Make read-only permission tests work on Windows

The read-only-folder approach does not fail on Windows: the directory
read-only attribute is ignored by the OS for file creation, so the
.zgroup / .zattrs write succeeded and no error was raised. (The original
noWritePermissions test passed on Windows only incidentally, because it
marked a pre-existing .zattrs *file* read-only via recursion.)

Make the specific target file read-only instead of its folder. A
read-only file is honored on both Windows and Unix, so the open-for-
write failure fires on all platforms. Write permission is restored on
teardown so the fixture can clean up.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jhughes-mw

test/tZarr.m

@@ -99,17 +99,24 @@ function verifyCreateGroupMakesFolder(testcase)
 
         function verifyCreateGroupOpenFailure(testcase)
             % Verify error when the .zgroup file cannot be opened for
-            % writing (e.g. the target folder is read-only). The read-only
-            % folder lives inside an isolated temporary folder fixture, so
-            % no real data is modified and cleanup is automatic (removal
-            % succeeds via the writable parent, so no permission restore is
-            % needed).
+            % writing. Everything lives inside an isolated temporary folder
+            % fixture, so no real data is modified.
+            %
+            % We make the existing .zgroup *file* read-only rather than its
+            % folder: a read-only folder does not prevent file creation on
+            % Windows (the directory read-only attribute is ignored there),
+            % whereas a read-only file is honored on both Windows and Unix.
             import matlab.unittest.fixtures.TemporaryFolderFixture
             tempFixture = testcase.applyFixture(TemporaryFolderFixture);
 
             groupPath = fullfile(tempFixture.Folder, "readOnlyGroup");
-            mkdir(groupPath);
-            fileattrib(groupPath, '-w', '', 's');
+            Zarr.createGroup(groupPath);            % writes .zgroup
+            zgroupFile = fullfile(groupPath, ".zgroup");
+
+            fileattrib(zgroupFile, '-w');
+            % Restore write permission before the fixture is torn down so its
+            % contents can be removed (runs before the fixture's rmdir).
+            testcase.addTeardown(@()fileattrib(zgroupFile, '+w'));
 
             testcase.verifyError(@()Zarr.createGroup(groupPath),...
                 "MATLAB:Zarr:fileOpenFailure");

test/tZarrAttributes.m

@@ -110,19 +110,28 @@ function notZarrObject(testcase)
         end
 
         function noWritePermissions(testcase)
-            % Verify error if there are no write permissions to the Zarr array.
-            % Create the array inside an isolated temporary folder fixture so
-            % no shared fixture data is modified and cleanup is automatic
-            % (removal succeeds via the writable parent, so no permission
-            % restore is needed).
+            % Verify error if the .zattrs file cannot be opened for writing.
+            % Everything lives inside an isolated temporary folder fixture so
+            % no shared fixture data is modified.
+            %
+            % We make the existing .zattrs *file* read-only rather than its
+            % folder: a read-only folder does not prevent file creation on
+            % Windows (the directory read-only attribute is ignored there),
+            % whereas a read-only file is honored on both Windows and Unix.
             import matlab.unittest.fixtures.TemporaryFolderFixture
             tempFixture = testcase.applyFixture(TemporaryFolderFixture);
 
             arrPath = fullfile(tempFixture.Folder, "roArr");
             zarrcreate(arrPath, testcase.ArrSize);
-
-            % Make the array folder read-only.
-            fileattrib(arrPath,'-w','','s');
+            % Write one attribute so the .zattrs file exists, then make it
+            % read-only so the next write cannot open it.
+            zarrwriteatt(arrPath, 'existingAttr', 1);
+            zattrsFile = fullfile(arrPath, '.zattrs');
+
+            fileattrib(zattrsFile, '-w');
+            % Restore write permission before the fixture is torn down so its
+            % contents can be removed (runs before the fixture's rmdir).
+            testcase.addTeardown(@()fileattrib(zattrsFile, '+w'));
 
             errID = 'MATLAB:zarrwriteatt:fileOpenFailure';
             testcase.verifyError(@()zarrwriteatt(arrPath,'myAttr','attrVal'), ...