[Collections] Fix racy refresh (#3327)

Motivation:
`testHappyRefresh` still crashes. See:
https://ci.swift.org/job/oss-swift-package-linux-ubuntu-16_04/5991/console

`refreshCollections` leads to target trie getting updated. `Trie` is not thread-safe.

Modifications:
Make `Trie` thread-safe. `TrieNode` should be thread-safe already.

Author: yim-lee

Sources/Basics/ConcurrencyHelpers.swift

@@ -1,7 +1,9 @@
 /*
  This source file is part of the Swift.org open source project
+
  Copyright (c) 2020 Apple Inc. and the Swift project authors
  Licensed under Apache License v2.0 with Runtime Library Exception
+
  See http://swift.org/LICENSE.txt for license information
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
  */

Sources/PackageCollections/Storage/Trie.swift

@@ -1,7 +1,9 @@
 /*
  This source file is part of the Swift.org open source project
- Copyright (c) 2020 Apple Inc. and the Swift project authors
+
+ Copyright (c) 2020-2021 Apple Inc. and the Swift project authors
  Licensed under Apache License v2.0 with Runtime Library Exception
+
  See http://swift.org/LICENSE.txt for license information
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
  */
@@ -14,6 +16,7 @@ struct Trie<Document: Hashable> {
     private typealias Node = TrieNode<Character, Document>
 
     private let root: Node
+    private let lock = Lock()
 
     init() {
         self.root = Node()
@@ -23,17 +26,19 @@ struct Trie<Document: Hashable> {
     func insert(word: String, foundIn document: Document) {
         guard !word.isEmpty else { return }
 
-        var currentNode = self.root
-        // Check if word already exists otherwise creates the node path
-        for character in word.lowercased() {
-            if let child = currentNode.children[character] {
-                currentNode = child
-            } else {
-                currentNode = currentNode.add(value: character)
+        self.lock.withLock {
+            var currentNode = self.root
+            // Check if word already exists otherwise creates the node path
+            for character in word.lowercased() {
+                if let child = currentNode.children[character] {
+                    currentNode = child
+                } else {
+                    currentNode = currentNode.add(value: character)
+                }
             }
-        }
 
-        currentNode.add(document: document)
+            currentNode.add(document: document)
+        }
     }
 
     /// Removes word occurrences found in the given document.
@@ -57,7 +62,9 @@ struct Trie<Document: Hashable> {
             }
         }
 
-        removeInSubTrie(root: self.root, document: document)
+        self.lock.withLock {
+            removeInSubTrie(root: self.root, document: document)
+        }
     }
 
     /// Removes word occurrences found in matching document(s).
@@ -81,7 +88,9 @@ struct Trie<Document: Hashable> {
             }
         }
 
-        removeInSubTrie(root: self.root, where: predicate)
+        self.lock.withLock {
+            removeInSubTrie(root: self.root, where: predicate)
+        }
     }
 
     /// Checks if the trie contains the exact word or words with matching prefix.
@@ -149,15 +158,17 @@ struct Trie<Document: Hashable> {
     private func findLastNodeOf(word: String) -> Node? {
         guard !word.isEmpty else { return nil }
 
-        var currentNode = self.root
-        // Traverse down the trie as far as we can
-        for character in word.lowercased() {
-            guard let child = currentNode.children[character] else {
-                return nil
+        return self.lock.withLock {
+            var currentNode = self.root
+            // Traverse down the trie as far as we can
+            for character in word.lowercased() {
+                guard let child = currentNode.children[character] else {
+                    return nil
+                }
+                currentNode = child
             }
-            currentNode = child
+            return currentNode
         }
-        return currentNode
     }
 }