Multithread processing source files

Previously every source file was formatted / linted one by one. On our
codebase this took a full project format from ~17 minutes to ~5 minutes.

Author: keith

Sources/swift-format/Frontend/Frontend.swift

@@ -92,7 +92,7 @@ class Frontend {
     if paths.isEmpty {
       processStandardInput()
     } else {
-      processPaths(paths)
+      processPaths(paths, parallel: lintFormatOptions.parallel)
     }
   }
 
@@ -124,27 +124,42 @@ class Frontend {
   }
 
   /// Processes source content from a list of files and/or directories provided as paths.
-  private func processPaths(_ paths: [String]) {
+  private func processPaths(_ paths: [String], parallel: Bool) {
     precondition(
       !paths.isEmpty,
       "processPaths(_:) should only be called when paths is non-empty.")
 
-    for path in FileIterator(paths: paths) {
+    let concurrentQueue = DispatchQueue(
+        label: "swift-format-queue", qos: .userInitiated, attributes: .concurrent)
+    let lock = NSLock()
+    let openAndProcess: (String) -> Void = { path in
       guard let sourceFile = FileHandle(forReadingAtPath: path) else {
-        diagnosticEngine.diagnose(Diagnostic.Message(.error, "Unable to open \(path)"))
-        continue
+        lock.lock()
+        defer { lock.unlock() }
+        self.diagnosticEngine.diagnose(Diagnostic.Message(.error, "Unable to open \(path)"))
+        return
       }
 
-      guard let configuration = configuration(
-        atPath: lintFormatOptions.configurationPath, orInferredFromSwiftFileAtPath: path)
+      guard let configuration = self.configuration(
+        atPath: self.lintFormatOptions.configurationPath, orInferredFromSwiftFileAtPath: path)
       else {
         // Already diagnosed in the called method.
-        continue
+        return
       }
 
       let fileToProcess = FileToProcess(
         fileHandle: sourceFile, path: path, configuration: configuration)
-      processFile(fileToProcess)
+      self.processFile(fileToProcess)
+    }
+
+    for path in FileIterator(paths: paths) {
+      if parallel {
+        concurrentQueue.async {
+          openAndProcess(path)
+        }
+      } else {
+        openAndProcess(path)
+      }
     }
   }
 

Sources/swift-format/Subcommands/LintFormatOptions.swift

@@ -49,6 +49,9 @@ struct LintFormatOptions: ParsableArguments {
     """)
   var ignoreUnparsableFiles: Bool
 
+  @Flag(help: "Whether or not to run formatting or linting in parallel consuming all possible resources")
+  var parallel: Bool
+
   /// The list of paths to Swift source files that should be formatted or linted.
   @Argument(help: "Zero or more input filenames.")
   var paths: [String]