FileManager: Fix fileExists(atPath:isDirectory:) with dangling symlink (#1514)

- When the path item is a symlink and the target of the symlink does
  not exist, return false instead of true.

- Combine the calls to stat() when the file is a symlink.

Author: spevans

Foundation/FileManager.swift

@@ -593,32 +593,26 @@ open class FileManager : NSObject {
 
     open func fileExists(atPath path: String, isDirectory: UnsafeMutablePointer<ObjCBool>?) -> Bool {
         var s = stat()
-        if lstat(path, &s) >= 0 {
-            if let isDirectory = isDirectory {
-                if (s.st_mode & S_IFMT) == S_IFLNK {
-                    if stat(path, &s) >= 0 {
-                        isDirectory.pointee = ObjCBool((s.st_mode & S_IFMT) == S_IFDIR)
-                    } else {
-                        return false
-                    }
-                } else {
-                    let isDir = (s.st_mode & S_IFMT) == S_IFDIR
-                    isDirectory.pointee = ObjCBool(isDir)
-                }
-            }
+        guard lstat(path, &s) >= 0 else {
+            return false
+        }
 
+        if (s.st_mode & S_IFMT) == S_IFLNK {
             // don't chase the link for this magic case -- we might be /Net/foo
             // which is a symlink to /private/Net/foo which is not yet mounted...
-            if (s.st_mode & S_IFMT) == S_IFLNK {
-                if (s.st_mode & S_ISVTX) == S_ISVTX {
-                    return true
-                }
-                // chase the link; too bad if it is a slink to /Net/foo
-                stat(path, &s)
+            if isDirectory == nil && (s.st_mode & S_ISVTX) == S_ISVTX {
+                return true
+            }
+            // chase the link; too bad if it is a slink to /Net/foo
+            guard stat(path, &s) >= 0 else {
+                return false
             }
-        } else {
-            return false
         }
+
+        if let isDirectory = isDirectory {
+            isDirectory.pointee = ObjCBool((s.st_mode & S_IFMT) == S_IFDIR)
+        }
+
         return true
     }
 

TestFoundation/TestFileManager.swift

@@ -23,6 +23,7 @@ class TestFileManager : XCTestCase {
             ("test_createFile", test_createFile ),
             ("test_moveFile", test_moveFile),
             ("test_fileSystemRepresentation", test_fileSystemRepresentation),
+            ("test_fileExists", test_fileExists),
             ("test_fileAttributes", test_fileAttributes),
             ("test_fileSystemAttributes", test_fileSystemAttributes),
             ("test_setFileAttributes", test_setFileAttributes),
@@ -161,7 +162,53 @@ class TestFileManager : XCTestCase {
         result.deallocate()
 #endif
     }
-    
+
+    func test_fileExists() {
+        let fm = FileManager.default
+        let tmpDir = fm.temporaryDirectory.appendingPathComponent("testFileExistsDir")
+        let testFile = tmpDir.appendingPathComponent("testFile")
+        let goodSymLink = tmpDir.appendingPathComponent("goodSymLink")
+        let badSymLink = tmpDir.appendingPathComponent("badSymLink")
+        let dirSymLink = tmpDir.appendingPathComponent("dirSymlink")
+
+        ignoreError { try fm.removeItem(atPath: tmpDir.path) }
+
+        do {
+            try fm.createDirectory(atPath: tmpDir.path, withIntermediateDirectories: false, attributes: nil)
+            XCTAssertTrue(fm.createFile(atPath: testFile.path, contents: Data()))
+            try fm.createSymbolicLink(atPath: goodSymLink.path, withDestinationPath: testFile.path)
+            try fm.createSymbolicLink(atPath: badSymLink.path, withDestinationPath: "no_such_file")
+            try fm.createSymbolicLink(atPath: dirSymLink.path, withDestinationPath: "..")
+
+            var isDirFlag: ObjCBool = false
+            XCTAssertTrue(fm.fileExists(atPath: tmpDir.path))
+            XCTAssertTrue(fm.fileExists(atPath: tmpDir.path, isDirectory: &isDirFlag))
+            XCTAssertTrue(isDirFlag.boolValue)
+
+            isDirFlag = true
+            XCTAssertTrue(fm.fileExists(atPath: testFile.path))
+            XCTAssertTrue(fm.fileExists(atPath: testFile.path, isDirectory: &isDirFlag))
+            XCTAssertFalse(isDirFlag.boolValue)
+
+            isDirFlag = true
+            XCTAssertTrue(fm.fileExists(atPath: goodSymLink.path))
+            XCTAssertTrue(fm.fileExists(atPath: goodSymLink.path, isDirectory: &isDirFlag))
+            XCTAssertFalse(isDirFlag.boolValue)
+
+            isDirFlag = true
+            XCTAssertFalse(fm.fileExists(atPath: badSymLink.path))
+            XCTAssertFalse(fm.fileExists(atPath: badSymLink.path, isDirectory: &isDirFlag))
+
+            isDirFlag = false
+            XCTAssertTrue(fm.fileExists(atPath: dirSymLink.path))
+            XCTAssertTrue(fm.fileExists(atPath: dirSymLink.path, isDirectory: &isDirFlag))
+            XCTAssertTrue(isDirFlag.boolValue)
+        } catch {
+            XCTFail(String(describing: error))
+        }
+        ignoreError { try fm.removeItem(atPath: tmpDir.path) }
+    }
+
     func test_fileAttributes() {
         let fm = FileManager.default
         let path = NSTemporaryDirectory() + "test_fileAttributes\(NSUUID().uuidString)"