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

README.md

@@ -82,6 +82,9 @@ as well as the following command line options:
 * `-i/--in-place`: Overwrites the input files when formatting instead of
   printing the results to standard output.
 
+* `-p/--parallel`: Process files in parallel, simultaneously across
+  multiple cores.
+
 * `-r/--recursive`: If specified, then the tool will process `.swift` source
   files in any directories listed on the command line and their descendants.
   Without this flag, it is an error to list a directory on the command line.

Sources/swift-format/Frontend/Frontend.swift

@@ -92,7 +92,7 @@ class Frontend {
     if paths.isEmpty {
       processStandardInput()
     } else {
-      processPaths(paths)
+      processPaths(paths, parallel: lintFormatOptions.parallel)
     }
   }
 
@@ -124,28 +124,41 @@ 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) {
-      guard let sourceFile = FileHandle(forReadingAtPath: path) else {
-        diagnosticEngine.diagnose(Diagnostic.Message(.error, "Unable to open \(path)"))
-        continue
+    if parallel {
+      let allFilePaths = Array(FileIterator(paths: paths))
+      DispatchQueue.concurrentPerform(iterations: allFilePaths.count) { index in
+        let path = allFilePaths[index]
+        openAndProcess(path)
       }
-
-      guard let configuration = configuration(
-        atPath: lintFormatOptions.configurationPath, orInferredFromSwiftFileAtPath: path)
-      else {
-        // Already diagnosed in the called method.
-        continue
+    } else {
+      for path in FileIterator(paths: paths) {
+        openAndProcess(path)
       }
+    }
+  }
+
+  /// Read and process the given path, optionally synchronizing diagnostic output.
+  private func openAndProcess(_ path: String) -> Void {
+    guard let sourceFile = FileHandle(forReadingAtPath: path) else {
+      diagnosticEngine.diagnose(Diagnostic.Message(.error, "Unable to open \(path)"))
+      return
+    }
 
-      let fileToProcess = FileToProcess(
-        fileHandle: sourceFile, path: path, configuration: configuration)
-      processFile(fileToProcess)
+    guard let configuration = configuration(
+      atPath: lintFormatOptions.configurationPath, orInferredFromSwiftFileAtPath: path)
+    else {
+      // Already diagnosed in the called method.
+      return
     }
+
+    let fileToProcess = FileToProcess(
+      fileHandle: sourceFile, path: path, configuration: configuration)
+    processFile(fileToProcess)
   }
 
   /// Returns the configuration that applies to the given `.swift` source file, when an explicit

Sources/swift-format/Subcommands/LintFormatOptions.swift

@@ -49,6 +49,12 @@ struct LintFormatOptions: ParsableArguments {
     """)
   var ignoreUnparsableFiles: Bool = false
 
+  /// Whether or not to run the formatter/linter in parallel.
+  @Flag(
+    name: .shortAndLong,
+    help: "Process files in parallel, simultaneously across multiple cores.")
+  var parallel: Bool = false
+
   /// The list of paths to Swift source files that should be formatted or linted.
   @Argument(help: "Zero or more input filenames.")
   var paths: [String] = []