Merge conflict resolution for MDEV-40365 & co. → 12.3

.github/copilot-instructions.md

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+# MariaDB Server — Code Review Instructions
+
+You are reviewing a change to MariaDB Server, a large C/C++ codebase with
+strict conventions. **Correctness** comes first, but style matters too: no
+automated formatter runs on this repo, so review is the only gate that
+enforces the coding style — flag style violations as well as bugs.
+
+## Priorities (in order)
+1. Correctness bugs and crashes (see checklist below).
+2. Backward/forward compatibility (on-disk formats, protocol, replication).
+3. Performance and scalability — but only where it plausibly matters (a hot
+   path, or large/unbounded data), not speculative micro-optimization. Look
+   for:
+   - Wrong algorithmic complexity for the expected scale — e.g. an O(N²) loop
+     where a hash or sort gives O(N)/O(N log N) and N can be large. Conversely,
+     do NOT flag a simple O(N²) over small, bounded input where a heavier data
+     structure would be slower and less readable — call out the trade-off, not
+     just the asymptotics.
+   - Expensive or blocking work inside a critical section — disk/network I/O,
+     `fsync`, memory allocation, logging, or acquiring another lock while
+     holding a mutex/latch/rwlock. Shrink the critical section instead.
+   - Avoidable per-row cost in a hot loop — repeated allocation, redundant
+     recomputation of a loop-invariant, needless charset/string conversion, or
+     a full scan where an index lookup exists.
+   - Unnecessary copying of large records/buffers where a reference, move, or
+     in-place operation suffices.
+4. Missing or inadequate test coverage.
+5. Style deviations from CODING_STANDARDS.md. These are NOT enforced by any
+   automated formatter, so review is the only gate — do flag them, as nits.
+
+## MariaDB-specific bug checklist
+- **Memory lifetime.** Allocator must match lifetime: `MEM_ROOT`
+  (`new (thd->mem_root) T`, `alloc_root`) for query-scoped; `my_malloc`/plain
+  `new` for large or long-lived. Flag: pointers into a `MEM_ROOT`/`String`/`blob_heap`
+  buffer that survive `free_root` or a record re-read; mismatched alloc/free
+  families — handing `MEM_ROOT` memory to `free`/`my_free` (it is bulk-freed at
+  `free_root`), or `my_free`-ing something from `new`. Note `delete obj` on a
+  `MEM_ROOT` object is fine and often required — classes like `Item` make
+  `operator delete` a no-op, so `delete` runs the destructor without freeing.
+- **Prepared-statement arena.** Objects that must survive re-execution belong
+  on `thd->stmt_arena` (`Query_arena_stmt`), not the runtime mem_root.
+  Statements whose shape can change need `CF_REEXECUTION_FRAGILE` /
+  `needs_reprepare`, or PS reuse crashes or returns stale results. Item
+  tree changes are either permanent on the stmt_arena or temporary
+  and must be registered.
+- **Error contract.** `bool`: false=success, true=error; int: 0=success.
+  Every fallible call checked; `my_error()` issued before returning true;
+  error path frees what it allocated (`goto err`).
+- **Scoped error interception.** `thd->is_error()` checked after an operation,
+  or `thd->clear_error()`, is usually an anti-pattern: both act on the whole
+  THD, so they also see (or wipe) a legitimate error raised *before* this
+  operation. To detect or suppress errors from a specific bounded operation,
+  push an `Internal_error_handler` (`thd->push_internal_handler()` /
+  `pop_internal_handler()`) around exactly that scope.
+- **NULL.** SQL NULL (`null_value`/`is_null()`) vs C NULL pointer; `maybe_null`
+  propagation; `val_str()`/`val_*` may return NULL. `item->null_value` /
+  `is_null()` is valid ONLY after the item has been evaluated by a `val_*()`
+  call in the current row — it is set as a side effect of evaluation, so
+  checking it before (or without) evaluating reads a stale/undefined flag.
+- **Replication.** Non-deterministic constructs and their SBR vs RBR logging
+  safety; binlog side effects; cross-version compatibility.
+- **Concurrency.** Mutex acquisition order and init order; `LOCK_*` globals;
+  init-once races. Respect `thd->killed`, `thd->check_killed()` is preferred.
+- **Compatibility.** `.frm`, redo/undo, system-table schema, wire protocol,
+  sysvar defaults — a format change without upgrade handling is blocking.
+- **Portability.** No `long`/`ulong` (use fixed-width or `size_t`); don't rely
+  on char being signed; alignment, endianness; integer overflow/truncation in
+  size arithmetic.
+- **Assertions.** `DBUG_ASSERT` is compiled out in release — for invariants
+  only, never to validate external/untrusted input. Use them also as a
+  self-enforcing code documentation ("note, ptr is never NULL here").
+
+## Testing
+- Expect an `mysql-test/` `.test`; it must FAIL without the code change
+  (demonstrate the regression), not merely pass with it.
+- **Check the test is adequate, not just present.** It must exercise the
+  logic the fix actually adds. For every new condition/branch, expect a case
+  that hits each side — a fix guarded by `if (value is NULL)` needs both a
+  NULL case (takes the new path) and a non-NULL case (doesn't). A new loop
+  bound, error path, or type case is untested until an input reaches it. Flag
+  new conditionals with no covering test.
+- **A `.result`-only change is a red flag, not noise.** When a `.test` is
+  unchanged but its `.result` is, ask: (1) Is the new output actually
+  correct, or does it silently bless a regression? (2) Did the change gut the
+  test — e.g. an `EXPLAIN` that once exercised a specific optimizer path now
+  shows a different plan, so the test no longer covers what it was written to
+  cover? Flag both.
+
+## PR hygiene
+- Subject line starts with `MDEV-NNNNN`; body wrapped at 72 cols.
+- Bug fix targets the oldest maintained branch that reproduces (≤3y since GA);
+  new feature targets the main branch.
+- Bug fix commit is minimal; cleanups (unrelated and related - prerequisite
+  cleanups) belong in separate commits, even if they are allowed to be in the
+  same PR
+
+## Do NOT
+- Restate what the diff does.
+- Report pure compiler/CI failures (they are already visible) — but style is
+  NOT auto-enforced here, so style review is expected, not redundant.
+- Suggest STL/`std::` replacements — this codebase uses `List<T>`,
+  `Dynamic_array`, `Hash_set`, `String`, `LEX_CSTRING`, etc. by policy.
+- Assert about code outside the diff — ask a question instead. Do not invent
+  API/symbol names; verify against the code shown.
+
+## Output
+- Cite `file:line`. Tag each finding **blocking / should-fix / nit**.
+- Prefer a few high-confidence findings over many speculative ones.
+
+## Skip
+Submodules and bundled third-party code (`zlib/`).
+Note: `.result` files are NOT skipped — see Testing.
+
+See CODING_STANDARDS.md for full style rules.