fix(tags): order add-path tag-count updates so concurrent imports cannot deadlock

[Maffooch]

Description

A concurrent POST /api/v2/import-scan/ (import tags enabled) failed with a Postgres deadlock surfaced to the client:

deadlock detected
CONTEXT:  while updating tuple in relation "dojo_tagulous_finding_tags"

dojo_tagulous_finding_tags is the tagulous tag table (it holds slug, count, protected), so the statement that deadlocked is the per-tag count = count + N UPDATE, which takes a row lock on the tag being applied.

bulk_add_tags_to_instances issued that count UPDATE once per tag, in the order the tags were supplied by the caller. During an import these UPDATEs run inside the import's transaction, so each tag-row lock is held until the import commits. Two imports running concurrently and applying an overlapping set of tags could therefore take the same tag-row locks in opposite orders, forming a lock cycle that Postgres breaks by aborting one side (SQLSTATE 40P01). Because both imports pass the same tag set, this shows up under normal CI-driven, parallel imports to the same product/engagement.

This is the add-path twin of the bulk_remove_all_tags deadlock fixed in #15486, which made the remove-path count decrements run in a deterministic tag-id order for exactly the same reason. The add path was never given the same ordering.

Fix

bulk_add_tags_to_instances now resolves every tag up front and then applies them — and so issues the count UPDATEs that take the row locks — in ascending tag-id order. A shared, deterministic lock sequence for every caller means the cycle can no longer form. The per-batch body (existing-relationship lookup, through-row bulk_create, count increment, prefetch-cache invalidation) is unchanged, so return value and resulting tag counts are identical; only the order in which tags are processed changes.

Scope note: bulk_add_tag_mapping (the parser-tag path) applies its count deltas in a single UPDATE … CASE WHEN statement rather than one UPDATE per tag, so it does not exhibit the multi-statement, caller-order lock acquisition that produces this cycle; it is intentionally left unchanged to keep this fix focused on the traced path.

Pro impact

DefectDojo Pro does not override bulk_add_tags_to_instances; it only calls it (importer batch tagging, rules, connectors). The ordering fix therefore applies to Pro's import/rules/connector tag paths transparently, with no Pro-side change required.

Test results

Added BulkAddTagsToInstancesLockOrderTest in unittests/test_tag_utils_bulk.py, mirroring the existing BulkRemoveAllTagsLockOrderTest added by #15486. Three tags are created in an order deliberately unrelated to their ids, then supplied to bulk_add_tags_to_instances in a non-ascending order; the test captures the tag rows locked by the count UPDATEs and asserts they are issued in ascending tag-id order. It fails on the old supplied-order code and passes once the updates are ordered.

Note: the full suite requires a Postgres-backed stack that could not be stood up in the ephemeral environment used to author this change; the new test is deterministic (it asserts lock/update ordering, not a live race) and validation is being carried through CI.

Documentation

No user-facing behavior changes; no documentation update required.

Checklist

• Bugfix submitted against the bugfix branch.
• Meaningful PR name.
• No model changes / no migration.
• Unit test added covering the ordering guarantee.

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