ssh: Support using the experimental nix copy command to copy

The new SSH store protocol (ssh-ng://) provides better performance.
This is now enabled for flake deployments.

src/nix/hive/mod.rs

@@ -205,6 +205,11 @@ impl Hive {
                 if let Some(ssh_config) = &ssh_config {
                     host.set_ssh_config(ssh_config.clone());
                 }
+
+                if self.is_flake() {
+                    host.set_use_nix3_copy(true);
+                }
+
                 host.upcast()
             });
             let ssh_host = host.is_some();

src/nix/host/ssh.rs

@@ -32,6 +32,9 @@ pub struct Ssh {
     /// Command to elevate privileges with.
     privilege_escalation_command: Vec<String>,
 
+    /// Whether to use the experimental `nix copy` command.
+    use_nix3_copy: bool,
+
     job: Option<JobHandle>,
 }
 
@@ -187,6 +190,7 @@ impl Ssh {
             port: None,
             ssh_config: None,
             privilege_escalation_command: Vec::new(),
+            use_nix3_copy: false,
             job: None,
         }
     }
@@ -203,6 +207,10 @@ impl Ssh {
         self.privilege_escalation_command = command;
     }
 
+    pub fn set_use_nix3_copy(&mut self, enable: bool) {
+        self.use_nix3_copy = enable;
+    }
+
     pub fn upcast(self) -> Box<dyn Host> {
         Box::new(self)
     }
@@ -252,7 +260,47 @@ impl Ssh {
         let ssh_options = self.ssh_options();
         let ssh_options_str = ssh_options.join(" ");
 
+        let mut command = if self.use_nix3_copy {
+            // experimental `nix copy` command with ssh-ng://
+            let mut command = Command::new("nix");
+
+            command.args(&[
+                "--extra-experimental-features",
+                "nix-command",
+                "copy",
+                "--no-check-sigs",
+            ]);
+
+            if options.use_substitutes {
+                command.arg("--substitute-on-destination");
+            }
+
+            if path.ends_with(".drv") {
+                command.arg("--derivation");
+            }
+
+            match direction {
+                CopyDirection::ToRemote => {
+                    command.arg("--to");
+                }
+                CopyDirection::FromRemote => {
+                    command.arg("--from");
+                }
+            }
+
+            let mut store_uri = format!("ssh-ng://{}", self.ssh_target());
+            if options.gzip {
+                store_uri += "?compress=true";
+            }
+            command.arg(store_uri);
+
+            command.arg(path.as_path());
+
+            command
+        } else {
+            // nix-copy-closure (ssh://)
             let mut command = Command::new("nix-copy-closure");
+
             match direction {
                 CopyDirection::ToRemote => {
                     command.arg("--to");
@@ -273,10 +321,12 @@ impl Ssh {
                 command.arg("--gzip");
             }
 
+            command.arg(&self.ssh_target()).arg(path.as_path());
+
             command
-            .arg(&self.ssh_target())
+        };
-            .arg(path.as_path())
+
-            .env("NIX_SSHOPTS", ssh_options_str);
+        command.env("NIX_SSHOPTS", ssh_options_str);
 
         command
     }