Suboptimal fake changes handling in online ALTER storage engine API

[Laurynas Biveinis]

Based on Facebook patch 5.6 port review:

1) prepare_inplace_alter_table_dict() handles fake changes as follows:

 if (UNIV_UNLIKELY(ctx->trx->fake_changes)) {
  trx_rollback_to_savepoint(ctx->trx, NULL);
  trx_free_for_mysql(ctx->trx);
  DBUG_RETURN(HA_ERR_WRONG_COMMAND);
 }

This is dead code, and ut_ad(!ctx->trx->fake_changes) is an adequate replacement. Fake changes have been checked in a previous online ALTER SE API call.

2) Generalizing the above, ha_innobase::check_if_supported_inplace_alter() can return HA_ALTER_INPLACE_NOT_SUPPORTED with reason ER_READ_ONLY_MODE if fake changes supported, and then the rest online ALTER SE API implementations can simply assert no fake changes in the alter trx context.

[Krunal Bauskar (krunal-bauskar)]

As per the discussion and investigation.

- #1 is dead code as the blocking is done at higher level for inplace ha_innobase::prepare_inplace_alter_table
- #2 is more logical but changes rejecting inplace will put the flow in copy mode giving a weird create table failure message vs "Got error 131 during COMMIT" std message seen from most stmt while operating in fake-change mode. So given this we will retain the existing checks but will get rid of dead code as part of this bug fix.

[Krunal Bauskar (krunal-bauskar)]

commit 87e5bedc9cd459afed84ee8d99e93d106868d95c
Merge: fd6682d 6044385
Author: Krunal Bauskar <email address hidden>
Date: Mon Sep 21 21:04:20 2015 +0530

    Merge pull request #187 from kbauskar/5.6-ps-bug1204422

    - bug#1204422 : Suboptimal fake changes handling in online ALTER stor…

commit 6044385e3aee2f10196fd27fb92b61ec83e7b494
Author: Krunal Bauskar <email address hidden>
Date: Mon Sep 21 13:32:10 2015 +0530

    - bug#1204422 : Suboptimal fake changes handling in online ALTER storage engine API

      Removal of dead-code.
      ALTER Table is blocked at higher level and so the lower level API doesn't need
      to block it as control is never expected to reach there.
      Replaced the dead-code with DEBUG ASSERT for safe check.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2037