sync-manager no longer queues syncs.

CHANGELOG.md

@@ -27,6 +27,10 @@
 - Verify signature of imported data when `.get()` is called with `sync_if_empty: true` ([#7518](https://github.com/mozilla/application-services/pull/7518)) 
 - Do not quote `_since` values with the v2 API ([#7523](https://github.com/mozilla/application-services/pull/7523))
 
+### Sync Manager
+
+- `SyncManager::sync()` now fails immediately with a new `SyncManagerError::Busy` when a sync is already in progress, instead of blocking until it finishes.
+
 # v154.0 (_2026-07-20_)
 
 ## ✨ What's Changed ✨

components/sync_manager/src/error.rs

@@ -10,6 +10,8 @@ pub enum SyncManagerError {
     UnknownEngine(String),
     #[error("Manager was compiled without support for {0:?}")]
     UnsupportedFeature(String),
+    #[error("Another sync is already in progress")]
+    Busy,
     // Used for things like 'failed to decode the provided sync key because it's
     // completely the wrong format', etc.
     #[error("Sync error: {0}")]

components/sync_manager/src/manager.rs

@@ -83,9 +83,14 @@ impl SyncManager {
     }
 
     /// Perform a sync.  See [SyncParams] and [SyncResult] for details on how this works
+    ///
+    /// Fails with [SyncManagerError::Busy] if a sync is already in progress.
     pub fn sync(&self, params: SyncParams) -> Result<SyncResult> {
         breadcrumb!("SyncManager::sync started");
-        let mut state = self.mem_cached_state.lock();
+        let Some(mut state) = self.mem_cached_state.try_lock() else {
+            breadcrumb!("SyncManager::sync is already in progress, bailing out early");
+            return Err(SyncManagerError::Busy);
+        };
         let engines = self.calc_engines_to_sync(&params.engines)?;
         let next_sync_after = state.as_ref().and_then(|mcs| mcs.get_next_sync_after());
         let result = if !backoff_in_effect(next_sync_after, &params) {
@@ -312,11 +317,43 @@ impl CommandProcessor for SyncClient {
 #[cfg(test)]
 mod test {
     use super::*;
+    use crate::types::{DeviceSettings, SyncAuthInfo};
 
     #[test]
     fn test_engine_id_sanity() {
         for engine_id in SyncEngineId::iter() {
             assert_eq!(engine_id, SyncEngineId::try_from(engine_id.name()).unwrap());
         }
     }
+
+    fn dummy_sync_params() -> SyncParams {
+        SyncParams {
+            reason: SyncReason::Scheduled,
+            engines: SyncEngineSelection::All,
+            enabled_changes: HashMap::new(),
+            local_encryption_keys: HashMap::new(),
+            auth_info: SyncAuthInfo {
+                kid: "kid".to_string(),
+                fxa_access_token: "token".to_string(),
+                sync_key: "sync-key".to_string(),
+                tokenserver_url: "https://example.com/token/1.0/sync/1.5".to_string(),
+            },
+            persisted_state: None,
+            device_settings: DeviceSettings {
+                fxa_device_id: "device-id".to_string(),
+                name: "Test Device".to_string(),
+                kind: sync15::DeviceType::Mobile,
+            },
+        }
+    }
+
+    #[test]
+    fn test_sync_is_busy_while_a_sync_is_in_progress() {
+        let manager = SyncManager::new();
+        let _in_progress = manager.mem_cached_state.lock();
+        assert!(matches!(
+            manager.sync(dummy_sync_params()),
+            Err(SyncManagerError::Busy)
+        ));
+    }
 }

components/sync_manager/src/syncmanager.udl

@@ -11,6 +11,7 @@ namespace syncmanager { };
 enum SyncManagerError {
     "UnknownEngine",
     "UnsupportedFeature",
+    "Busy",
     "Sync15Error",
     "UrlParseError",
     "InterruptedError",