fix(audio): pin system-audio capture to the sink monitor

pw-record --target <sink> also linked the default microphone to the
stream unless stream.capture.sink was set, so system-audio capture could
open an input device. Set the property in PIPEWIRE_PROPS for the spawned
pw-record only, keeping any value the user already has.

Verified on PipeWire 1.6.10 with a null sink: the stream linked the sink
monitor and no Audio/Source node.

Fixes #41

src/vocalinux/audio/pipewire.py

@@ -25,7 +25,7 @@ import shutil
 import subprocess
 import time
 from dataclasses import dataclass
-from typing import Any, Callable, List, Optional, Tuple
+from typing import Any, Callable, Dict, List, Mapping, Optional, Tuple
 
 from ..utils.host_process import host_env
 from .errors import CaptureStalledError
@@ -52,6 +52,28 @@ _MONITOR_CLASSES = frozenset({"Audio/Sink", "Audio/Sink/Virtual"})
 #: Metadata keys carrying the effective and configured default sink.
 _DEFAULT_SINK_KEYS = ("default.audio.sink", "default.configured.audio.sink")
 
+#: Stream property that makes ``--target <sink>`` record the sink's monitor.
+#: Without it PipeWire also links the default microphone to the stream (issue
+#: #41), so system-audio capture must always set it.
+_CAPTURE_SINK_PROPERTY = "stream.capture.sink = true"
+
+
+def _capture_sink_env(base: Mapping[str, str]) -> Dict[str, str]:
+    """Return ``base`` with ``PIPEWIRE_PROPS`` pinning capture to the sink monitor.
+
+    This is the form verified on the real machine (PipeWire 1.6.10): the stream
+    linked the sink's monitor and no microphone. A user's existing
+    ``PIPEWIRE_PROPS`` is kept, and only this child process sees the change.
+    """
+    env = dict(base)
+    existing = env.get("PIPEWIRE_PROPS", "").strip()
+    if existing.startswith("{") and existing.endswith("}"):
+        existing = existing[1:-1].strip()
+    parts = [part for part in (existing, _CAPTURE_SINK_PROPERTY) if part]
+    env["PIPEWIRE_PROPS"] = "{ " + " ".join(parts) + " }"
+    return env
+
+
 _COMMAND_TIMEOUT_SECONDS = 3.0
 
 CommandResult = Tuple[int, str, str]
@@ -374,7 +396,7 @@ class PipeWireCaptureSource:
             ],
             stdout=subprocess.PIPE,
             stderr=subprocess.DEVNULL,
-            env=host_env(),
+            env=_capture_sink_env(host_env()),
         )
 
     def reopen(

tests/test_pipewire_capture.py

@@ -347,6 +347,46 @@ class TestPipeWireCaptureSource(unittest.TestCase):
         assert "env" in kwargs  # host binaries get the de-bundled environment
         assert capture.stream is process
 
+    def test_open_pins_capture_to_the_sink_monitor(self) -> None:
+        # Without stream.capture.sink, pw-record --target <sink> also links the
+        # default microphone (issue #41). Set it through PIPEWIRE_PROPS, the
+        # form verified on the real machine.
+        calls: List[Tuple[List[str], Dict[str, Any]]] = []
+        with (
+            patch("vocalinux.audio.pipewire.resolve_pipewire_source", return_value=_source()),
+            patch("vocalinux.audio.pipewire.host_env", return_value={"PATH": "/usr/bin"}),
+        ):
+            capture = PipeWireCaptureSource(
+                device_index=PIPEWIRE_INDEX_BASE,
+                popen=_popen_factory(_FakeProcess(), calls),
+                which=lambda n: "/usr/bin/pw-record",
+            )
+            capture.open()
+        args, kwargs = calls[0]
+        assert args[args.index("--target") + 1] == "alsa_output.dac"
+        assert args[-1] == "-"
+        assert "stream.capture.sink = true" in kwargs["env"]["PIPEWIRE_PROPS"]
+        assert kwargs["env"]["PATH"] == "/usr/bin"
+
+    def test_open_keeps_user_pipewire_props_when_pinning_the_monitor(self) -> None:
+        calls: List[Tuple[List[str], Dict[str, Any]]] = []
+        user_props = "{ node.latency = 1024/48000 }"
+        with (
+            patch("vocalinux.audio.pipewire.resolve_pipewire_source", return_value=_source()),
+            patch(
+                "vocalinux.audio.pipewire.host_env",
+                return_value={"PIPEWIRE_PROPS": user_props},
+            ),
+        ):
+            capture = PipeWireCaptureSource(
+                device_index=PIPEWIRE_INDEX_BASE,
+                popen=_popen_factory(_FakeProcess(), calls),
+                which=lambda n: "/usr/bin/pw-record",
+            )
+            capture.open()
+        props = calls[0][1]["env"]["PIPEWIRE_PROPS"]
+        assert props == "{ node.latency = 1024/48000 stream.capture.sink = true }"
+
     def test_open_requires_pw_record(self) -> None:
         capture = PipeWireCaptureSource(device_index=PIPEWIRE_INDEX_BASE, which=lambda name: None)
         with self.assertRaises(FileNotFoundError):