Use inotify to wait for the loop device being created

Cargo.toml

@@ -20,6 +20,9 @@ direct_io = []
 errno = "0.2.8"
 libc = "0.2.105"
 
+[target.'cfg(target_os = "android")'.dependencies]
+inotify = "0.10"
+
 [build-dependencies]
 bindgen = { version = "0.60.1", default-features = false, features = ["runtime"] }
 

src/lib.rs

@@ -120,7 +120,7 @@ impl LoopControl {
         })?;
         let dev = format!("{}{}", LOOP_PREFIX, dev_num);
         #[cfg(target_os = "android")]
-        wait_for_device(&dev);
+        let _ = wait_until_file_created(&dev);
         LoopDevice::open(&dev)
     }
 
@@ -520,17 +520,46 @@ fn ioctl_to_error(ret: i32) -> io::Result<i32> {
 // created by userspace daemon ueventd. There could be a noticeable delay
 // between LOOP_CTL_GET_FREE issued and loop device created, so we need to
 // wait until it is created and then continue.
-// The timeout (5s) and time quantum (20ms) is picked randomly, it bares no
-// special meaning but they worked fine empirically.
 #[cfg(target_os = "android")]
-fn wait_for_device<P: AsRef<Path>>(device: P) {
+fn wait_until_file_created<P: AsRef<Path>>(dev: P) -> io::Result<()> {
+    let path = dev.as_ref();
+    if path.exists() {
+        return Ok(());
+    }
+
+    use inotify::{Inotify, WatchMask};
+    let mut inotify = Inotify::init()?;
+    // We need to watch the parent directory because the file may not exist yet
+    // And we can be sure that the parent must exist
+    let parent_dir = path.parent().ok_or(io::Error::new(
+        io::ErrorKind::Other,
+        format!("Cannot get parent directory of {}", path.display()),
+    ))?;
+    inotify.add_watch(parent_dir, WatchMask::CREATE)?;
+
+    // the file may have been created when we are adding the watch
+    if path.exists() {
+        return Ok(());
+    }
+    // 1024 is enough for most cases
+    let mut buffer = [0; 1024];
     let start = std::time::Instant::now();
-    let timeout = std::time::Duration::from_secs(5);
-    let quantum = std::time::Duration::from_millis(20);
-    while !device.as_ref().exists() {
+    let timeout = std::time::Duration::from_secs(2);
+    loop {
         if start.elapsed() > timeout {
             break;
         }
-        std::thread::sleep(quantum);
+        if let Ok(events) = inotify.read_events(&mut buffer) {
+            for event in events {
+                if let Some(ev_path) = path.file_name() {
+                    return Ok(());
+                }
+            }
+        }
     }
-}
+
+    Err(io::Error::new(
+        io::ErrorKind::Other,
+        format!("Timeout waiting for file {} being created!", path.display()),
+    ))
+}