Add support for currentDirectoryPath in Process class.

This patch adds support for the `currentDirectoryPath` property on `Process`

Since `posix_spawn` doesn't make it easy to change the working directory, a
call to `chdir` is issued just before the process executes. It looks like
`lldb` follows a similar pattern in one of its tools
(`tools/darwin-debug/darwin-debug.cpp`).

Author: mattrajca

Foundation/Process.swift

@@ -387,11 +387,30 @@ open class Process: NSObject {
             posix(posix_spawn_file_actions_addclose(&fileActions, fd))
         }
 
+        let fileManager = FileManager()
+        let previousDirectoryPath = fileManager.currentDirectoryPath
+        if !fileManager.changeCurrentDirectoryPath(currentDirectoryPath) {
+            // Foundation throws an NSException when changing the working directory fails,
+            // and unfortunately launch() is not marked `throws`, so we get away with a
+            // fatalError.
+            switch errno {
+            case ENOENT:
+                fatalError("Process: The specified working directory does not exist.")
+            case EACCES:
+                fatalError("Process: The specified working directory cannot be accessed.")
+            default:
+                fatalError("Process: The specified working directory cannot be set.")
+            }
+        }
+
         // Launch
 
         var pid = pid_t()
         posix(posix_spawn(&pid, launchPath, &fileActions, nil, argv, envp))
 
+        // Reset the previous working directory path.
+        fileManager.changeCurrentDirectoryPath(previousDirectoryPath)
+
         // Close the write end of the input and output pipes.
         if let pipe = standardInput as? Pipe {
             pipe.fileHandleForReading.closeFile()

TestFoundation/TestProcess.swift

@@ -28,6 +28,7 @@ class TestProcess : XCTestCase {
                    ("test_pipe_stdin", test_pipe_stdin),
                    ("test_pipe_stdout", test_pipe_stdout),
                    ("test_pipe_stderr", test_pipe_stderr),
+                   ("test_current_working_directory", test_current_working_directory),
                    // disabled for now
                    // ("test_pipe_stdout_and_stderr_same_pipe", test_pipe_stdout_and_stderr_same_pipe),
                    ("test_file_stdout", test_file_stdout),
@@ -262,6 +263,19 @@ class TestProcess : XCTestCase {
             XCTFail("Test failed: \(error)")
         }
     }
+
+    func test_current_working_directory() {
+        do {
+            let previousWorkingDirectory = FileManager.default.currentDirectoryPath
+
+            // `bash` will not be found if the current working directory is not set correctly.
+            let _ = try runTask(["bash", "-c", "exit 0"], currentDirectoryPath: "/bin")
+
+            XCTAssertEqual(previousWorkingDirectory, FileManager.default.currentDirectoryPath)
+        } catch let error {
+            XCTFail("Test failed: \(error)")
+        }
+    }
 }
 
 private func mkstemp(template: String, body: (FileHandle) throws -> Void) rethrows {
@@ -284,14 +298,18 @@ private enum Error: Swift.Error {
     case InvalidEnvironmentVariable(String)
 }
 
-private func runTask(_ arguments: [String], environment: [String: String]? = nil) throws -> String {
+private func runTask(_ arguments: [String], environment: [String: String]? = nil, currentDirectoryPath: String? = nil) throws -> String {
     let process = Process()
 
     var arguments = arguments
     process.launchPath = arguments.removeFirst()
     process.arguments = arguments
     process.environment = environment
 
+    if let directoryPath = currentDirectoryPath {
+        process.currentDirectoryPath = directoryPath
+    }
+
     let pipe = Pipe()
     process.standardOutput = pipe
     process.standardError = pipe