Only one writeable campaign version at a time

server/app/mailer/persistence/versions.py

@@ -257,6 +257,36 @@ def is_version_locked(version: CampaignVersion) -> bool:
     )
 
 
+def ensure_current_working_version(campaign: Campaign, version: CampaignVersion, *, action: str = "modify") -> None:
+    """Require the campaign's single active working version.
+
+    Historical versions remain reviewable, but they never become writable in
+    place. Continuing from immutable history must create a new working copy,
+    and that copy becomes the campaign's sole current version.
+    """
+
+    if campaign.current_version_id != version.id:
+        raise LockedCampaignVersionError(
+            f"Historical campaign versions are read-only and cannot be used to {action}. "
+            "Open the current working version instead."
+        )
+
+
+def campaign_has_active_working_version(session: Session, campaign: Campaign) -> bool:
+    """Return True while the campaign already has a non-final working version.
+
+    Validation locks and temporary user locks are still the same working
+    version; they must be unlocked rather than forked into parallel drafts.
+    """
+
+    if not campaign.current_version_id:
+        return False
+    current = session.get(CampaignVersion, campaign.current_version_id)
+    if not current or current.campaign_id != campaign.id:
+        return False
+    return not is_audit_safe_version(current)
+
+
 def _apply_campaign_metadata(campaign: Campaign, raw_json: dict[str, Any]) -> None:
     campaign_meta = raw_json.get("campaign") if isinstance(raw_json.get("campaign"), dict) else {}
     if campaign_meta:
@@ -279,17 +309,30 @@ def fork_campaign_version_for_edit(
     source_base_path: str | None = None,
     autosave: bool = True,
 ) -> CampaignVersion:
-    """Create a new editable working version from a locked/validated version.
+    """Create the next sole working version from immutable campaign history.
 
-    This preserves the audit value of the validated/sent version while allowing
-    users to continue editing a campaign. New content starts with the supplied
-    raw_json when provided, otherwise with a clone of the source version.
+    Validation and temporary user locks are still the active working version
+    and must be unlocked in place. A copy is allowed only once the current
+    version is permanently user-locked or delivery-final.
     """
 
     source = get_campaign_version_for_tenant(session, tenant_id=tenant_id, campaign_id=campaign_id, version_id=version_id)
     campaign = session.get(Campaign, campaign_id)
     assert campaign is not None
 
+    if campaign_has_active_working_version(session, campaign):
+        current = session.get(CampaignVersion, campaign.current_version_id)
+        current_number = current.version_number if current else "current"
+        raise LockedCampaignVersionError(
+            f"Campaign already has active working version #{current_number}. "
+            "Unlock or continue editing that version instead of creating a parallel draft."
+        )
+    if campaign.current_version_id and source.id != campaign.current_version_id:
+        raise LockedCampaignVersionError(
+            "Historical versions remain review-only and cannot become a new branch. "
+            "Create the next working copy from the campaign's current immutable version."
+        )
+
     base_json = raw_json if raw_json is not None else copy.deepcopy(source.raw_json)
     runtime_json = normalize_campaign_paths(base_json, source_base_path) if source_base_path else copy.deepcopy(base_json)
 
@@ -381,6 +424,7 @@ def unlock_validated_campaign_version(
     version = get_campaign_version_for_tenant(session, tenant_id=tenant_id, campaign_id=campaign_id, version_id=version_id)
     campaign = session.get(Campaign, campaign_id)
     assert campaign is not None
+    ensure_current_working_version(campaign, version, action="unlock")
 
     if is_temporary_user_locked_version(version):
         raise LockedCampaignVersionError("This version has a temporary user lock. Remove that lock before unlocking validation.")
@@ -440,6 +484,7 @@ def update_campaign_version(
     version = get_campaign_version_for_tenant(session, tenant_id=tenant_id, campaign_id=campaign_id, version_id=version_id)
     campaign = session.get(Campaign, campaign_id)
     assert campaign is not None
+    ensure_current_working_version(campaign, version, action="edit")
 
     if is_version_locked(version):
         raise LockedCampaignVersionError(
@@ -511,6 +556,9 @@ def update_campaign_review_state(
         campaign_id=campaign_id,
         version_id=version_id,
     )
+    campaign = session.get(Campaign, campaign_id)
+    assert campaign is not None
+    ensure_current_working_version(campaign, version, action="record review state for")
     if is_version_final_locked(version):
         raise LockedCampaignVersionError("Delivery has started; message review state can no longer be changed.")
     build_summary = version.build_summary if isinstance(version.build_summary, dict) else {}
@@ -555,6 +603,9 @@ def lock_campaign_version_temporarily(
         campaign_id=campaign_id,
         version_id=version_id,
     )
+    campaign = session.get(Campaign, campaign_id)
+    assert campaign is not None
+    ensure_current_working_version(campaign, version, action="lock")
     if is_version_final_locked(version):
         raise LockedCampaignVersionError("Delivery/final versions are permanently locked and cannot receive a temporary user lock.")
     if is_permanent_user_locked_version(version):
@@ -587,6 +638,9 @@ def unlock_user_locked_campaign_version(
         campaign_id=campaign_id,
         version_id=version_id,
     )
+    campaign = session.get(Campaign, campaign_id)
+    assert campaign is not None
+    ensure_current_working_version(campaign, version, action="unlock")
     state = campaign_version_user_lock_state(version)
     if state == USER_LOCK_PERMANENT:
         raise LockedCampaignVersionError("Permanently locked versions cannot be unlocked. Create an editable copy instead.")
@@ -623,6 +677,9 @@ def permanently_lock_campaign_version(
         campaign_id=campaign_id,
         version_id=version_id,
     )
+    campaign = session.get(Campaign, campaign_id)
+    assert campaign is not None
+    ensure_current_working_version(campaign, version, action="lock permanently")
     if is_version_final_locked(version):
         raise LockedCampaignVersionError("This version is already permanently locked by its delivery/final state.")
     if is_permanent_user_locked_version(version):