Pool has a QoS

Author: groue

GRDB/Core/DatabasePool.swift

@@ -57,14 +57,17 @@ public final class DatabasePool: DatabaseWriter {
         readerConfiguration.allowsUnsafeTransactions = false
 
         var readerCount = 0
-        readerPool = Pool(maximumCount: configuration.maximumReaderCount, makeElement: {
-            readerCount += 1 // protected by Pool (TODO: document this protection behavior)
-            return try SerializedDatabase(
-                path: path,
-                configuration: readerConfiguration,
-                defaultLabel: "GRDB.DatabasePool",
-                purpose: "reader.\(readerCount)")
-        })
+        readerPool = Pool(
+            maximumCount: configuration.maximumReaderCount,
+            qos: configuration.readQoS,
+            makeElement: {
+                readerCount += 1 // protected by Pool (TODO: document this protection behavior)
+                return try SerializedDatabase(
+                    path: path,
+                    configuration: readerConfiguration,
+                    defaultLabel: "GRDB.DatabasePool",
+                    purpose: "reader.\(readerCount)")
+            })
 
         // Activate WAL Mode unless readonly
         if !configuration.readonly {

GRDB/Utils/Pool.swift

@@ -53,13 +53,24 @@ final class Pool<T> {
     private let barrierQueue: DispatchQueue
     private let semaphoreWaitingQueue: DispatchQueue // Inspired by https://khanlou.com/2016/04/the-GCD-handbook/
 
-    init(maximumCount: Int, makeElement: @escaping () throws -> T) {
+    /// Creates a Pool.
+    ///
+    /// - parameters:
+    ///     - maximumCount: The maximum number of elements.
+    ///     - qos: The quality of service of asynchronous accesses.
+    ///     - makeElement: A function that creates an element. It is called
+    ///       on demand.
+    init(
+        maximumCount: Int,
+        qos: DispatchQoS = .unspecified,
+        makeElement: @escaping () throws -> T)
+    {
         GRDBPrecondition(maximumCount > 0, "Pool size must be at least 1")
         self.makeElement = makeElement
         self.itemsSemaphore = DispatchSemaphore(value: maximumCount)
         self.itemsGroup = DispatchGroup()
-        self.barrierQueue = DispatchQueue(label: "GRDB.Pool.barrier", attributes: [.concurrent])
-        self.semaphoreWaitingQueue = DispatchQueue(label: "GRDB.Pool.wait")
+        self.barrierQueue = DispatchQueue(label: "GRDB.Pool.barrier", qos: qos, attributes: [.concurrent])
+        self.semaphoreWaitingQueue = DispatchQueue(label: "GRDB.Pool.wait", qos: qos)
     }
 
     /// Returns a tuple (element, release)