Prevent concurrent expunging / compacting of Drafts folders

If `compactDrafts` was waiting for all the folders to become idle and
`compactDrafts` was called again during the waiting, then both would
end up running in parallel and attempting to expunge / compact all the
Drafts folders. Fix this by making subsequent invocations wait for
prior invocations to complete, and by localizing the list of folders
to clean at the start of the loop other code can add new folders to
clean without disrupting the current clean cycle.

background.js

@@ -496,19 +496,57 @@ const SendLater = {
     if (force) SendLater.draftsToClean.slforce = true;
   },
 
+  // There are two different race conditions we're concerned about here. First,
+  // while we're waiting for all of the drafts folders we're cleaning to become
+  // idle, someone else could call cleanDrafts a second time. Second, while
+  // we're awaiting for something in the loop of cleaning all the folders,
+  // someone could add another folder to the list.
+  // To address the first, any time cleanDrafts is invoked it needs to await
+  // for the prior running invocation to finish. We set this up in a loop that
+  // keeps awaiting until there's nothing to wait for, because if multiple
+  // invocations are awaiting for it to finish before they start, one of them
+  // could regain control before we do and start another clean cycle.
+  // To address the second, we make the list of drafts to clean local before we
+  // start the cleaning process, and reinitialize the shared list to an empty
+  // array, so if someone else adds a folder to the list once we've started
+  // cleaning it'll get picked up in the next invocation of cleanDrafts.
+
+  // This is just used so that different invocations of cleanDrafts can be
+  // distinguished from each other in the logs.
+  cdid: 0,
+  cleanDraftsPromise: null,
+
   async cleanDrafts() {
-    if (!SendLater.draftsToClean.length) return;
-    if (SendLater.draftsToClean.slforce || SendLater.prefCache.compactDrafts) {
-      await messenger.SL3U.waitUntilIdle(SendLater.draftsToClean);
-      for (let folder of SendLater.draftsToClean) {
+    let _id = SendLater.cdid++;
+    SLStatic.trace(`cleanDrafts[${_id}]: start`);
+    let waited;
+    while (SendLater.cleanDraftsPromise) {
+      waited = true;
+      SLStatic.debug(
+        `cleanDrafts[${_id}]: waiting for previous clean to finish`,
+      );
+      await SendLater.cleanDraftsPromise;
+    }
+    if (waited) SLStatic.debug(`cleanDrafts[${_id}]: finished waiting`);
+    SendLater.cleanDraftsPromise = SendLater.cleanDraftsReal();
+    await SendLater.cleanDraftsPromise;
+    SendLater.cleanDraftsPromise = null;
+    SLStatic.trace(`cleanDrafts[${_id}]: end`);
+  },
+
+  async cleanDraftsReal() {
+    draftsToClean = SendLater.draftsToClean;
+    SendLater.draftsToClean = [];
+    if (!draftsToClean.length) return;
+    if (draftsToClean.slforce || SendLater.prefCache.compactDrafts) {
+      await messenger.SL3U.waitUntilIdle(draftsToClean);
+      for (let folder of draftsToClean) {
         SLStatic.debug("Cleaning folder:", folder);
         await messenger.SL3U.expungeOrCompactFolder(folder);
       }
-      SendLater.draftsToClean.slforce = false;
     } else {
       SLStatic.debug("Not cleaning folders, preference is disabled");
     }
-    SendLater.draftsToClean.length = 0;
   },
 
   async deleteMessage(hdr) {