fix: 修正 UAC 链接顺序及原生 ALSA 启动恢复

src/audio/uac/playback.rs

@@ -9,9 +9,11 @@ use tracing::{info, warn};
 use crate::error::{AppError, Result};
 
 const RETRY_BACKOFF: Duration = Duration::from_secs(1);
-const PERIOD_FRAMES: Frames = 960;
-const BUFFER_FRAMES: Frames = 4_800;
-const START_THRESHOLD_PERIODS: Frames = 4;
+const PERIOD_FRAMES: Frames = 1_024;
+// Request the same compatibility buffer as the known-working ALSA player.
+// The gadget driver may negotiate a smaller buffer; always use its result.
+const BUFFER_FRAMES: Frames = 32_768;
+const IDLE_REOPEN_TIMEOUT: Duration = Duration::from_secs(5);
 const SINK_STALL_TIMEOUT: Duration = Duration::from_millis(200);
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -58,9 +60,17 @@ struct PlaybackInner {
 }
 
 enum SessionSink {
-    Closed { retry_at: Option<Instant> },
-    Probing { pcm: PCM, stalled: bool },
-    Active { pcm: PCM, last_progress: Instant },
+    Closed {
+        retry_at: Option<Instant>,
+    },
+    Probing {
+        pcm: PlaybackPcm,
+        stalled: bool,
+    },
+    Active {
+        pcm: PlaybackPcm,
+        last_progress: Instant,
+    },
 }
 
 impl SessionSink {
@@ -77,12 +87,14 @@ impl SessionSink {
 
 struct SessionRuntime {
     sink: SessionSink,
+    last_frame: Option<Instant>,
 }
 
 impl SessionRuntime {
     fn new() -> Self {
         Self {
             sink: SessionSink::Closed { retry_at: None },
+            last_frame: None,
         }
     }
 
@@ -92,12 +104,22 @@ impl SessionRuntime {
 
     fn close(&mut self) {
         self.sink = SessionSink::Closed { retry_at: None };
+        self.last_frame = None;
     }
 
     /// Advance playback only when a WebSocket frame arrives. All ALSA handles
     /// are non-blocking, so a slow or absent USB host drops the current frame
     /// instead of occupying a worker thread or accumulating stale speech.
     fn write(&mut self, config: &UacPlaybackConfig, samples: &[i16]) -> bool {
+        // Reopen on resume without an idle timer thread. stop()/session drop
+        // still close the PCM synchronously, even when no frames arrive.
+        if self
+            .last_frame
+            .is_some_and(|last| last.elapsed() >= IDLE_REOPEN_TIMEOUT)
+        {
+            self.close();
+        }
+        self.last_frame = Some(Instant::now());
         let sink = std::mem::replace(&mut self.sink, SessionSink::Closed { retry_at: None });
         let (next_sink, accepted) = drive_sink(sink, config, samples);
         self.sink = next_sink;
@@ -222,8 +244,8 @@ fn drive_sink(
                 return (SessionSink::Closed { retry_at }, false);
             }
 
-            match open_pcm(config).and_then(|pcm| {
-                prime_pcm_with_silence(&pcm, config.channels as usize)?;
+            match open_pcm(config).and_then(|mut pcm| {
+                pcm.prime_with_silence(config.channels as usize)?;
                 Ok(pcm)
             }) {
                 Ok(pcm) => drive_probe(pcm, false, config, samples),
@@ -246,64 +268,71 @@ fn drive_sink(
 }
 
 fn drive_probe(
-    pcm: PCM,
+    mut pcm: PlaybackPcm,
     stalled: bool,
     config: &UacPlaybackConfig,
     samples: &[i16],
 ) -> (SessionSink, bool) {
-    match sink_is_consuming(&pcm) {
+    match pcm.consumption_progress() {
         Ok(false) => (SessionSink::Probing { pcm, stalled }, false),
         Ok(true) => {
-            if let Err(error) = reset_pcm_buffer(&pcm) {
-                warn!("Failed to activate UAC playback; retrying later: {error}");
-                return retry_later();
-            }
+            // Keep the stream that has just started consuming. Dropping and
+            // preparing it here creates another startup/underrun window.
             info!("UAC target started consuming microphone audio");
             drive_active(pcm, Instant::now(), config, samples)
         }
-        Err(error) => {
-            warn!("Failed to probe UAC playback; retrying later: {error}");
-            retry_later()
-        }
+        Err(error) => recover_sink(pcm, config, error),
     }
 }
 
 fn drive_active(
-    pcm: PCM,
+    mut pcm: PlaybackPcm,
     last_progress: Instant,
     config: &UacPlaybackConfig,
     samples: &[i16],
 ) -> (SessionSink, bool) {
-    match write_pcm_nonblocking(&pcm, samples, config.channels as usize) {
-        Ok(WriteOutcome::Progress) => (
-            SessionSink::Active {
-                pcm,
-                last_progress: Instant::now(),
-            },
-            true,
-        ),
-        Ok(WriteOutcome::Recovered) => (
-            SessionSink::Active {
-                pcm,
-                last_progress: Instant::now(),
-            },
-            false,
-        ),
-        Ok(WriteOutcome::Blocked) if last_progress.elapsed() < SINK_STALL_TIMEOUT => {
-            (SessionSink::Active { pcm, last_progress }, false)
-        }
-        Ok(WriteOutcome::Blocked) => {
-            if let Err(error) = reset_pcm_buffer(&pcm)
-                .and_then(|_| prime_pcm_with_silence(&pcm, config.channels as usize))
-            {
+    let last_progress = match pcm.consumption_progress() {
+        Ok(true) => Instant::now(),
+        Ok(false) => last_progress,
+        Err(error) => return recover_sink(pcm, config, error),
+    };
+    if last_progress.elapsed() >= SINK_STALL_TIMEOUT {
+        // Discard queued speech before probing an unavailable host again.
+        if let Err(error) = pcm.reset_and_prime(config.channels as usize) {
             warn!("Failed to reset stalled UAC playback: {error}");
             return retry_later();
         }
         info!("UAC target stopped consuming audio; waiting for playback activity");
-            (SessionSink::Probing { pcm, stalled: true }, false)
+        return (SessionSink::Probing { pcm, stalled: true }, false);
     }
+
+    match pcm.write_samples(samples, config.channels as usize) {
+        Ok(accepted) => (SessionSink::Active { pcm, last_progress }, accepted),
+        Err(error) => recover_sink(pcm, config, error),
+    }
+}
+
+fn recover_sink(
+    mut pcm: PlaybackPcm,
+    config: &UacPlaybackConfig,
+    error: alsa::Error,
+) -> (SessionSink, bool) {
+    match error.errno() {
+        libc::EAGAIN | libc::EINTR => (SessionSink::Probing { pcm, stalled: true }, false),
+        libc::EPIPE | libc::ESTRPIPE => {
+            // prepare restarts after XRUN/suspend without snd_pcm_recover's
+            // potentially unbounded resume loop. Start again with silence,
+            // and require fresh consumption before reporting Active.
+            match pcm.reset_and_prime(config.channels as usize) {
+                Ok(()) => (SessionSink::Probing { pcm, stalled: true }, false),
                 Err(error) => {
-            warn!("UAC playback write failed; retrying later: {error}");
+                    warn!("Failed to recover UAC playback: {error}");
+                    retry_later()
+                }
+            }
+        }
+        _ => {
+            warn!("UAC playback failed; reopening later: {error}");
             retry_later()
         }
     }
@@ -318,7 +347,7 @@ fn retry_later() -> (SessionSink, bool) {
     )
 }
 
-fn open_pcm(config: &UacPlaybackConfig) -> Result<PCM> {
+fn open_pcm(config: &UacPlaybackConfig) -> Result<PlaybackPcm> {
     let pcm = PCM::new(&config.device_name, Direction::Playback, true).map_err(|error| {
         AppError::AudioError(format!(
             "Failed to open UAC device {}: {error}",
@@ -348,10 +377,9 @@ fn open_pcm(config: &UacPlaybackConfig) -> Result<PCM> {
         let params = pcm.sw_params_current().map_err(|error| {
             AppError::AudioError(format!("Failed to read UAC SwParams: {error}"))
         })?;
-        let start_threshold =
-            (period_frames as Frames * START_THRESHOLD_PERIODS).min(buffer_frames as Frames);
         params
-            .set_start_threshold(start_threshold)
+            .set_start_threshold(buffer_frames as Frames)
+            .and_then(|_| params.set_stop_threshold(buffer_frames as Frames))
             .and_then(|_| params.set_avail_min(period_frames as Frames))
             .and_then(|_| pcm.sw_params(&params))
             .map_err(|error| {
@@ -365,101 +393,219 @@ fn open_pcm(config: &UacPlaybackConfig) -> Result<PCM> {
         "UAC playback opened on {} (buffer={} frames, period={} frames)",
         config.device_name, buffer_frames, period_frames
     );
-    Ok(pcm)
+    Ok(PlaybackPcm {
+        pcm,
+        buffer_frames: buffer_frames as Frames,
+        period_frames: period_frames as Frames,
+        submitted_frames: 0,
+        consumed_frames: 0,
+    })
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-enum WriteOutcome {
-    Progress,
-    Blocked,
-    Recovered,
+struct PlaybackPcm {
+    pcm: PCM,
+    buffer_frames: Frames,
+    period_frames: Frames,
+    submitted_frames: u64,
+    consumed_frames: u64,
 }
 
-fn write_pcm_nonblocking(pcm: &PCM, samples: &[i16], channels: usize) -> Result<WriteOutcome> {
-    let total_frames = samples.len() / channels;
-    match pcm.avail() {
-        Ok(available) if available < total_frames as Frames => return Ok(WriteOutcome::Blocked),
-        Ok(_) => {}
-        Err(error) => {
-            recover_pcm(pcm, error)?;
-            return Ok(WriteOutcome::Recovered);
+impl PlaybackPcm {
+    fn consumption_progress(&mut self) -> std::result::Result<bool, alsa::Error> {
+        // avail synchronizes the hardware pointer. Successful writes alone
+        // only show that the ring buffer has room, not that USB is consuming.
+        let available = self.pcm.avail()?;
+        match self.pcm.state() {
+            State::XRun => return Err(alsa::Error::new("UAC PCM state", libc::EPIPE)),
+            State::Suspended => return Err(alsa::Error::new("UAC PCM state", libc::ESTRPIPE)),
+            State::Disconnected => return Err(alsa::Error::new("UAC PCM state", libc::ENODEV)),
+            State::Running => {}
+            _ => return Ok(false),
         }
+        let consumed = consumed_frames(self.submitted_frames, self.buffer_frames, available);
+        let progressed = consumed > self.consumed_frames;
+        self.consumed_frames = consumed;
+        Ok(progressed)
     }
 
-    let io = pcm
-        .io_i16()
-        .map_err(|error| AppError::AudioError(format!("UAC PCM I/O failed: {error}")))?;
-    match io.writei(samples) {
-        Ok(0) => Ok(WriteOutcome::Blocked),
-        Ok(_) => Ok(WriteOutcome::Progress),
-        Err(error) if error.errno() == libc::EAGAIN => Ok(WriteOutcome::Blocked),
-        Err(error) => {
-            recover_pcm(pcm, error)?;
-            Ok(WriteOutcome::Recovered)
-        }
-    }
-}
-
-/// Once a full playback buffer gains at least one period of free space, the
-/// USB host has enabled the UAC streaming interface and is consuming samples.
-fn sink_is_consuming(pcm: &PCM) -> Result<bool> {
-    if pcm.state() == State::XRun {
-        return Ok(true);
-    }
-
-    match pcm.avail() {
-        Ok(available) => Ok(available >= PERIOD_FRAMES),
-        Err(error) if error.errno() == libc::EPIPE => Ok(true),
-        Err(error) => Err(AppError::AudioError(format!(
-            "Failed to query UAC playback availability: {error}"
-        ))),
-    }
-}
-
-fn recover_pcm(pcm: &PCM, error: alsa::Error) -> Result<()> {
-    let errno = error.errno();
-    pcm.try_recover(error, true).map_err(|recover_error| {
-        AppError::AudioError(format!("Failed to recover UAC playback: {recover_error}"))
+    fn write_samples(
+        &mut self,
+        samples: &[i16],
+        channels: usize,
+    ) -> std::result::Result<bool, alsa::Error> {
+        let io = self.pcm.io_i16()?;
+        let written = write_frames(samples, channels, self.period_frames as usize, |chunk| {
+            let written = io.writei(chunk)?;
+            self.submitted_frames += written as u64;
+            Ok(written)
+        })?;
+        Ok(written == samples.len() / channels)
+    }
+
+    fn prime_with_silence(&mut self, channels: usize) -> Result<()> {
+        // Use the negotiated capacity, not BUFFER_FRAMES. A near request is
+        // often clamped by u_audio's DMA buffer limit.
+        let silence = vec![0i16; self.buffer_frames as usize * channels];
+        let complete = self
+            .write_samples(&silence, channels)
+            .map_err(|error| AppError::AudioError(format!("Failed to prime UAC PCM: {error}")))?;
+        if !complete {
+            return Err(AppError::AudioError(
+                "UAC PCM priming was interrupted".into(),
+            ));
+        }
+        // Most hardware starts automatically at the threshold. Some PCM
+        // plugins remain Prepared despite accepting the complete prefill.
+        // Start explicitly only after priming, and never restart a running PCM.
+        if self.pcm.state() == State::Prepared {
+            self.pcm.start().map_err(|error| {
+                AppError::AudioError(format!("Failed to start primed UAC PCM: {error}"))
             })?;
-    if matches!(errno, libc::EPIPE | libc::ESTRPIPE) {
-        warn!("Recovered UAC playback after ALSA error {errno}");
         }
         Ok(())
+    }
+
+    fn reset_and_prime(&mut self, channels: usize) -> Result<()> {
+        self.pcm
+            .drop()
+            .and_then(|_| self.pcm.prepare())
+            .map_err(|error| AppError::AudioError(format!("Failed to reset UAC PCM: {error}")))?;
+        self.submitted_frames = 0;
+        self.consumed_frames = 0;
+        self.prime_with_silence(channels)
+    }
 }
 
-fn reset_pcm_buffer(pcm: &PCM) -> Result<()> {
-    pcm.drop()
-        .and_then(|_| pcm.prepare())
-        .map_err(|error| AppError::AudioError(format!("Failed to reset UAC PCM: {error}")))
+fn consumed_frames(submitted: u64, buffer: Frames, available: Frames) -> u64 {
+    let queued = (buffer - available.clamp(0, buffer)) as u64;
+    submitted.saturating_sub(queued)
 }
 
-/// Prime the non-blocking ALSA buffer with silence. Subsequent WebSocket
-/// frames inspect buffer progress to detect when the USB host starts reading.
-fn prime_pcm_with_silence(pcm: &PCM, channels: usize) -> Result<()> {
-    let silence = vec![0i16; BUFFER_FRAMES as usize * channels];
-    let io = pcm
-        .io_i16()
-        .map_err(|error| AppError::AudioError(format!("UAC PCM I/O failed: {error}")))?;
-    let mut frame_offset = 0usize;
-    while frame_offset < BUFFER_FRAMES as usize {
-        match io.writei(&silence[frame_offset * channels..]) {
+/// Bound each write to one negotiated period and advance by actual frames,
+/// including short writes. Never wait for space or retain stale audio.
+fn write_frames(
+    samples: &[i16],
+    channels: usize,
+    period_frames: usize,
+    mut write: impl FnMut(&[i16]) -> std::result::Result<usize, alsa::Error>,
+) -> std::result::Result<usize, alsa::Error> {
+    let total_frames = samples.len() / channels;
+    let mut offset = 0;
+    while offset < total_frames {
+        let end = (offset + period_frames).min(total_frames);
+        match write(&samples[offset * channels..end * channels]) {
             Ok(0) => break,
-            Ok(written) => frame_offset += written,
-            Err(error) if error.errno() == libc::EAGAIN => break,
-            Err(error) => {
-                return Err(AppError::AudioError(format!(
-                    "Failed to prime UAC PCM with silence: {error}"
-                )));
+            Ok(written) => offset += written,
+            Err(error) if matches!(error.errno(), libc::EAGAIN | libc::EINTR) => break,
+            Err(error) => return Err(error),
         }
     }
-    }
-    Ok(())
+    Ok(offset)
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn writes_short_frames_without_skipping_stereo_samples() {
+        let samples: Vec<i16> = (0..24).collect();
+        let mut received = Vec::new();
+        let written = write_frames(&samples, 2, 4, |chunk| {
+            assert!(chunk.len() <= 8);
+            // Simulate a device accepting only one frame per write.
+            received.extend_from_slice(&chunk[..2]);
+            Ok(1)
+        })
+        .unwrap();
+        assert_eq!(written, 12);
+        assert_eq!(received, samples);
+    }
+
+    #[test]
+    fn full_device_stops_writing_without_waiting_or_claiming_whole_packet() {
+        let mut calls = 0;
+        let written = write_frames(&[0; 24], 2, 4, |_| {
+            calls += 1;
+            if calls == 1 {
+                Ok(2)
+            } else {
+                Err(alsa::Error::new("test write", libc::EAGAIN))
+            }
+        })
+        .unwrap();
+        assert_eq!(written, 2);
+        assert_eq!(calls, 2);
+    }
+
+    #[test]
+    fn writing_into_free_space_is_not_host_consumption() {
+        // A partially filled or full buffer can exist without any USB I/O.
+        assert_eq!(consumed_frames(1024, 4096, 3072), 0);
+        assert_eq!(consumed_frames(4096, 4096, 0), 0);
+        // Consuming a period, followed by filling it again, preserves progress.
+        assert_eq!(consumed_frames(4096, 4096, 1024), 1024);
+        assert_eq!(consumed_frames(5120, 4096, 0), 1024);
+    }
+
+    fn null_config() -> UacPlaybackConfig {
+        UacPlaybackConfig {
+            device_name: "null".into(),
+            sample_rate: 48_000,
+            channels: 2,
+        }
+    }
+
+    #[test]
+    fn native_pcm_uses_negotiated_start_threshold_and_recovers_to_probing() {
+        // ALSA's null plugin exercises real libasound configuration and I/O
+        // without requiring a USB controller. It cannot verify DWC3 behavior.
+        let config = null_config();
+        let mut pcm = open_pcm(&config).unwrap();
+        assert_eq!(
+            pcm.pcm
+                .sw_params_current()
+                .unwrap()
+                .get_start_threshold()
+                .unwrap(),
+            pcm.buffer_frames
+        );
+        pcm.prime_with_silence(2).unwrap();
+        assert!(pcm.consumption_progress().unwrap());
+        let (sink, accepted) =
+            recover_sink(pcm, &config, alsa::Error::new("test xrun", libc::EPIPE));
+        assert!(!accepted);
+        assert!(matches!(sink, SessionSink::Probing { stalled: true, .. }));
+    }
+
+    #[test]
+    fn stop_closes_an_open_native_pcm_before_returning() {
+        let playback = UacPlayback::start(null_config()).unwrap();
+        let session = playback.acquire_session().unwrap();
+        session.try_write(&[0; 2048]).unwrap();
+        assert!(!matches!(
+            session.runtime.lock().unwrap().sink,
+            SessionSink::Closed { .. }
+        ));
+        playback.stop();
+        assert!(matches!(
+            session.runtime.lock().unwrap().sink,
+            SessionSink::Closed { retry_at: None }
+        ));
+    }
+
+    #[test]
+    fn idle_resume_reopens_instead_of_reusing_previous_sink() {
+        let config = null_config();
+        let mut runtime = SessionRuntime::new();
+        runtime.sink = SessionSink::Closed {
+            retry_at: Some(Instant::now() + Duration::from_secs(60)),
+        };
+        runtime.last_frame = Some(Instant::now() - IDLE_REOPEN_TIMEOUT);
+        runtime.write(&config, &[0; 2048]);
+        assert!(!matches!(runtime.sink, SessionSink::Closed { .. }));
+    }
+
     #[test]
     fn permits_only_one_microphone_session() {
         let playback = UacPlayback::start(UacPlaybackConfig::default()).unwrap();

src/otg/manager.rs

@@ -3,9 +3,9 @@ use std::path::PathBuf;
 use tracing::{debug, error, info, warn};
 
 use super::configfs::{
-    configfs_path, create_dir, create_symlink, find_udc, is_configfs_available, remove_dir,
-    remove_file, write_file, write_file_if_exists, DEFAULT_GADGET_NAME, DEFAULT_USB_BCD_DEVICE,
-    DEFAULT_USB_PRODUCT_ID, DEFAULT_USB_VENDOR_ID, USB_BCD_USB,
+    configfs_path, create_dir, find_udc, is_configfs_available, remove_dir, write_file,
+    write_file_if_exists, DEFAULT_GADGET_NAME, DEFAULT_USB_BCD_DEVICE, DEFAULT_USB_PRODUCT_ID,
+    DEFAULT_USB_VENDOR_ID, USB_BCD_USB,
 };
 use super::function::GadgetFunction;
 use super::hid::HidFunction;
@@ -221,9 +221,7 @@ impl OtgGadgetManager {
     }
 
     pub fn bind(&mut self, udc: &str) -> Result<()> {
-        if let Err(e) = self.recreate_config_links() {
-            warn!("Failed to recreate gadget config links before bind: {}", e);
-        }
+        self.recreate_config_links()?;
 
         debug!("Binding gadget to UDC: {}", udc);
         write_file(&self.gadget_path.join("UDC"), &udc)?;
@@ -385,39 +383,22 @@ impl OtgGadgetManager {
             return Ok(());
         }
 
-        let entries = std::fs::read_dir(&functions_path).map_err(|e| {
+        // ConfigFS binds functions in link insertion order. Preserve the
+        // setup order (UAC before HID), including on rebind: directory
+        // iteration order is unspecified and can change endpoint allocation.
+        for func in &self.functions {
+            let dest = self.config_path.join(func.name());
+            if dest.symlink_metadata().is_ok() {
+                fs::remove_file(&dest).map_err(|error| {
                     AppError::Internal(format!(
-                "Failed to read functions directory {}: {}",
-                functions_path.display(),
-                e
+                        "Failed to remove config link {}: {error}",
+                        dest.display()
                     ))
                 })?;
-
-        for entry in entries.flatten() {
-            let name = entry.file_name();
-            let name = match name.to_str() {
-                Some(n) => n,
-                None => continue,
-            };
-            if !name.contains(".usb") {
-                continue;
-            }
-
-            let src = functions_path.join(name);
-            let dest = self.config_path.join(name);
-
-            if dest.exists() {
-                if let Err(e) = remove_file(&dest) {
-                    warn!(
-                        "Failed to remove existing config link {}: {}",
-                        dest.display(),
-                        e
-                    );
-                    continue;
             }
         }
-
-            create_symlink(&src, &dest)?;
+        for func in &self.functions {
+            func.link(&self.config_path, &self.gadget_path)?;
         }
 
         Ok(())
@@ -470,6 +451,81 @@ pub async fn wait_for_hid_devices(device_paths: &[PathBuf], timeout_ms: u64) ->
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::path::Path;
+    use std::sync::{Arc, Mutex};
+
+    struct RecordedFunction {
+        name: &'static str,
+        links: Arc<Mutex<Vec<String>>>,
+    }
+
+    impl GadgetFunction for RecordedFunction {
+        fn name(&self) -> &str {
+            self.name
+        }
+        fn create(&self, _: &Path) -> Result<()> {
+            Ok(())
+        }
+        fn link(&self, config: &Path, gadget: &Path) -> Result<()> {
+            super::super::configfs::create_symlink(
+                &gadget.join("functions").join(self.name),
+                &config.join(self.name),
+            )?;
+            self.links.lock().unwrap().push(self.name.into());
+            Ok(())
+        }
+        fn unlink(&self, _: &Path) -> Result<()> {
+            Ok(())
+        }
+        fn cleanup(&self, _: &Path) -> Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn rebind_links_uac_before_hid_in_registration_order() {
+        let temp = tempfile::tempdir().unwrap();
+        let mut manager = OtgGadgetManager::new();
+        manager.gadget_path = temp.path().to_path_buf();
+        manager.config_path = temp.path().join("configs/c.1");
+        fs::create_dir_all(&manager.config_path).unwrap();
+        let links = Arc::new(Mutex::new(Vec::new()));
+        for name in ["uac1.usb0", "hid.usb0", "mass_storage.usb0"] {
+            fs::create_dir_all(temp.path().join("functions").join(name)).unwrap();
+            // Include a dangling pre-existing link, as well as testing rebind.
+            std::os::unix::fs::symlink("/nonexistent-uac-test", manager.config_path.join(name))
+                .unwrap();
+            manager
+                .add_function(Box::new(RecordedFunction {
+                    name,
+                    links: Arc::clone(&links),
+                }))
+                .unwrap();
+        }
+        for _ in 0..2 {
+            links.lock().unwrap().clear();
+            manager.recreate_config_links().unwrap();
+            assert_eq!(
+                *links.lock().unwrap(),
+                ["uac1.usb0", "hid.usb0", "mass_storage.usb0"]
+            );
+        }
+    }
+
+    #[test]
+    fn link_failure_prevents_udc_binding() {
+        let temp = tempfile::tempdir().unwrap();
+        let mut manager = OtgGadgetManager::new();
+        manager.gadget_path = temp.path().to_path_buf();
+        manager.config_path = temp.path().join("configs/c.1");
+        fs::create_dir_all(temp.path().join("functions/hid.usb0")).unwrap();
+        // A directory occupying a config link cannot be removed as a file.
+        fs::create_dir_all(manager.config_path.join("hid.usb0")).unwrap();
+        fs::write(temp.path().join("UDC"), "").unwrap();
+        manager.add_keyboard(false).unwrap();
+        assert!(manager.bind("test-udc").is_err());
+        assert_eq!(fs::read_to_string(temp.path().join("UDC")).unwrap(), "");
+    }
 
     #[test]
     fn test_manager_creation() {