plugin session local variable not reset at plugin reinstall

[tomas-villagesql]

MySQL version / commit

trunk (but would exist since forever)

Steps to reproduce

test case below, needs a master.opt:

~/githome/mysql-server/build-debug % cat ../mysql-test/t/plugin_thdvar_reinstall-master.opt
$EXAMPLE_PLUGIN_OPT

The example storage engine declares MYSQL_THDVAR_INT(signed_int_thdvar) with

a registered default of -10. This test distinguishes the two possible reads

after reinstall by moving BOTH the session value AND the global value off the

default to distinct values before uninstall:

registered default : -10

session value set : 42

global value set : 99

After reinstall, a correct server reads @@session = -10 (the registered

default). The bug makes @@session read 42 (the stale pre-uninstall session

value). @@global reads -10 (correctly reset to the registered default, NOT the

99 that was set) -- proving the stale @@session value is the old session value

and not merely an echo of the global.

Mechanism: the per-THD bookmark is permanent (offset reused across

uninstall/reinstall); UNINSTALL clears only the global slot; per-connection

slots in dynamic_variables_ptr are never cleared, and reinstall reuses the

same offset. Reproduces on unmodified MySQL (no SET SESSION crash: INT type).

--source include/have_example_plugin.inc

--echo # Install the example engine (declares example_signed_int_thdvar)
--replace_regex /.dll/.so/
eval INSTALL PLUGIN example SONAME '$EXAMPLE_PLUGIN';

--echo # Registered default
SELECT @@session.example_signed_int_thdvar AS first_install_default;

--echo # Set a distinct per-session value on THIS connection
SET SESSION example_signed_int_thdvar = 42;
SELECT @@session.example_signed_int_thdvar AS after_set;

--echo # Move GLOBAL off its registered default (-10) to a distinct value (99),
--echo # so a post-reinstall read can tell "reset to default" (-10) from
--echo # "stale session value survived" (42) from "echo of global" (99).
SET GLOBAL example_signed_int_thdvar = 99;
SELECT @@global.example_signed_int_thdvar AS global_after_set;

UNINSTALL PLUGIN example;

--echo # While uninstalled the variable does not exist
--error ER_UNKNOWN_SYSTEM_VARIABLE
SELECT @@session.example_signed_int_thdvar AS when_uninstalled;

--echo # Reinstall on the SAME connection/server. @@session MUST be the
--echo # registered default (-10). BUG: it reads 42 (the stale session value).
--replace_regex /.dll/.so/
eval INSTALL PLUGIN example SONAME '$EXAMPLE_PLUGIN';
SELECT @@session.example_signed_int_thdvar AS after_reinstall;
SELECT @@global.example_signed_int_thdvar AS after_reinstall_global;

UNINSTALL PLUGIN example;

Expected vs actual result

Install the example engine (declares example_signed_int_thdvar)

INSTALL PLUGIN example SONAME 'ha_example.so';

Registered default

SELECT @@session.example_signed_int_thdvar AS first_install_default;
first_install_default
-10

Set a distinct per-session value on THIS connection

SET SESSION example_signed_int_thdvar = 42;
SELECT @@session.example_signed_int_thdvar AS after_set;
after_set
42

Move GLOBAL off its registered default (-10) to a distinct value (99),

so a post-reinstall read can tell "reset to default" (-10) from

"stale session value survived" (42) from "echo of global" (99).

SET GLOBAL example_signed_int_thdvar = 99;
SELECT @@global.example_signed_int_thdvar AS global_after_set;
global_after_set
99
UNINSTALL PLUGIN example;

While uninstalled the variable does not exist

SELECT @@session.example_signed_int_thdvar AS when_uninstalled;
ERROR HY000: Unknown system variable 'example_signed_int_thdvar'

Reinstall on the SAME connection/server. @@session MUST be the

registered default (-10). BUG: it reads 42 (the stale session value).

INSTALL PLUGIN example SONAME 'ha_example.so';
SELECT @@session.example_signed_int_thdvar AS after_reinstall;
after_reinstall
42
----> would expect -10 here
SELECT @@global.example_signed_int_thdvar AS after_reinstall_global;
after_reinstall_global
-10
UNINSTALL PLUGIN example;

Platform / compiler

No response