From f9df7170ac929ccc915642c2e6415e768973bb54 Mon Sep 17 00:00:00 2001
From: Thomas Karpiniec <tom.karpiniec@outlook.com>
Date: Sat, 15 Jun 2024 12:30:32 +0100
Subject: [PATCH 1/1] Commit to reporting error if original interface listing
 fails

---
 src/lib.rs         |  7 +++++++
 src/watch_linux.rs | 34 ++++++++++++++++++----------------
 2 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 5c8d73d..cb3a117 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -145,6 +145,13 @@ pub fn list_interfaces() -> Result<HashMap<IfIndex, Interface>, Error> {
 ///
 /// The callback will fire once immediately with an initial interface list, and a diff as if
 /// there were originally no interfaces present.
+///
+/// This function will return an error if there is a problem configuring the watcher, or if there
+/// is an error retrieving the initial interface list.
+///
+/// We assume that if listing the interfaces worked the first time, then it will continue to work
+/// for as long as the watcher is running. If listing interfaces begins to fail later, those
+/// failures will be swallowed and the callback will not be called for that change event.
 pub fn watch_interfaces<F: FnMut(Update) + Send + 'static>(
     callback: F,
 ) -> Result<WatchHandle, Error> {
diff --git a/src/watch_linux.rs b/src/watch_linux.rs
index e584211..95ef106 100644
--- a/src/watch_linux.rs
+++ b/src/watch_linux.rs
@@ -51,24 +51,26 @@ fn start_watcher_thread<F: FnMut(Update) + Send + 'static>(
     bind(sockfd.as_raw_fd(), &sa_nl).map_err(|_| Error::Internal)?; // TODO: proper errors
     let (pipe_rd, pipe_wr) = pipe().map_err(|_| Error::Internal)?;
 
-    std::thread::spawn(move || {
-        let mut prev_list = List::default();
-        let mut buf = [0u8; 4096];
-        let mut handle_update = move |new_list: List| {
-            if new_list == prev_list {
-                return;
-            }
-            let update = Update {
-                interfaces: new_list.0.clone(),
-                diff: new_list.diff_from(&prev_list),
-            };
-            (callback)(update);
-            prev_list = new_list;
+    let mut prev_list = List::default();
+    let mut handle_update = move |new_list: List| {
+        if new_list == prev_list {
+            return;
+        }
+        let update = Update {
+            interfaces: new_list.0.clone(),
+            diff: new_list.diff_from(&prev_list),
         };
+        (callback)(update);
+        prev_list = new_list;
+    };
 
-        if let Ok(initial) = crate::list::list_interfaces() {
-            handle_update(initial);
-        };
+    // Now that netlink socket is open, provide an initial update.
+    // By having this outside the thread we can return an error synchronously if it
+    // looks like we're going to have trouble listing interfaces.
+    handle_update(crate::list::list_interfaces()?);
+
+    std::thread::spawn(move || {
+        let mut buf = [0u8; 4096];
 
         loop {
             let mut fds = [
-- 
2.39.5