ADR-049: Cross-Platform WiFi Interface Detection and Graceful Degradation

Context

Users report RuntimeError: Cannot read /proc/net/wireless when running WiFi DensePose in environments where the Linux wireless proc filesystem is unavailable:

• Docker containers on macOS/Windows (Linux kernel detected, but no wireless subsystem)
• WSL2 without USB WiFi passthrough
• Headless Linux servers without WiFi hardware
• Embedded Linux boards without wireless-extensions support

The current architecture has two layers of defense:

1. ws_server.py (line 345-355) checks os.path.exists("/proc/net/wireless") before instantiating LinuxWifiCollector and falls back to SimulatedCollector if missing.
2. rssi_collector.py LinuxWifiCollector._validate_interface() (line 178-196) raises a hard RuntimeError if /proc/net/wireless is missing or the interface isn't listed.

However, there are gaps:

• Direct usage: Any code that instantiates LinuxWifiCollector directly (outside ws_server.py) hits the unguarded RuntimeError with no fallback.
• Error message: The RuntimeError message tells users to "use SimulatedCollector instead" but doesn't explain how.
• No auto-detection: The collector selection logic is duplicated between ws_server.py and install.sh with no shared platform-detection utility.
• Partial /proc/net/wireless: The file may exist (e.g., kernel module loaded) but contain no interfaces, producing a confusing "interface not found" error instead of a clean fallback.

Decision

1. Platform-Aware Collector Factory

Introduce a create_collector() factory function in rssi_collector.py that encapsulates the platform detection and fallback chain:

def create_collector(
    preferred: str = "auto",
    interface: str = "wlan0",
    sample_rate_hz: float = 10.0,
) -> BaseCollector:
    """
    Create the best available WiFi collector for the current platform.

    Resolution order (when preferred="auto"):
      1. ESP32 CSI (if UDP port 5005 is receiving frames)
      2. Platform-native WiFi:
         - Linux: LinuxWifiCollector (requires /proc/net/wireless + active interface)
         - Windows: WindowsWifiCollector (netsh wlan)
         - macOS: MacosWifiCollector (CoreWLAN)
      3. SimulatedCollector (always available)

    Raises nothing — always returns a usable collector.
    """

2. Soft Validation in LinuxWifiCollector

Replace the hard RuntimeError in _validate_interface() with a class method that returns availability status without raising:

@classmethod
def is_available(cls, interface: str = "wlan0") -> tuple[bool, str]:
    """Check if Linux WiFi collection is possible. Returns (available, reason)."""
    if not os.path.exists("/proc/net/wireless"):
        return False, "/proc/net/wireless not found (Docker, WSL, or no wireless subsystem)"
    with open("/proc/net/wireless") as f:
        content = f.read()
    if interface not in content:
        names = cls._parse_interface_names(content)
        return False, f"Interface '{interface}' not in /proc/net/wireless. Available: {names}"
    return True, "ok"

The existing _validate_interface() continues to raise RuntimeError for direct callers who need fail-fast behavior, but create_collector() uses is_available() to probe without exceptions.

3. Structured Fallback Logging

When auto-detection skips a collector, log at WARNING level with actionable context:

WiFi collector: LinuxWifiCollector unavailable (/proc/net/wireless not found — likely Docker/WSL).
WiFi collector: Falling back to SimulatedCollector. For real sensing, connect ESP32 nodes via UDP:5005.

4. Consolidate Platform Detection

Remove duplicated platform-detection logic from ws_server.py and install.sh. Both should use create_collector() (Python) or a shared detect_wifi_platform() shell function.

Consequences

Positive

• Zero-crash startup: create_collector("auto") never raises — Docker, WSL, and headless users get SimulatedCollector automatically with a clear log message.
• Single detection path: Platform logic lives in one place (rssi_collector.py), reducing drift between ws_server.py, install.sh, and future entry points.
• Better DX: Error messages explain why a collector is unavailable and what to do (connect ESP32, install WiFi driver, etc.).

Negative

• SimulatedCollector may mask hardware issues: Users with real WiFi hardware that fails detection might unknowingly run on simulated data. Mitigated by the WARNING-level log.
• Breaking change for direct LinuxWifiCollector callers: Code that catches RuntimeError from _validate_interface() as a signal needs to migrate to is_available() or create_collector(). This is a minor change — there are no known external consumers.

Neutral

• _validate_interface() behavior is unchanged for existing direct callers — this is additive.

Implementation Notes

1. Add create_collector() and BaseCollector.is_available() to v1/src/sensing/rssi_collector.py
2. Refactor ws_server.py _init_collector() to call create_collector()
3. Update install.sh detect_wifi_hardware() to use shared detection logic
4. Add unit tests for each platform path (mock /proc/net/wireless presence/absence)
5. Comment on issue #148 with the fix

References

• Issue #148: RuntimeError: Cannot read /proc/net/wireless
• ADR-013: Feature-Level Sensing on Commodity Gear
• ADR-025: macOS CoreWLAN WiFi Sensing
• Linux /proc/net/wireless documentation