hive-priv: stream-forward the profile-swap step's output too

Was skipping live-log forwarding for the swap step on the reasoning
that it's near-instant in the happy case; mara pointed out a failure
there is exactly when the caller most wants the line in the build log,
not just a summary error. Factored the log+forward pattern into one
helper, used by both the nix-env --set and systemctl reload steps.

hive-priv/src/main.rs

@@ -772,7 +772,7 @@ async fn container_flake_action(
     let toplevel = nix_build_toplevel(name, stream.then_some(&mut *writer)).await?;
     let system_name = container_system_name(name);
     if verb == "update" {
-        return swap_container_profile(&system_name, &toplevel).await;
+        return swap_container_profile(&system_name, &toplevel, stream, writer).await;
     }
     let args = [verb, &system_name, "--system-path", &toplevel];
     if stream {
@@ -790,10 +790,16 @@ async fn container_flake_action(
 /// it's currently running — the container's own next start already reads
 /// from the profile, so a stopped container needs nothing further.
 ///
-/// Deliberately not stream-forwarded to `writer` — both operations here
+/// Both steps are near-instant in the happy case, but still stream-
-/// are near-instant (a profile symlink swap, a `systemctl reload`), unlike
+/// forwarded like the rest of this operation: a failure here (e.g. a
-/// the multi-minute build `container_flake_action` already streams.
+/// wedged `systemctl reload`) is exactly when the caller most wants the
-async fn swap_container_profile(system_name: &str, toplevel: &str) -> Result<(String, String)> {
+/// live line in the build log, not just a summary error afterward.
+async fn swap_container_profile(
+    system_name: &str,
+    toplevel: &str,
+    stream: bool,
+    writer: &mut OwnedWriteHalf,
+) -> Result<(String, String)> {
     let profile = format!("/nix/var/nix/profiles/per-container/{system_name}/system");
     let set_out = Command::new("nix-env")
         .args(["-p", &profile, "--set", toplevel])
@@ -802,12 +808,7 @@ async fn swap_container_profile(system_name: &str, toplevel: &str) -> Result<(St
         .context("invoke nix-env --set")?;
     let mut stdout = String::from_utf8_lossy(&set_out.stdout).into_owned();
     let mut stderr = String::from_utf8_lossy(&set_out.stderr).into_owned();
-    for line in stdout.lines() {
+    log_and_forward(&stdout, &stderr, stream, writer).await;
-        tracing::info!(target: "nixos-container", "{line}");
-    }
-    for line in stderr.lines() {
-        tracing::warn!(target: "nixos-container", "{line}");
-    }
     if !set_out.status.success() {
         bail!(
             "nix-env -p {profile} --set failed ({}): {}",
@@ -831,12 +832,7 @@ async fn swap_container_profile(system_name: &str, toplevel: &str) -> Result<(St
             .context("invoke systemctl reload")?;
         let r_stdout = String::from_utf8_lossy(&reload_out.stdout).into_owned();
         let r_stderr = String::from_utf8_lossy(&reload_out.stderr).into_owned();
-        for line in r_stdout.lines() {
+        log_and_forward(&r_stdout, &r_stderr, stream, writer).await;
-            tracing::info!(target: "nixos-container", "{line}");
-        }
-        for line in r_stderr.lines() {
-            tracing::warn!(target: "nixos-container", "{line}");
-        }
         if !reload_out.status.success() {
             bail!(
                 "systemctl reload {unit} failed ({}): {}",
@@ -850,6 +846,26 @@ async fn swap_container_profile(system_name: &str, toplevel: &str) -> Result<(St
     Ok((stdout, stderr))
 }
 
+/// Log a command's captured stdout/stderr the same way every other
+/// `nixos-container`-adjacent shellout in this file does, and — when
+/// `stream` is set — forward each line to the client as a live
+/// [`PrivEvent::Line`] too, so a caller watching the build log sees these
+/// lines exactly like any other step's, not just a summary on failure.
+async fn log_and_forward(stdout: &str, stderr: &str, stream: bool, writer: &mut OwnedWriteHalf) {
+    for line in stdout.lines() {
+        tracing::info!(target: "nixos-container", "{line}");
+        if stream {
+            write_line_event(writer, PrivStream::Stdout, line).await;
+        }
+    }
+    for line in stderr.lines() {
+        tracing::warn!(target: "nixos-container", "{line}");
+        if stream {
+            write_line_event(writer, PrivStream::Stderr, line).await;
+        }
+    }
+}
+
 /// The explicit `nixosConfigurations.<name>.config.system.build.toplevel`
 /// flake attr path — same construction `hive-c0re`'s own
 /// `lifecycle::prebuild_toplevel` uses, kept here as a pure function so