test: make atomic snapshot gate non-vacuous

agent-session-id: dev3.dotfiles-cos-misc-agent-runtime-simplification
agent-tool: Codex
agent-tool-version: 0.145.0
agent-model: gpt-5.6-sol
agent-runtime-profile: /home/schickling/.config/coding-agents/profile.json
agent-skills-manifest: /nix/store/nk9iml2841l1yjjg0f6f0d3y60zkg1nn-agent-skills-corpus/share/agent-skills/manifest.json
tooling-profile: dotfiles@4a0515f

Author: schickling-assistant

flake.nix

@@ -125,14 +125,13 @@
 
         # Narrow sandbox-safe integration gate for the atomic snapshot boundary. The main package
         # deliberately omits the broad doctor suite because some doctor cases exercise facilities
-        # unavailable in the Nix sandbox; this derivation executes the one hermetic regression by
-        # its unique test name so it cannot be present-but-invisible to CI.
+        # unavailable in the Nix sandbox. A dedicated target containing exactly one test makes the
+        # gate structurally non-vacuous: a missing target is a cargo error, never a zero-match pass.
         st2AtomicPtySnapshot = st2.overrideAttrs (_: {
           pname = "st2-atomic-pty-snapshot-check";
           cargoTestFlags = [
             "--test"
-            "doctor"
-            "doctor_rejects_a_partial_pty_snapshot_atomically"
+            "atomic_pty_snapshot"
           ];
         });
 

tests/atomic_pty_snapshot.rs

@@ -0,0 +1,54 @@
+#![cfg(unix)]
+
+//! Hermetic producer-consumer boundary: a truncated `pty list --json` response is never consumed
+//! as a valid prefix. This is a dedicated single-test target so CI cannot pass on a vacuous libtest
+//! name filter.
+
+use std::fs;
+use std::os::unix::fs::PermissionsExt;
+use std::path::Path;
+use std::process::Command;
+
+fn executable(path: &Path, body: &str) {
+    fs::write(path, body).unwrap();
+    fs::set_permissions(path, fs::Permissions::from_mode(0o755)).unwrap();
+}
+
+#[test]
+fn doctor_rejects_a_partial_pty_snapshot_atomically() {
+    let tmp = tempfile::tempdir().unwrap();
+    let catalog = tmp.path().join("catalog");
+    let declaration = catalog.join("agents/h/gone/agent.kdl");
+    let bin = tmp.path().join("bin");
+    fs::create_dir_all(declaration.parent().unwrap()).unwrap();
+    fs::create_dir_all(&bin).unwrap();
+    fs::write(
+        declaration,
+        "agent \"gone\" { host \"h\"; retired #true; command \"true\" }\n",
+    )
+    .unwrap();
+    executable(
+        &bin.join("pty"),
+        "#!/bin/sh\nprintf '[{\"name\":\"h.gone.agent\",\"status\":\"running\"},'\n",
+    );
+
+    let output = Command::new(env!("CARGO_BIN_EXE_st2"))
+        .arg("doctor")
+        .arg("--catalog")
+        .arg(&catalog)
+        .args(["--host", "h"])
+        .env("PATH", &bin)
+        .env("XDG_STATE_HOME", tmp.path().join("state"))
+        .env("PTY_ROOT", tmp.path().join("pty"))
+        .output()
+        .unwrap();
+
+    assert!(!output.status.success());
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(stdout.contains("✗ task runtime readable"), "{stdout}");
+    assert!(stdout.contains("parsing `pty list --json`"), "{stdout}");
+    assert!(
+        !stdout.contains("retirement complete"),
+        "a valid prefix must never be consumed as a partial snapshot:\n{stdout}"
+    );
+}

tests/doctor.rs

@@ -193,36 +193,6 @@ fn doctor_bounds_a_hung_pty_probe_and_reports_the_runtime_error() {
     assert!(stdout.contains("timed out after 2.0s"), "{stdout}");
 }
 
-#[test]
-fn doctor_rejects_a_partial_pty_snapshot_atomically() {
-    let tmp = tempfile::tempdir().unwrap();
-    let catalog = tmp.path().join("catalog");
-    let declaration = catalog.join("agents/h/gone/agent.kdl");
-    let bin = tmp.path().join("bin");
-    fs::create_dir_all(declaration.parent().unwrap()).unwrap();
-    fs::create_dir_all(&bin).unwrap();
-    fs::write(
-        declaration,
-        "agent \"gone\" { host \"h\"; retired #true; command \"true\" }\n",
-    )
-    .unwrap();
-    executable(
-        &bin.join("pty"),
-        "#!/bin/sh\nprintf '[{\"name\":\"h.gone.agent\",\"status\":\"running\"},'\n",
-    );
-
-    let output = doctor(&catalog, &bin, &tmp.path().join("state"));
-
-    assert!(!output.status.success());
-    let stdout = String::from_utf8_lossy(&output.stdout);
-    assert!(stdout.contains("✗ task runtime readable"), "{stdout}");
-    assert!(stdout.contains("parsing `pty list --json`"), "{stdout}");
-    assert!(
-        !stdout.contains("retirement complete"),
-        "a valid prefix must never be consumed as a partial snapshot:\n{stdout}"
-    );
-}
-
 #[test]
 fn retired_declaration_is_healthy_when_tasks_and_presence_are_absent() {
     let tmp = tempfile::tempdir().unwrap();