Don't leak gpg-agent when signing with gpg

mkosi signs SHA256SUMS by running gpg, which autostarts a gpg-agent if none is
running. As mkosi's sandbox has no PID namespace, that agent daemonizes and is
leaked when the sandbox goes away. Unprivileged this is worse, as the leaked
agents hold systemd-nsresourced dynamic UID ranges and eventually exhaust the
pool (builds then fail with io.systemd.NamespaceResource.NoDynamicRange), but
the process is leaked either way.

Shut the agent down after signing, but only if we started it: an agent the
user already had running for their GNUPGHOME (e.g. their own ~/.gnupg) is left
untouched. gpg-connect-agent exits 0 even with no agent, so detect a running
agent by its GETINFO output rather than the exit code.

The gpg signing integration test also starts its own agent via
gpg --quick-gen-key; kill that too (its GNUPGHOME is a throwaway directory).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: martinpitt

mkosi/__init__.py

@@ -2565,12 +2565,35 @@ def calculate_signature_gpg(context: Context) -> None:
     ]  # fmt: skip
 
     with complete_step("Signing SHA256SUMS…"):
+        # gpg autostarts a gpg-agent to sign and, as the sandbox has no PID namespace, that agent is
+        # leaked when the sandbox goes away. If we started it (no agent was running yet), shut it down
+        # again afterwards; leave an agent the user already had running (e.g. for ~/.gnupg) alone.
+        # gpg-connect-agent exits 0 even with no agent, so detect one via its GETINFO output: a running
+        # agent prints "D <version>\nOK" on stdout, otherwise stdout stays empty.
+        agent_running = bool(
+            run(
+                ["gpg-connect-agent", "--no-autostart", "GETINFO version", "/bye"],
+                env=env,
+                sandbox=context.sandbox(options=options),
+                stdout=subprocess.PIPE,
+                check=False,
+            ).stdout.strip()
+        )
+
         run(
             cmdline,
             env=env,
             sandbox=context.sandbox(options=options),
         )
 
+        if not agent_running:
+            run(
+                ["gpgconf", "--kill", "gpg-agent"],
+                env=env,
+                sandbox=context.sandbox(options=options),
+                check=False,
+            )
+
 
 def calculate_signature_sop(context: Context) -> None:
     if context.config.key is None:

tests/integration_signing.py

@@ -66,6 +66,11 @@ async def test_signing_checksums_with_gpg(self) -> None:
                     stdout=o,
                 )
 
+            # gpg --quick-gen-key started a gpg-agent that we're done with now (the build manages its own).
+            # Shut it down so it doesn't linger past the test. GNUPGHOME is a throwaway directory, so this
+            # only affects this test's agent, never one running for the user's real GNUPGHOME.
+            run(cmdline=["gpgconf", "--kill", "gpg-agent"], env=env, check=False)
+
             image.build(options=["--checksum=true", "--sign=true", f"--key={signing_key}"], env=env)
 
             signed_file = image.output_dir / "image.SHA256SUMS"