dav: improve pipelining

Signed-off-by: Varun Patil <radialapps@gmail.com>

src/services/dav/base.ts

@@ -150,14 +150,43 @@ async function getFilesInternal(fileIds: number[]): Promise<IFileInfo[]> {
  * Run promises in parallel, but only n at a time
  * @param promises Array of promise generator funnction (async functions)
  * @param n Number of promises to run in parallel
+ * @details Each promise returned MUST resolve and not throw an error
+ * @returns Generator of lists of results. Each list is of length n.
  */
 export async function* runInParallel<T>(promises: (() => Promise<T>)[], n: number) {
-  while (promises.length > 0) {
-    const promisesToRun = promises.splice(0, n);
-    const resultsForThisBatch = await Promise.all(promisesToRun.map((p) => p()));
-    yield resultsForThisBatch;
+  promises.reverse(); // reverse so we can use pop() efficiently
+
+  const results: T[] = [];
+  const running: Promise<void>[] = [];
+
+  while (true) {
+    // add one promise per iteration
+    if (promises.length) {
+      let task!: Promise<void>;
+      running.push(
+        (task = (async () => {
+          // run the promise
+          results.push(await promises.pop()!());
+
+          // remove the promise from the running list
+          running.splice(running.indexOf(task), 1);
+        })())
+      );
+    }
+
+    // wait for one of the promises to finish
+    if (running.length >= n || !promises.length) {
+      await Promise.race(running);
+    }
+
+    // yield the results if the threshold is reached
+    if (results.length >= n || !running.length) {
+      yield results.splice(0, results.length);
+    }
+
+    // stop when all promises are done
+    if (!running.length) break;
   }
-  return;
 }
 
 /**