Add minimal IgnoreFile abstraction.

Sources/SwiftFormat/Core/IgnoreFile.swift

@@ -0,0 +1,82 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+public class IgnoreFile {
+  /// Name of the ignore file to look for.
+  /// The presence of this file in a directory will cause the formatter
+  /// to skip formatting files in that directory and its subdirectories.
+  fileprivate static let fileName = ".swift-format-ignore"
+
+  /// Errors that can be thrown by the IgnoreFile initializer.
+  public enum Error: Swift.Error {
+    /// Error thrown when an ignore file has invalid content.
+    case invalidContent(URL)
+  }
+
+  /// Create an instance from the contents of the file at the given URL.
+  /// Throws an error if the file content can't be read, or is not valid.
+  public init(contentsOf url: URL) throws {
+    let content = try String(contentsOf: url, encoding: .utf8)
+      guard content.trimmingCharacters(in: .whitespacesAndNewlines) == "*" else {
+        throw Error.invalidContent(url)
+      }
+  } 
+
+  /// Create an instance for the given directory, if a valid
+  /// ignore file with the standard name is found in that directory.
+  /// Returns nil if no ignore file is found.
+  /// Throws an error if an invalid ignore file is found.
+  ///
+  /// Note that this initializer does not search parent directories for ignore files.
+  public convenience init?(forDirectory directory: URL) throws {
+    let url = directory.appendingPathComponent(IgnoreFile.fileName)
+    guard FileManager.default.isReadableFile(atPath: url.path) else {
+      return nil
+    }
+
+    try self.init(contentsOf: url)
+  }
+
+  /// Create an instance to use for the given URL.
+  /// We search for an ignore file starting from the given URL's container,
+  /// and moving up the directory tree, until we reach the root directory.
+  /// Returns nil if no ignore file is found.
+  /// Throws an error if an invalid ignore file is found somewhere 
+  /// in the directory tree.
+  public convenience init?(for url: URL) throws {
+      var containingDirectory = url.absoluteURL.standardized
+      repeat {
+        containingDirectory.deleteLastPathComponent()
+        let url = containingDirectory.appendingPathComponent(IgnoreFile.fileName)
+        if FileManager.default.isReadableFile(atPath: url.path) {
+          try self.init(contentsOf: url)
+          return
+        }
+      } while !containingDirectory.isRoot
+      return nil
+  }
+
+  /// Should the given URL be processed?
+  /// Currently the only valid ignore file content is "*",
+  /// which means that all files should be ignored.
+  func shouldProcess(_ url: URL) -> Bool {
+    return false
+  }
+
+  /// Returns true if the name of the given URL matches 
+  /// the standard ignore file name.
+  public static func isStandardIgnoreFile(_ url: URL) -> Bool {
+    return url.lastPathComponent == fileName
+  }
+}

Sources/SwiftFormat/Utilities/FileIterator.swift

@@ -17,11 +17,6 @@ import Foundation
 @_spi(Internal)
 public struct FileIterator: Sequence, IteratorProtocol {
 
-  /// Name of the ignore file to look for.
-  /// The presence of this file in a directory will cause the formatter
-  /// to skip formatting files in that directory and its subdirectories.
-  fileprivate static let ignoreFileName = ".swift-format-ignore"
-
   /// List of file and directory URLs to iterate over.
   private let urls: [URL]
 
@@ -88,8 +83,18 @@ public struct FileIterator: Sequence, IteratorProtocol {
           fallthrough
 
         case .typeDirectory:
-          let ignoreFile = next.appendingPathComponent(Self.ignoreFileName)
-          if FileManager.default.fileExists(atPath: ignoreFile.path) {
+          do {
+            if let ignoreFile = try IgnoreFile(forDirectory: next), !ignoreFile.shouldProcess(next) {
+              // skip this directory and its subdirectories if it should be ignored
+              continue
+            }
+          } catch IgnoreFile.Error.invalidContent(let url) {
+            // we hit an invalid ignore file
+            // we skip the directory, but return the path of the ignore file
+            // so that we can report an error
+            output = url
+          } catch {
+            // we hit another unexpected error; just skip the directory
             continue
           }
 
@@ -182,20 +187,17 @@ private func fileType(at url: URL) -> FileAttributeType? {
 
 /// Returns true if the file should be processed.
 /// Directories are always processed.
-/// Other files are processed if there is not a 
-/// ignore file in the containing directory or any of its parents.
+/// For other files, we look for an ignore file in the containing 
+/// directory or any of its parents.
+/// If there is no ignore file, we process the file.
+/// If an ignore file is found, we consult it to see if the file should be processed.
+/// An invalid ignore file is treated here as if it does not exist, but
+/// will be reported as an error when we try to process the directory.
 private func inputShouldBeProcessed(at url: URL) -> Bool {
   guard fileType(at: url) != .typeDirectory else {
     return true
   }
 
-  var containingDirectory = url.absoluteURL.standardized
-  repeat {
-    containingDirectory.deleteLastPathComponent()
-    let candidateFile = containingDirectory.appendingPathComponent(FileIterator.ignoreFileName)
-    if FileManager.default.isReadableFile(atPath: candidateFile.path) {
-      return false
-    }
-  } while !containingDirectory.isRoot
-  return true
+  let ignoreFile = try? IgnoreFile(for: url)
+  return ignoreFile?.shouldProcess(url) ?? true
 }

Sources/swift-format/Frontend/Frontend.swift

@@ -165,6 +165,13 @@ class Frontend {
   /// Read and prepare the file at the given path for processing, optionally synchronizing
   /// diagnostic output.
   private func openAndPrepareFile(at url: URL) -> FileToProcess? {
+    guard !IgnoreFile.isStandardIgnoreFile(url) else {
+      diagnosticsEngine.emitError(
+        "Invalid ignore file \(url.relativePath): currently the only supported content for ignore files is a single asterisk `*`, which matches all files."
+      )
+      return nil
+    }
+
     guard let sourceFile = try? FileHandle(forReadingFrom: url) else {
       diagnosticsEngine.emitError(
         "Unable to open \(url.relativePath): file is not readable or does not exist"