fix(core): asset hash is different between linux and windows (#16945)

The hash for a specific file in a directory include its relative path.
This gives different results on Linux vs Windows because of the
different path separator. The solution is to normalize the relative path
using forward slashes.

Affects directory assets with subdirectories.

Closes #14555
Closes #16928


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/lib/fs/fingerprint.ts

@@ -53,24 +53,26 @@ export function fingerprint(fileOrDirectory: string, options: FingerprintOptions
   return hash.digest('hex');
 
   function _processFileOrDirectory(symbolicPath: string, isRootDir: boolean = false, realPath = symbolicPath) {
-    const relativePath = path.relative(fileOrDirectory, symbolicPath);
-
     if (!isRootDir && ignoreStrategy.ignores(symbolicPath)) {
       return;
     }
 
     const stat = fs.lstatSync(realPath);
 
+    // Use relative path as hash component. Normalize it with forward slashes to ensure
+    // same hash on Windows and Linux.
+    const hashComponent = path.relative(fileOrDirectory, symbolicPath).replace(/\\/g, '/');
+
     if (stat.isSymbolicLink()) {
       const linkTarget = fs.readlinkSync(realPath);
       const resolvedLinkTarget = path.resolve(path.dirname(realPath), linkTarget);
       if (shouldFollow(follow, rootDirectory, resolvedLinkTarget)) {
         _processFileOrDirectory(symbolicPath, false, resolvedLinkTarget);
       } else {
-        _hashField(hash, `link:${relativePath}`, linkTarget);
+        _hashField(hash, `link:${hashComponent}`, linkTarget);
       }
     } else if (stat.isFile()) {
-      _hashField(hash, `file:${relativePath}`, contentFingerprint(realPath));
+      _hashField(hash, `file:${hashComponent}`, contentFingerprint(realPath));
     } else if (stat.isDirectory()) {
       for (const item of fs.readdirSync(realPath).sort()) {
         _processFileOrDirectory(path.join(symbolicPath, item), false, path.join(realPath, item));

packages/@aws-cdk/core/test/fs/fs-fingerprint.test.ts

@@ -178,4 +178,22 @@ describe('fs fingerprint', () => {
 
     });
   });
+
+  test('normalizes relative path', () => {
+    // Simulate a Windows path.relative()
+    const originalPathRelative = path.relative;
+    const pathRelativeSpy = jest.spyOn(path, 'relative').mockImplementation((from: string, to: string): string => {
+      return originalPathRelative(from, to).replace(/\//g, '\\');
+    });
+
+    const hash1 = FileSystem.fingerprint(path.join(__dirname, 'fixtures', 'test1'));
+
+    // Restore Linux behavior
+    pathRelativeSpy.mockRestore();
+
+    const hash2 = FileSystem.fingerprint(path.join(__dirname, 'fixtures', 'test1'));
+
+    // Relative paths are normalized
+    expect(hash1).toEqual(hash2);
+  });
 });