agent: fix race condition when test watcher

create_tmpfs won't pass as the race condition in watcher umount. quote
James's words here:

1. Rust runs all tests in parallel.
2. Mounts are a process-wide, not a per-thread resource.
The only test that calls watcher.mount() is create_tmpfs().
However, other tests create BindWatcher objects.
3. BindWatcher's drop() implementation calls self.cleanup(),
which calls unmount for the mountpoint create_tmpfs() asserts.
4. The other tests are calling unmount whenever a BindWatcher goes
out of scope.

To avoid that issue, let the tests using BindWatcher in watcher and
sandbox.rs run sequentially.

Fixes: #2809
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

src/agent/Cargo.lock

@@ -546,6 +546,7 @@ dependencies = [
  "scopeguard",
  "serde",
  "serde_json",
+ "serial_test",
  "slog",
  "slog-scope",
  "slog-stdlog",

src/agent/Cargo.toml

@@ -20,6 +20,7 @@ scan_fmt = "0.2.3"
 scopeguard = "1.0.0"
 thiserror = "1.0.26"
 regex = "1"
+serial_test = "0.5.1"
 
 # Async helpers
 async-trait = "0.1.42"

src/agent/src/sandbox.rs

@@ -464,7 +464,10 @@ mod tests {
         baremount(src, dst, "bind", MsFlags::MS_BIND, "", logger)
     }
 
+    use serial_test::serial;
+
     #[tokio::test]
+    #[serial]
     async fn set_sandbox_storage() {
         let logger = slog::Logger::root(slog::Discard, o!());
         let mut s = Sandbox::new(&logger).unwrap();
@@ -499,6 +502,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn remove_sandbox_storage() {
         skip_if_not_root!();
 
@@ -555,6 +559,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn unset_and_remove_sandbox_storage() {
         skip_if_not_root!();
 
@@ -606,6 +611,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn unset_sandbox_storage() {
         let logger = slog::Logger::root(slog::Discard, o!());
         let mut s = Sandbox::new(&logger).unwrap();
@@ -689,6 +695,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn get_container_entry_exist() {
         skip_if_not_root!();
         let logger = slog::Logger::root(slog::Discard, o!());
@@ -702,6 +709,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn get_container_no_entry() {
         let logger = slog::Logger::root(slog::Discard, o!());
         let mut s = Sandbox::new(&logger).unwrap();
@@ -711,6 +719,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn add_and_get_container() {
         skip_if_not_root!();
         let logger = slog::Logger::root(slog::Discard, o!());
@@ -722,6 +731,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn update_shared_pidns() {
         skip_if_not_root!();
         let logger = slog::Logger::root(slog::Discard, o!());
@@ -740,6 +750,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn add_guest_hooks() {
         let logger = slog::Logger::root(slog::Discard, o!());
         let mut s = Sandbox::new(&logger).unwrap();
@@ -763,6 +774,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn test_sandbox_set_destroy() {
         let logger = slog::Logger::root(slog::Discard, o!());
         let mut s = Sandbox::new(&logger).unwrap();

src/agent/src/watcher.rs

@@ -979,7 +979,10 @@ mod tests {
         );
     }
 
+    use serial_test::serial;
+
     #[tokio::test]
+    #[serial]
     async fn create_tmpfs() {
         skip_if_not_root!();
 
@@ -994,6 +997,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn spawn_thread() {
         skip_if_not_root!();
 
@@ -1023,6 +1027,7 @@ mod tests {
     }
 
     #[tokio::test]
+    #[serial]
     async fn verify_container_cleanup_watching() {
         skip_if_not_root!();