From: Thomas Karpiniec <tom.karpiniec@outlook.com>
Date: Sat, 8 Jun 2024 07:19:14 +0000 (+1000)
Subject: Clean up race condition in initial watch on Windows
X-Git-Tag: v0.1.0~22
X-Git-Url: https://code.octet-stream.net/netwatcher/commitdiff_plain/74d0fddfbb570514118b4ddc228ce19ccd8bb969?hp=cc3e849e9ff8568609f160d4bcdb83966ffdb6c5

Clean up race condition in initial watch on Windows
---

diff --git a/src/watch_win.rs b/src/watch_win.rs
index c88cd7c..67e0d4c 100644
--- a/src/watch_win.rs
+++ b/src/watch_win.rs
@@ -40,18 +40,10 @@ impl Drop for WatchHandle {
 }
 
 pub(crate) fn watch_interfaces<F: FnMut(Update) + 'static>(
-    mut callback: F,
+    callback: F,
 ) -> Result<WatchHandle, Error> {
-    let null_list = List::default();
-    let prev_list = crate::list::list_interfaces()?;
-    callback(Update {
-        interfaces: prev_list.0.clone(),
-        diff: prev_list.diff_from(&null_list),
-    });
-
-    // TODO: Can wo do something about the race condition?
     let state = Box::pin(Mutex::new(WatchState {
-        prev_list,
+        prev_list: List::default(),
         cb: Box::new(callback),
     }));
     let state_ctx = &*state.as_ref() as *const _ as *const c_void;
@@ -67,7 +59,14 @@ pub(crate) fn watch_interfaces<F: FnMut(Update) + 'static>(
         )
     };
     match res {
-        NO_ERROR => Ok(WatchHandle { hnd, _state: state }),
+        NO_ERROR => {
+            // Trigger an initial update.
+            // This is allowed to race with true updates because it
+            // will always calculate a diff and discard no-ops.
+            handle_notif(&mut *state.lock().unwrap());
+            // Then return the handle
+            Ok(WatchHandle { hnd, _state: state })
+        }
         ERROR_INVALID_HANDLE => Err(Error::Internal),
         ERROR_INVALID_PARAMETER => Err(Error::Internal),
         ERROR_NOT_ENOUGH_MEMORY => Err(Error::Internal),
@@ -80,24 +79,24 @@ unsafe extern "system" fn notif(
     _row: *const MIB_UNICASTIPADDRESS_ROW,
     _notification_type: MIB_NOTIFICATION_TYPE,
 ) {
-    println!("There was a change!");
-    let Ok(new_list) = crate::list::list_interfaces() else {
-        println!("Failed to get list of interfaces on change");
-        return;
-    };
     let state_ptr = ctx as *const Mutex<WatchState>;
     unsafe {
         let state_guard = &mut *state_ptr.as_ref().unwrap().lock().unwrap();
-        if new_list == state_guard.prev_list {
-            // TODO: Hitting this a lot, is it true?
-            println!("Interfaces seem to be the same, ignoring");
-            return;
-        }
-        let update = Update {
-            interfaces: new_list.0.clone(),
-            diff: new_list.diff_from(&state_guard.prev_list),
-        };
-        (state_guard.cb)(update);
-        state_guard.prev_list = new_list;
+        handle_notif(state_guard);
+    }
+}
+
+fn handle_notif(state: &mut WatchState) {
+    let Ok(new_list) = crate::list::list_interfaces() else {
+        return;
+    };
+    if new_list == state.prev_list {
+        return;
     }
+    let update = Update {
+        interfaces: new_list.0.clone(),
+        diff: new_list.diff_from(&state.prev_list),
+    };
+    (state.cb)(update);
+    state.prev_list = new_list;
 }