Add lock on queue traversal

jobs-sled/src/lib.rs

@@ -17,6 +17,7 @@ pub struct SledStorage {
     running_inverse: Tree<u64>,
     queue: Tree<String>,
     stats: Tree<Stats>,
+    lock: Tree<u64>,
     db: sled::Db,
 }
 
@@ -36,16 +37,24 @@ impl Storage for SledStorage {
     }
 
     fn fetch_job_from_queue(&mut self, queue: &str) -> Result<Option<JobInfo>> {
-        let job = self
-            .queue
-            .iter()
-            .filter_map(|res| res.ok())
-            .filter_map(|(id, in_queue)| if queue == in_queue { Some(id) } else { None })
-            .filter_map(|id| self.jobinfo.get(id).ok())
-            .filter_map(|opt| opt)
-            .next();
+        let queue_tree = self.queue.clone();
+        let job_tree = self.jobinfo.clone();
 
-        Ok(job)
+        self.lock_queue(queue, move || {
+            let job = queue_tree
+                .iter()
+                .filter_map(|res| res.ok())
+                .filter_map(|(id, in_queue)| if queue == in_queue { Some(id) } else { None })
+                .filter_map(|id| job_tree.get(id).ok())
+                .filter_map(|opt| opt)
+                .next();
+
+            if let Some(ref job) = job {
+                queue_tree.del(&job_key(job.id()))?;
+            }
+
+            Ok(job)
+        })
     }
 
     fn queue_job(&mut self, queue: &str, id: u64) -> Result<()> {
@@ -104,9 +113,33 @@ impl SledStorage {
             running_inverse: open_tree(&db, "background-jobs-running-inverse")?,
             queue: open_tree(&db, "background-jobs-queue")?,
             stats: open_tree(&db, "background-jobs-stats")?,
+            lock: open_tree(&db, "background-jobs-lock")?,
             db,
         })
     }
+
+    fn lock_queue<T, F>(&self, queue: &str, f: F) -> Result<T>
+    where
+        F: Fn() -> Result<T>,
+    {
+        let id = self.db.generate_id()?;
+
+        let mut prev; 
+        while {
+            prev = self.lock.fetch_and_update(queue, move |opt| match opt {
+                Some(_) => opt,
+                None => Some(id),
+            })?;
+
+            prev.is_some()
+        } {}
+
+        let res = (f)();
+
+        self.lock.fetch_and_update(queue, |_| None)?;
+
+        res
+    }
 }
 
 fn job_key(id: u64) -> String {