Strip any UNC long path prefix from _wgetcwd

- This keeps any UNC long file or device prefix (\\?\) out of
  URL that were relative and then resolved to absolute urls.

Author: daveinglis

Sources/CoreFoundation/CFPlatform.c

@@ -35,6 +35,7 @@
 #include <shellapi.h>
 #include <shlobj.h>
 #include <shlwapi.h>
+#include <pathcch.h>
 #include <WinIoCtl.h>
 #include <direct.h>
 #include <process.h>
@@ -1140,9 +1141,22 @@ CF_EXPORT char *_NS_getcwd(char *dstbuf, size_t size) {
     if (!buf) {
         return NULL;
     }
+    
+    // Strip UNC long path prefixe (\\?\) from the wide character buffer using PathCchStripPrefix
+    wchar_t *pathToConvert = buf;
+    size_t pathLen = wcslen(buf);
+    
+    // Use PathCchStripPrefix to remove UNC long path prefixes
+    HRESULT hr = PathCchStripPrefix(buf, pathLen + 1);
+    if (SUCCEEDED(hr)) {
+        pathToConvert = buf;
+    } else {
+        // If PathCchStripPrefix fails, fall back to the original buffer
+        pathToConvert = buf;
+    }
 
     // Convert result to UTF8
-    copyToNarrowFileSystemRepresentation(buf, (CFIndex)size, dstbuf);
+    copyToNarrowFileSystemRepresentation(pathToConvert, (CFIndex)size, dstbuf);
     free(buf);
     return dstbuf;
 }

Tests/Foundation/TestCFPlatformGetcwd.swift

@@ -0,0 +1,111 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+
+#if os(Windows)
+// Import Windows C functions
+import WinSDK
+
+// Declare _NS_getcwd function for testing
+@_silgen_name("_NS_getcwd")
+func _NS_getcwd(_ buffer: UnsafeMutablePointer<CChar>, _ size: Int) -> UnsafeMutablePointer<CChar>?
+#endif
+
+class TestCFPlatformGetcwd : XCTestCase {
+    
+    #if os(Windows)
+    func test_NS_getcwd_UNC_prefix_stripping() {
+        // Test that _NS_getcwd properly strips UNC long path prefixes using PathCchStripPrefix
+        
+        // Create a temporary directory to work with
+        let fm = FileManager.default
+        let tempDir = fm.temporaryDirectory.appendingPathComponent("test_getcwd_\(UUID().uuidString)")
+        
+        do {
+            try fm.createDirectory(at: tempDir, withIntermediateDirectories: true)
+            defer { try? fm.removeItem(at: tempDir) }
+            
+            // Get original directory for restoration
+            var originalBuffer = [CChar](repeating: 0, count: Int(MAX_PATH))
+            guard _NS_getcwd(&originalBuffer, originalBuffer.count) != nil else {
+                XCTFail("Failed to get original directory")
+                return
+            }
+            let originalDir = String(cString: originalBuffer)
+            
+            defer {
+                // Restore original directory
+                originalDir.withCString { _chdir($0) }
+            }
+            
+            // Test with UNC long path prefix \\?\
+            let uncLongPathPrefix = "\\\\?\\" + tempDir.path
+            let uncLongPathCString = uncLongPathPrefix.cString(using: .utf8)!
+            let uncChdirResult = uncLongPathCString.withUnsafeBufferPointer { buffer in
+                return _chdir(buffer.baseAddress!)
+            }
+            XCTAssertEqual(uncChdirResult, 0, "Failed to change directory using UNC long path prefix")
+            
+            // Test _NS_getcwd directly after changing to UNC prefixed path
+            var buffer = [CChar](repeating: 0, count: Int(MAX_PATH))
+            guard let result = _NS_getcwd(&buffer, buffer.count) else {
+                XCTFail("_NS_getcwd returned null")
+                return
+            }
+            
+            let currentDir = String(cString: result)
+            
+            // Verify that the path doesn't contain UNC prefixes (this is the key test!)
+            XCTAssertFalse(currentDir.hasPrefix("\\\\?\\"), "Current directory path should not contain \\\\?\\ UNC prefix after stripping")
+            
+            // Verify that we can still access the directory (it's a valid path)
+            XCTAssertTrue(fm.fileExists(atPath: currentDir), "Current directory path should be valid and accessible")
+            
+            // Verify the path ends with our test directory name
+            XCTAssertTrue(currentDir.hasSuffix(tempDir.lastPathComponent), "Current directory should end with our test directory name")
+                       
+            // Test with a deeper nested directory using UNC prefix to ensure stripping works with longer paths
+            let deepDir = tempDir.appendingPathComponent("level1").appendingPathComponent("level2").appendingPathComponent("level3")
+            try fm.createDirectory(at: deepDir, withIntermediateDirectories: true)
+            
+            let deepUncPath = "\\\\?\\" + deepDir.path
+            let deepUncCString = deepUncPath.cString(using: .utf8)!
+            let deepChdirResult = deepUncCString.withUnsafeBufferPointer { buffer in
+                return _chdir(buffer.baseAddress!)
+            }
+            XCTAssertEqual(deepChdirResult, 0, "Failed to change to deep directory with UNC prefix")
+            
+            // Test _NS_getcwd with deep UNC prefixed path
+            var deepBuffer = [CChar](repeating: 0, count: Int(MAX_PATH))
+            guard let deepResult = _NS_getcwd(&deepBuffer, deepBuffer.count) else {
+                XCTFail("_NS_getcwd returned null for deep UNC path")
+                return
+            }
+            
+            let deepCurrentDir = String(cString: deepResult)
+            
+            // Verify UNC prefixes are stripped from deeper paths too
+            XCTAssertFalse(deepCurrentDir.hasPrefix("\\\\?\\"), "Deep directory path should not contain \\\\?\\ UNC prefix after stripping")
+            XCTAssertTrue(fm.fileExists(atPath: deepCurrentDir), "Deep directory path should be valid and accessible")
+            XCTAssertTrue(deepCurrentDir.hasSuffix("level3"), "Deep directory should end with level3")
+            
+        } catch {
+            XCTFail("Failed to set up test environment: \(error)")
+        }
+    }
+           
+    func test_NS_getcwd_small_buffer() {
+        // Test that _NS_getcwd handles small buffer correctly
+        var smallBuffer = [CChar](repeating: 0, count: 1)
+        let result = _NS_getcwd(&smallBuffer, smallBuffer.count)
+        // This should either return null or handle the small buffer gracefully
+        // The exact behavior depends on the implementation, but it shouldn't crash
+        XCTAssertTrue(result == nil || result != nil, "Function should not crash with small buffer")
+    }
+    #endif
+}