GCC 4.6 has new -Wunused-but-set-variable flag, which is enabled
by -Wall, that causes GCC to emit a warning whenever a local variable
is assigned to, but otherwise unused (aside from its declaration).
Since the maintainer mode uses -Wall and -Werror, source code which
triggers these warnings will be rejected. That is, these warnings
become hard errors.
The solution is to fix the code which triggers these specific warnings.
In most of the cases, this is a welcome cleanup as code which triggers
this warning is probably dead anyway.
Before this fix, a thread instrumented for the performance schema,
that would perform file io operations, could crash inside the LF_HASH
implementation, in cases when my_thread_init is not called.
The crash itself has not been reported in 5.5 but similar crashes have
been found in 5.6-based development branches, using LF_HASH for
more instrumentation.
The possibility of a crash in 5.5 is confirmed by code analysis.
The problem is that, when my_thread_init() is not called,
which can happen for threads in storage engines or thirs party code,
my_thread_var is NULL.
Using my_thread_var->stacks_ends_here in mysys/lf_alloc-pin.c is unsafe.
Given that my_thread_var is used:
- only for stacks_ends_here
- only on platform with HAVE_ALLOCA
- only when there is enough room on the stack
and given that the LF_HASH implementation has a fallback
algorythm implemented already when using alloca is not possible,
using my_thread_var->stacks_ends_here is in fact not a strict requirement,
and can be relaxed.
The fix is to:
- test explicitly if my_thread_var is NULL, to account for cases
when my_thread_init() is not used by the calling thread.
- not use alloca in this case, and rely on the fall back code already in place.
so that the LF_HASH can be supported even without my_thread_init().
The implementation of mysys/lf_alloc-pin.c has been fixed to support this new usage.
The units tests in unittest/mysys/lf-t.c have been adjusted accordingly.
