fix: add @Exclusive tag to prevent cluster-mutating tests from running in parallel

tests/ctst/HOW_TO_WRITE_TESTS.md

@@ -36,13 +36,16 @@ restart of multiple services, must be created before the test starts, or in a
 dedicated set of tests, that is, a set of test executed before all tests having
 a specific tag used for identifying the feature(s).
 
-Note: Testing the reconfiguration of the environment is recommended, but it
-should be done carefully to not affect other tests.
+When a scenario *must* reconfigure the environment (e.g., create or modify
+locations, add overlay endpoints), tag it with `@Exclusive`. This ensures no
+other scenario runs in parallel while the exclusive scenario executes, and the
+exclusive scenario only starts once all running scenarios have finished. See
+the implementation in `common/hooks.ts`.
 
-## 4. Do not use `atMostOnePicklePerTag`.
+## 4. Avoid unnecessary parallel restrictions.
 
-If a set of scenario requires the use of `atMostOnePicklePerTag`, they won't
-be executed in parallel, which is:
+Adding `atMostOnePicklePerTag` for a set of scenarios means they won't be
+executed in parallel, which is:
 
 - Not realistic for the production environment.
 - Not efficient for the test execution: the duration of tests will suffer.
@@ -59,6 +62,11 @@ possible. Solutions exist:
   relative checks.
 - As a last resort, we might have a dedicated test suite.
 
+Note: `@Exclusive` (see **Rule #3**) is an exception — it is reserved for
+scenarios that mutate cluster-wide state (location creation, overlay changes)
+and cannot be made idempotent because the operator reconciliation affects all
+running pods. Use it sparingly.
+
 ## 5. Focus on validating features.
 
 We only want to assert against externally visible state, as given in the

tests/ctst/common/hooks.ts

@@ -32,7 +32,21 @@ const noParallelRun = atMostOnePicklePerTag([
     ...replicationLockTags
 ]);
 
-setParallelCanAssign(noParallelRun);
+const EXCLUSIVE_TAG = '@Exclusive';
+
+function hasTag(pickle: { tags: readonly { name: string }[] }, tagName: string): boolean {
+    return pickle.tags.some(t => t.name === tagName);
+}
+
+setParallelCanAssign((pickle, runningPickles) => {
+    if (runningPickles.some(p => hasTag(p, EXCLUSIVE_TAG))) {
+        return false;
+    }
+    if (hasTag(pickle, EXCLUSIVE_TAG)) {
+        return runningPickles.length === 0;
+    }
+    return noParallelRun(pickle, runningPickles);
+});
 
 Before(async function (this: Zenko, scenario: ITestCaseHookParameter) {
     this.resetSaved();

tests/ctst/features/azureArchive.feature

@@ -45,6 +45,7 @@ Feature: Azure Archive
     @Flaky
     @AzureArchive
     @ColdStorage
+    @Exclusive
     Scenario Outline: Create, read, update and delete azure archive location
     Given an azure archive location "<locationName>"
     And a "<versioningConfiguration>" bucket

tests/ctst/features/bucketWebsite.feature

@@ -3,6 +3,7 @@ Feature: Bucket Websites
     @2.6.0
     @PreMerge
     @BucketWebsite
+    @Exclusive
     Scenario Outline: Bucket Website CRUD
         # The scenario should test that we can put a bucket website configuration on a bucket
         # send an index.html

tests/ctst/features/pra.feature

@@ -5,6 +5,7 @@ Feature: PRA operations
     @Dmf
     @PRA
     @ColdStorage
+    @Exclusive
     Scenario Outline: PRA (nominal case)
     # Prepare objects in the primary site
     Given a "<versioningConfiguration>" bucket