SR-14953: Data.write(to:) fails when writing to /dev/stdout

- FileHandle.synchronize() would throw when calling fsync() on special
  files. Ignore EINVAL and EROFS if fsync() fails, so that the behaviour
  matches Darwin Foundation.

Author: spevans

Sources/Foundation/FileHandle.swift

@@ -595,7 +595,14 @@ open class FileHandle : NSObject {
             throw _NSErrorWithWindowsError(GetLastError(), reading: false)
         }
         #else
-        guard fsync(_fd) >= 0 else { throw _NSErrorWithErrno(errno, reading: false) }
+        // Linux, macOS, OpenBSD return -1 and errno == EINVAL if trying to sync a special file,
+        // eg a fifo, character device etc which can be ignored.
+        // Additionally, Linux may return EROFS if tying to sync on a readonly filesystem, which also can be ignored.
+        // macOS can also return ENOTSUP for pipes but dont ignore it, so that the behaviour matches Darwin's Foundation.
+        if fsync(_fd) < 0 {
+            if errno == EINVAL || errno == EROFS { return }
+            throw _NSErrorWithErrno(errno, reading: false)
+        }
         #endif
     }
 

Tests/Foundation/Tests/TestFileHandle.swift

@@ -613,6 +613,12 @@ class TestFileHandle : XCTestCase {
     }
 #endif
 
+    func testSynchronizeOnSpecialFile() throws {
+        // .synchronize() on a special file shouldnt fail
+        let fh = try XCTUnwrap(FileHandle(forWritingAtPath: "/dev/stdout"))
+        XCTAssertNoThrow(try fh.synchronize())
+    }
+
     static var allTests : [(String, (TestFileHandle) -> () throws -> ())] {
         var tests: [(String, (TestFileHandle) -> () throws -> ())] = [
             ("testReadUpToCount", testReadUpToCount),
@@ -632,6 +638,7 @@ class TestFileHandle : XCTestCase {
             ("test_waitForDataInBackgroundAndNotify", test_waitForDataInBackgroundAndNotify),
             /* ⚠️ */ ("test_readWriteHandlers", testExpectedToFail(test_readWriteHandlers,
             /* ⚠️ */     "<rdar://problem/50860781> sporadically times out")),
+            ("testSynchronizeOnSpecialFile", testSynchronizeOnSpecialFile),
         ]
 
 #if NS_FOUNDATION_ALLOWS_TESTABLE_IMPORT

Tests/Foundation/Tests/TestNSData.swift

@@ -237,6 +237,7 @@ class TestNSData: LoopbackServerTest {
             ("test_writeToURLOptions", test_writeToURLOptions),
             ("test_writeToURLPermissions", test_writeToURLPermissions),
             ("test_writeToURLPermissionsWithAtomic", test_writeToURLPermissionsWithAtomic),
+            ("test_writeToURLSpecialFile", test_writeToURLSpecialFile),
             ("test_edgeNoCopyDescription", test_edgeNoCopyDescription),
             ("test_initializeWithBase64EncodedDataGetsDecodedData", test_initializeWithBase64EncodedDataGetsDecodedData),
             ("test_initializeWithBase64EncodedDataWithNonBase64CharacterIsNil", test_initializeWithBase64EncodedDataWithNonBase64CharacterIsNil),
@@ -623,6 +624,11 @@ class TestNSData: LoopbackServerTest {
 #endif
     }
 
+    func test_writeToURLSpecialFile() {
+        let url = URL(fileURLWithPath: "/dev/stdout")
+        XCTAssertNoThrow(try Data("Output to STDOUT\n".utf8).write(to: url))
+    }
+
     func test_emptyDescription() {
         let expected = "<>"