test_git counts only its own ssh-agents (#172)

The unit tier runs under xdist, so another worker's push could start an
ssh-agent between a test's count and its check. The test sshd now tags
every session with NISSE_TEST_SSHD=<its root>, and the tests count only
agents carrying their own sshd's tag. A new test starts a foreign agent
alongside and checks it isn't counted.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-dev/tests/test_git.py

@@ -7,6 +7,7 @@ from pathlib import Path
 import pytest
 
 from nisse_core.execution.ssh import SSHHost
+from nisse_core.execution.testing import started_by
 from nisse_dev.git import GitError, GitRepo, HTTPSToken, SSHKey
 
 
@@ -113,9 +114,9 @@ def test_the_key_is_gone_when_the_command_ends(
     repo.configure_identity("t", "t@example.test")
     (repo.root / "a.txt").write_text("a")
     repo.commit_all("first")
-    before = _agents()
+    before = _agents(ssh_host)
     repo.push(url, "main", credential=key)
-    assert _agents() == before  # the throwaway ssh-agent was killed
+    assert _agents(ssh_host) == before  # the throwaway ssh-agent was killed
 
 
 def test_a_failed_push_still_kills_its_agent(
@@ -124,19 +125,35 @@ def test_a_failed_push_still_kills_its_agent(
     _, key = ssh_remote
     repo = GitRepo(ssh_host, tmp_path / "w", known_hosts=str(tmp_path / "kh"))
     repo.init()
-    before = _agents()
+    before = _agents(ssh_host)
     with pytest.raises(GitError):
         repo.push(
             f"ssh://nobody@127.0.0.1:{ssh_host.port}/nowhere.git",
             "main",
             credential=key,
         )
-    assert _agents() == before
+    assert _agents(ssh_host) == before
 
 
-def _agents() -> set[str]:
-    """Live ssh-agents. A killed one lingers as a zombie in a CI container,
-    where nothing reaps orphans, so zombies don't count."""
+def test_an_agent_started_elsewhere_is_not_counted(ssh_host: SSHHost) -> None:
+    # The unit tier runs in parallel: another worker's push starts its own
+    # ssh-agent at any moment (#172). Only this sshd's count.
+    before = _agents(ssh_host)
+    out = subprocess.run(
+        ["ssh-agent", "-s"], capture_output=True, text=True, check=True
+    ).stdout
+    pid = out.split("SSH_AGENT_PID=")[1].split(";")[0]
+    try:
+        assert _agents(ssh_host) == before
+        assert pid in _agents(None)
+    finally:
+        subprocess.run(["kill", pid], check=False)
+
+
+def _agents(host: SSHHost | None) -> set[str]:
+    """Live ssh-agents started through ``host``'s sshd, or any when None. A
+    killed one lingers as a zombie in a CI container, where nothing reaps
+    orphans, so zombies don't count."""
     out = subprocess.run(
         ["pgrep", "-x", "ssh-agent"], capture_output=True, text=True, check=False
     ).stdout
@@ -146,7 +163,9 @@ def _agents() -> set[str]:
             stat = Path(f"/proc/{pid}/stat").read_text()
         except FileNotFoundError:
             continue
-        if stat.rsplit(")", 1)[1].split()[0] != "Z":
+        if stat.rsplit(")", 1)[1].split()[0] == "Z":
+            continue
+        if host is None or started_by(host, pid):
             live.add(pid)
     return live