Correctly extract parent path on Windows (#227)

* Delegated path filtration to dirnameImpl

Closes #221

Author: fzhinkin

core/android/src/files/FileSystemAndroid.kt

@@ -12,6 +12,9 @@ import platform.posix.dirname
 
 @OptIn(ExperimentalForeignApi::class)
 internal actual fun dirnameImpl(path: String): String {
+    if (!path.contains(SystemPathSeparator)) {
+        return ""
+    }
     return dirname(path)?.toKString() ?: ""
 }
 

core/apple/src/files/FileSystemApple.kt

@@ -25,6 +25,9 @@ public actual val SystemTemporaryDirectory: Path
     get() = Path(NSTemporaryDirectory())
 
 internal actual fun dirnameImpl(path: String): String {
+    if (!path.contains(SystemPathSeparator)) {
+        return ""
+    }
     memScoped {
         return dirname(path.cstr.ptr)?.toKString() ?: ""
     }

core/linux/src/files/FileSystemLinux.kt

@@ -14,6 +14,9 @@ import platform.posix.dirname
 
 @OptIn(ExperimentalForeignApi::class)
 internal actual fun dirnameImpl(path: String): String {
+    if (!path.contains(SystemPathSeparator)) {
+        return ""
+    }
     memScoped {
         return dirname(path.cstr.ptr)?.toKString() ?: ""
     }

core/mingw/src/files/FileSystemMingw.kt

@@ -15,6 +15,8 @@ import platform.windows.MOVEFILE_REPLACE_EXISTING
 import platform.windows.MoveFileExA
 import platform.windows.PathIsRelativeA
 
+private const val WindowsPathSeparator: Char = '\\'
+
 internal actual fun atomicMoveImpl(source: Path, destination: Path) {
     if (MoveFileExA(source.path, destination.path, MOVEFILE_REPLACE_EXISTING.convert()) == 0) {
         // TODO: get formatted error message
@@ -23,6 +25,9 @@ internal actual fun atomicMoveImpl(source: Path, destination: Path) {
 }
 
 internal actual fun dirnameImpl(path: String): String {
+    if (!path.contains(SystemPathSeparator) && !path.contains(WindowsPathSeparator)) {
+        return ""
+    }
     memScoped {
         return dirname(path.cstr.ptr)?.toKString() ?: ""
     }

core/mingw/test/files/SmokeFileTestWindows.kt

@@ -5,9 +5,7 @@
 
 package kotlinx.io.files
 
-import kotlin.test.Test
-import kotlin.test.assertFalse
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 class SmokeFileTestWindows {
     @Test
@@ -20,4 +18,13 @@ class SmokeFileTestWindows {
         assertFalse(Path("bla\\bla\\bla").isAbsolute)
         assertTrue(Path("\\\\server\\share").isAbsolute)
     }
+
+    @Test
+    fun getParent() {
+        assertNull(Path("C:\\").parent)
+        assertNull(Path("a\\b").parent?.parent)
+        assertEquals(Path("\\\\server"), Path("\\\\server\\share").parent)
+        assertEquals(Path("C:\\"), Path("C:\\Program Files").parent)
+        assertEquals(Path("C:\\Program Files"), Path("C:\\Program Files/Java").parent)
+    }
 }

core/native/src/files/PathsNative.kt

@@ -23,7 +23,6 @@ public actual class Path internal constructor(
         get() {
             when {
                 path.isBlank() -> return null
-                !path.contains(SystemPathSeparator) -> return null
             }
             val parentName = dirnameImpl(path)
             return when {