, 49, 56, 46, 50, 0, 0])
+            // Trusted (1 == client trusted
+            //.write(&[1, 0, 0, 0, 0, 0, 0, 0])
+            .build();
+        let picked_version = server_handshake_client(&mut test_conn, "2.18.2", Trust::Trusted)
+            .await
+            .unwrap();
+
+        assert_eq!(picked_version, ProtocolVersion::from_parts(1, 24))
     }
 
     #[tokio::test]
diff --git a/users/picnoir/tvix-daemon/src/main.rs b/users/picnoir/tvix-daemon/src/main.rs
index dc49b209e009..580f07652097 100644
--- a/users/picnoir/tvix-daemon/src/main.rs
+++ b/users/picnoir/tvix-daemon/src/main.rs
@@ -67,14 +67,14 @@ where
     R: AsyncReadExt + AsyncWriteExt + Unpin + std::fmt::Debug,
 {
     match server_handshake_client(&mut conn, "2.18.2", Trust::Trusted).await {
-        Ok(client_protocol_version) => {
+        Ok(picked_protocol_version) => {
             let mut client_connection = ClientConnection {
                 conn,
-                version: client_protocol_version,
+                version: picked_protocol_version,
                 client_settings: None,
             };
             debug!("Client hanshake succeeded");
-            debug!(client_protocol_version = ?client_protocol_version);
+            debug!(picked_protocol_version = ?picked_protocol_version);
             // TODO: implement logging. For now, we'll just send
             // STDERR_LAST, which is good enough to get Nix respond to
             // us.