[gbirman]

PR 3 of the channel and comment unification stack. It moves the historical "Comment" / "Thread" rows into comms_messages / comms_message_threads while the application stays live, so that when PR 4's flag turns the new document UI on, every old discussion is already there. Nothing is dropped; the legacy tables, endpoints, and channel_id go in PR 7.

Operating model

Teo's cutover was one transaction between two migrations with every writer paused. This PR keeps its logic and replaces the operating model with a re-runnable projection, comment_import, a feature-gated binary in crates/macro_db_migrator (the same home Teo chose; the repo's other operator one-offs are shell scripts fed by tooling/scripts/rds_database_url.sh, which is how this one gets its DATABASE_URL too).

• Live and batched. One transaction per batch of documents (250 by default, --documents-per-batch). Each batch takes SHARE ROW EXCLUSIVE on "Comment" and "Thread" for the length of that transaction with a 5 s lock_timeout and retries, so legacy comment writes wait for at most one batch and are never rejected. Nothing takes ACCESS EXCLUSIVE. Two runners cannot overlap: a session advisory lock makes the second exit.
• Idempotent. Every run recomputes the projection of each legacy thread and comment and upserts only rows whose projected state differs. A second run prints Nothing to import and writes nothing. Ids are allocated once (UUIDv7, in chunks of 1,000, in the same transaction as the rows they point at) and never change.
• Delta-aware. Legacy writers stay live until the final pass. New comments get mappings and are inserted; edited or deleted (soft or hard) comments update the imported row; a thread deleted or revived in legacy flips its thread state. Comparison is by content, updatedAt, edited_at, deletedAt, metadata, and order.
• Final pass. LEGACY_COMMENT_WRITES_ENABLED=false on the document storage service makes comment create, edit, delete and anchor delete (which also deletes the thread) answer 503 with a plain message; reads, highlight creation, and anchor geometry edits keep working. Sequence: run the importer any number of times, flip the switch, run once more until it reports nothing to import, then enable the new UI. docs/legacy-comment-import.md is the operator doc.

What is remapped

Messages (sender_id from owner, imported_author from sender, import_metadata, import_order; roots by legacy order, then createdAt, then id, including deleted roots, whose replies survive), thread state (resolution, legacy metadata, anchor derived as pdf_placeable > live pdf_highlight > markdown markId when it is a UUID and not DISCUSSION:), root_id on both PDF anchor tables, commentId/threadId in notification.metadata for the three document comment event types, and numeric comments[].id values inside saved PDF DocumentInstanceModificationData payloads. The PDF payload rewrite moved from a temporary plpgsql function to Rust so it is unit-tested and never aborts on an id without a mapping.

GET /messages/document/{id}/legacy/{numeric_id} from PR 2 already resolves through the mapping tables; this PR adds the repository test that seeds mappings and checks comment ids, thread ids, wrong-document and channel parents, and that the new foreign key rejects a mapping to a message that was never written.

Foreign keys

migrated_comment_id.message_id → comms_messages(id) and migrated_comment_thread_id.root_id → comms_message_threads(root_id) are added in a normal migration, DEFERRABLE INITIALLY DEFERRED. They hold for the empty tables at deploy time and at every importer commit because mapping allocation and the rows it points at share a transaction. The migration also indexes both mapping tables by document_id: the importer batches by document and the existing Document cascade had no index on the referencing column.

Tradeoffs

• Empty legacy threads are marked deleted at import. Teo's fixture leaves an empty thread as a live thread whose only message is a tombstone, which the Discussion view would render as an empty "This message was deleted." row. There is nothing to show, and keeping such threads live would also occupy a Markdown mark in the unique active-mark index. The thread keeps its structural root and mapping so anchors and old links still resolve, and if a legacy comment lands in that thread later the projection revives the thread (the structural root stays a tombstone and the new comment is a reply to it; one test covers this).
• Newer message store writes win. A legacy row only overwrites an imported message or thread state when its timestamp is at least as new as the stored updated_at. Legacy deletions do not bump updatedAt, so they still apply; an edit or resolution made through the new API after the legacy row went stale is never reverted. This protects an accidental rerun after the new UI is on, but a legacy write and a new-store write on the same thread while both are live is still a race, hence the write switch.
• The write switch is a config flag, not a feature flag, because the freeze is an operator step during a deploy window and must default to writes enabled. It is #[macro_config_default(true)], so it needs no Doppler value until the final pass; I could not register LEGACY_COMMENT_WRITES_ENABLED in Doppler from this machine.
• The lock covers the whole batch transaction, not just allocation. With a persistent FK from mapping to message, allocation and import cannot be split into separate transactions, so the brief's "only around allocation" becomes "around one short batch". Batch size bounds the hold time.
• Preflight aborts on a live thread with more than one PDF anchor and on two live legacy threads with comments sharing a Markdown mark on one document; both are data an operator has to resolve. Everything else (notifications and PDF ids without a mapping, marks that are not UUIDs, root order drift) is reported as a warning and never blocks a run.
• The PDF save path's comment rewrite in save_document is commented out on main, so hard deletes of legacy rows only happen through document deletion. The importer still tombstones messages and threads whose legacy row disappeared, because it is one statement each and keeps the projection total.

Left for PR 7

Dropping "Comment", "Thread", "ThreadAnchor", the anchors' "threadId", channel_id, the parent-sync trigger, the legacy annotation endpoints, and the write switch; the importer binary goes with the tables it reads. Unrelated work included: none.

Verification

• cargo test -p macro_db_migrator --features comment-import (9 tests: roots, replies, deleted roots, empty threads, Markdown and PDF anchors, notification and PDF payload remaps, no-op second run, delta run with a new, an edited, a deleted comment and a deleted and a revived thread, protection of newer message store writes, tombstones for removed legacy rows, concurrent runner, both preflight aborts, rollback of a failed batch with retry, mappings across the 1,000-row chunk and across document batches, and Teo's 30,000-row query-plan regression for the parent-scoped indexes), cargo test -p messages --all-features, cargo test -p macro_db_client, cargo test -p document_storage_service write_switch, all with SQLX_OFFLINE unset. just check with CHECK_BASE=origin/gab/messages/2-crate; the default gate diffs against main and only reports biome findings in PR 1 and PR 2's generated TypeScript, which this PR does not touch. Clippy on the three crates with all features and targets is clean apart from two pre-existing warnings in PR 2's tests.
• Dev snapshot rehearsal: not done. This machine has neither Doppler nor AWS credentials, so no hosted database is reachable. If you can point me at a snapshot (or run comment_import -- --check and then comment_import against it yourself), the summary it prints has the source and imported counts; I would like to record runtime and counts here before this merges.

---

Note

High Risk
Bulk writes to production message and notification data during live traffic, with correctness hinging on batch locking, timestamp merge rules, and preflight checks for ambiguous legacy threads.

Overview
Adds a live, re-runnable operator tool (comment_import, feature-gated on macro_db_migrator) that projects legacy "Comment" / "Thread" data into comms_messages and comms_message_threads in document batches, with advisory locking, legacy table locks, idempotent upserts, and remaps for notifications, PDF anchors, and saved PDF payloads.

A new migration adds deferred foreign keys from migrated_comment_id / migrated_comment_thread_id to the message store plus document_id indexes for batch work. The messages repo gains a test that legacy numeric ids resolve only through valid mappings.

Document storage gets legacy_comment_writes_enabled (default on) and middleware that returns 503 on comment create/edit/delete and anchor delete when writes are frozen for the final import pass. Operator steps are documented in docs/legacy-comment-import.md.

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📝 Summary

Summary by CodeRabbit

• New Features

  • Added a resumable, idempotent comment-import tool with batch processing, preflight checks, retry handling, and detailed run summaries.
  • Added support for migrating legacy comments, threads, anchors, notifications, and related document references.
  • Added legacy comment ID resolution across migrated messages and threads.

• Bug Fixes

  • Improved data integrity with foreign-key validation and cascading cleanup.

• Documentation

  • Added operational guidance for running and monitoring the comment import.

• Configuration

  • Added a switch to temporarily disable legacy comment creation, editing, and deletion, returning a clear service-unavailable response.

Walkthrough

The pull request adds a feature-gated comment_import CLI that migrates legacy comments and threads into shared message tables. It adds deferred cascading foreign keys and document indexes for mapping tables. The importer supports batching, retries, idempotent updates, tombstones, anchor and payload remapping, warnings, and preflight checks. Integration tests cover migration scenarios and query plans. Document comment writes can be disabled through configuration and return HTTP 503. Documentation describes operation and rollout.

Changes

Area	Change
Database	Added mapping foreign keys and document indexes.
Migrator	Added CLI, importer logic, reports, fixtures, and tests.
Service	Added the legacy comment write switch and configuration field.
Documentation	Added importer operation and rollout instructions.

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to b9248

The importer remains mergeable with bounded risk, but nested saved-PDF comment IDs may not migrate and production observability may be misconfigured; the localized fixes should preferably land first.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the conventional commit format, is 61 characters long, and accurately describes the idempotent legacy document comment importer.
Description check	✅ Passed	The description clearly explains the importer, migration behavior, write switch, remapping, tests, operating model, and deferred follow-up work.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Preview: https://gab-messages-3-import-ed5pnd.preview.macro.com/app (609f150)

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Inline comments:
In `@crates/macro_db_migrator/src/bin/comment_import.rs`:
- Line 30: Update the MacroEntrypoint initialization to use
Environment::new_or_prod() instead of Environment::Local, so the runtime
environment selects the appropriate staging or production initialization branch
while preserving local behavior when applicable.

In `@crates/macro_db_migrator/src/bin/comment_import/runner.rs`:
- Around line 690-702: Update the comment traversal in remap_comment_ids so each
comment item is recursively processed after handling its own legacy id. Preserve
the existing mapping and counter behavior, and merge the recursive RemapStats
into the parent stats so nested comments are remapped and counted.

In `@crates/macro_db_migrator/src/bin/comment_import/test.rs`:
- Around line 540-545: Update the advisory-lock probe to bind the connection
returned by other_connection as a mutable prober, execute
pg_advisory_unlock(LOCK_ID) on that same connection after asserting acquisition,
and explicitly close prober before the subsequent run call.

In `@docs/legacy-comment-import.md`:
- Line 39: Update the Step 2 response text in the legacy comment import
documentation to exactly match the complete 503 response represented by
DISABLED_MESSAGE, including “Refresh the app in a few minutes and try again.”

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