Summary

Closes one of the most common "split / merge / reorder" cases that PARTITIONING.md previously sent to manual ALTERs: catalog ends in VALUES LESS THAN MAXVALUE and desired slots one or more new partitions strictly between the catalog prefix and the catch-all (catch-all stays in place with the same body, prefix unchanged). Until now this hit the generic split / merge / reorder plan-time error, even though the shape is unambiguous — the new partitions slice the catch-all's range without changing total coverage. myschema now emits a single ALTER TABLE … REORGANIZE PARTITION pmax INTO (extras…, pmax).

A naive ADD PARTITION would be rejected by MySQL with Error 1481 ("MAXVALUE can only be used in last partition definition") and the manual workaround was a REORGANIZE pmax INTO (new…, pmax) typed out by hand. After this PR the operator just edits the Schemafile and myschema apply does the rewrite.

Trigger envelope (intentionally narrow)

Auto-emit fires only when ALL of:

• scalar RANGE (RANGE COLUMNS excluded — the monotonicity check punts on tuple boundaries, and vitess can't even round-trip VALUES LESS THAN (MAXVALUE, MAXVALUE) today, so the catalog re-parse fails before this branch is reached);
• catalog has at least one partition AND the last one is VALUES LESS THAN MAXVALUE;
• desired's last partition is also VALUES LESS THAN MAXVALUE AND partitionDefEqual to catalog's catch-all (case-insensitive name + byte-equal body);
• desired has strictly more partitions than catalog (m > n — at least one extra to insert);
• catalog's prefix [0..n-2] is partitionDefEqual to desired's prefix [0..n-2] for each pair (no compound diffs — body changes on prefix partitions disqualify);
• extras' (desired[n-1..m-2]) names don't reuse any name already present in catalog (rules out reorder / dup-name desired SQL — vitess accepts duplicate partition names in CREATE TABLE so this guard is real).

Anything outside the envelope falls through to the existing split / merge / reorder error so the operator runs the right ALTER by hand. The new partitions' boundary ordering (strictly increasing, MAXVALUE only at the tail) is enforced upstream by validateDesiredRangeMonotonic (round 23 on PR #53), so non-monotonic inserts surface there with the boundary-ordering error, not as a misfire of this auto-emit.

Implementation

• diff/partitions.go adds detectCatchAllInteriorInsert (returns the catalog catch-all + the extras when the trigger fires, false otherwise) and formatCatchAllInteriorInsertReorganize (renders the single REORGANIZE statement with model.Ident quoting on the OLD-name list and the catalog's catch-all body in INTO so case-only renames don't leak).
• The new branch sits between the existing matched-name-list (per-partition definition rewrite) path and the existing partitionByNameOrderPreserving (split / merge / reorder error) path, so it's purely additive — every other code path through diffPartitions is byte-for-byte unchanged.
• The LIST VALUES IN (DEFAULT) equivalent isn't covered here; that would need a parallel detector keyed on InType plus a different boundary-coverage proof (out of scope).

Tests

12 new YAML fixtures pin the spec coverage matrix. All run as part of make test:

Positive (auto-emits, apply + verify_no_drift):

• partition_catchall_insert_one.yml — canonical 1-extra case (with pre-existing rows in pmax that get redistributed correctly).
• partition_catchall_insert_multiple.yml — multiple extras packed into a single REORGANIZE.
• partition_catchall_insert_only_catchall.yml — catalog has only the catch-all (n=1 boundary of the prefix-equal check).
• partition_catchall_insert_case_only_rename.yml — case-only catch-all name diff (pmax vs PMAX) still triggers via partitionDefEqual's EqualFold; OLD-name list uses catalog's casing.

Negative — auto-detect must NOT fire, existing error preserved (plan):

• partition_interior_insert_no_catchall_errors.yml — catalog has no MAXVALUE.
• partition_catchall_drop_with_replacement_errors.yml — desired drops the catch-all entirely.
• partition_catchall_renamed_errors.yml — catch-all name differs beyond case.
• partition_catchall_body_change_with_insert_errors.yml — catch-all body differs (strict spec, no compound diffs).
• partition_catchall_prefix_body_change_with_insert_errors.yml — prefix partition body differs.
• partition_interior_insert_not_before_catchall_errors.yml — insert NOT directly before catch-all.
• partition_catchall_insert_extras_name_overlap_errors.yml — extras' name reuses a catalog name (vitess accepts duplicate partition names in CREATE TABLE so this guard is real).

Validation order (existing check fires before auto-detect):

• partition_catchall_insert_non_monotonic_errors.yml — validateDesiredRangeMonotonic catches the bad boundary first; the auto-detect never runs.

partition_maxvalue_catchall_insert_errors.yml (which pinned the now-replaced "split / merge / reorder" behaviour for the canonical case) is removed — partition_catchall_insert_one.yml covers the same shape with the new auto-emit assertion.

All 78 existing partition fixtures + 7 partition Go-level tests still pass — every other partition path is unaffected.

Docs

• PARTITIONING.md:
  • "Suffix add — RANGE / LIST" caveat: rewrite the catch-all paragraph from "fails with split / merge / reorder; drop the catch-all by hand" to "RANGE auto-handled (see Catch-all interior insert below); LIST DEFAULT not auto-handled".
  • New "Catch-all interior insert (RANGE)" sub-section under "Generated diffs" describing the trigger envelope, the SQL shape, the row-redistribution semantics, the MySQL Error 1481 it sidesteps, and the LIST-DEFAULT not-yet-covered limitation.
  • "Diffs that still error → Split / merge / reorder": removed "interior inserts in front of a catch-all" (now auto-handled) and added precise sub-bullets for the cases that still fall through (no catalog catch-all, mid-prefix insert, LIST DEFAULT, compound diffs on top of an insert).
• diff/partitions.go doc comments updated to match.
• CAVEATS.md / AGENTS.md / TODO.md already point at PARTITIONING.md — no edits needed.

Test plan

• make lint
• make test (8.0)
• make test-scenario