Avoid unused value (src/fe_utils/print.c)

Hello Ranier,

03.06.2023 13:14, Ranier Vilela wrote:

Hi,

This is for Postgres 17 (head).

Per Coverity.
At function print_unaligned_text, variable "need_recordsep", is
unnecessarily set to true and false.

Clang' scan-build detects 58 errors "Dead assignment", including that one.
Maybe it would be more sensible to eliminate all errors of this class?

Best regards,
Alexander

Em sáb., 3 de jun. de 2023 às 09:00, Alexander Lakhin <exclusion@gmail.com>
escreveu:

Hello Ranier,

03.06.2023 13:14, Ranier Vilela wrote:

Hi,

This is for Postgres 17 (head).

Per Coverity.
At function print_unaligned_text, variable "need_recordsep", is
unnecessarily set to true and false.

Clang' scan-build detects 58 errors "Dead assignment", including that one.
Maybe it would be more sensible to eliminate all errors of this class?

Hi Alexander,

Sure.
I hope that when you or I are a committer,
we can fix a whole class of bugs together.

best regards,
Ranier Vilela

On Sat, Jun 03, 2023 at 03:00:01PM +0300, Alexander Lakhin wrote:

Clang' scan-build detects 58 errors "Dead assignment", including that one.
Maybe it would be more sensible to eliminate all errors of this class?

Depends on if this makes any code changed a bit easier to understand I
guess, so that would be a case-by-case analysis. Saying that, the
proposed patch seems right while it makes slightly easier to
understand the footer print part.
--
Michael

Hi Michael,

04.06.2023 01:42, Michael Paquier wrote:

On Sat, Jun 03, 2023 at 03:00:01PM +0300, Alexander Lakhin wrote:

Clang' scan-build detects 58 errors "Dead assignment", including that one.
Maybe it would be more sensible to eliminate all errors of this class?

Depends on if this makes any code changed a bit easier to understand I
guess, so that would be a case-by-case analysis. Saying that, the
proposed patch seems right while it makes slightly easier to
understand the footer print part.

It also aligns the code with print_unaligned_vertical(), but I can't see why
need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases when
need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

Best regards,
Alexander

Em dom., 4 de jun. de 2023 às 01:00, Alexander Lakhin <exclusion@gmail.com>
escreveu:

Hi Michael,

04.06.2023 01:42, Michael Paquier wrote:

On Sat, Jun 03, 2023 at 03:00:01PM +0300, Alexander Lakhin wrote:

Clang' scan-build detects 58 errors "Dead assignment", including that

one.

Maybe it would be more sensible to eliminate all errors of this class?

Depends on if this makes any code changed a bit easier to understand I
guess, so that would be a case-by-case analysis. Saying that, the
proposed patch seems right while it makes slightly easier to
understand the footer print part.

It also aligns the code with print_unaligned_vertical(), but I can't see
why
need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases when
need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

Not sure about this.
I tested with patch and the results is:

psql -U postgres --no-align
psql (16beta1)
WARNING: Console code page (850) differs from Windows code page (1252)
8-bit characters might not work correctly. See psql reference
page "Notes for Windows users" for details.
Type "help" for help.

postgres=# select * from pgbench_accounts limit 100;
aid|bid|abalance|filler
1|1|0|
2|1|0|
3|1|0|
4|1|0|
5|1|0|
6|1|0|
7|1|0|
8|1|0|
9|1|0|
etc, etc

psql -U postgres --no-align -P recordsep=";"
psql (16beta1)
WARNING: Console code page (850) differs from Windows code page (1252)
8-bit characters might not work correctly. See psql reference
page "Notes for Windows users" for details.
Type "help" for help.

postgres=# select * from pgbench_accounts limit 100;
aid|bid|abalance|filler;1|1|0|
;2|1|0|
;3|1|0|

;4|1|0|
;5|1|0|
;6|1|0|
;7|1|0|
;8|1|0|

;9|1|0|
;10|1|0|
;11|1|0|
;12|1|0|

;13|1|0|
;14|1|0|
;15|1|0|
;16|1|0|
;17|1|0|
etc, etc

regards,
Ranier Vilela

Hi,

Alexander wrote:

It also aligns the code with print_unaligned_vertical(), but I can't see why
need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases when
need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

As I understand cont->cells is supoused to have all cont->ncolumns * cont->nrows
entries filled so the loop "print cells" always assigns need_recordsep = true,
except when there are no cells at all or cancel_pressed == true.
If cancel_pressed == true then footers are not printed. So to have
need_recordsep == false before the loop "for (footers)" table should be empty,
and need_recordsep should be false before the loop "print cells". It can only
be false there when cont->opt->start_table == true and opt_tuples_only == true
so that headers are not printed. But when opt_tuples_only == true footers are
not printed either.

So technically removing "need_recordsep = true;" won't change the outcome. But
it's not obvious, so I also have doubts about removing this line. If someday
print options are changed, for example to support printing footers and not
printing headers, or anything else changes in this function, the output might
be unexpected with this line removed.

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/

Hello Karina,

30.06.2023 17:25, Karina Litskevich wrote:

Hi,

Alexander wrote:

It also aligns the code with print_unaligned_vertical(), but I can't see why
need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases when
need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

As I understand cont->cells is supoused to have all cont->ncolumns * cont->nrows
entries filled so the loop "print cells" always assigns need_recordsep = true,
except when there are no cells at all or cancel_pressed == true.
If cancel_pressed == true then footers are not printed. So to have
need_recordsep == false before the loop "for (footers)" table should be empty,
and need_recordsep should be false before the loop "print cells". It can only
be false there when cont->opt->start_table == true and opt_tuples_only == true
so that headers are not printed. But when opt_tuples_only == true footers are
not printed either.

So technically removing "need_recordsep = true;" won't change the outcome. But
it's not obvious, so I also have doubts about removing this line. If someday
print options are changed, for example to support printing footers and not
printing headers, or anything else changes in this function, the output might
be unexpected with this line removed.

I think that the question that should be answered before moving forward
here is: what this discussion is expected to result in?
If the goal is to avoid an unused value to make Coverity/clang`s scan-build
a little happier, then maybe just don't touch other line, that is not
recognized as dead (at least by scan-build; I wonder what Coverity says
about that line).
Otherwise, if the goal is to do review the code and make it cleaner, then
why not get rid of "if (need_recordsep)" there?

Best regards,
Alexander

Em sex., 30 de jun. de 2023 às 11:26, Karina Litskevich <
litskevichkarina@gmail.com> escreveu:

Hi,

Alexander wrote:

It also aligns the code with print_unaligned_vertical(), but I can't see

why

need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases when
need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

Hi Karina,

As I understand cont->cells is supoused to have all cont->ncolumns *
cont->nrows
entries filled so the loop "print cells" always assigns need_recordsep =
true,
except when there are no cells at all or cancel_pressed == true.
If cancel_pressed == true then footers are not printed. So to have
need_recordsep == false before the loop "for (footers)" table should be
empty,
and need_recordsep should be false before the loop "print cells". It can
only
be false there when cont->opt->start_table == true and opt_tuples_only ==
true
so that headers are not printed. But when opt_tuples_only == true footers
are
not printed either.

So technically removing "need_recordsep = true;" won't change the outcome.

Thanks for the confirmation.

But
it's not obvious, so I also have doubts about removing this line. If
someday
print options are changed, for example to support printing footers and not
printing headers, or anything else changes in this function, the output
might
be unexpected with this line removed.

That part I didn't understand.
How are we going to make this function less readable by removing the
complicating part.

best regards,
Ranier Vilela

Em sex., 30 de jun. de 2023 às 13:00, Alexander Lakhin <exclusion@gmail.com>
escreveu:

Hello Karina,

30.06.2023 17:25, Karina Litskevich wrote:

Hi,

Alexander wrote:

It also aligns the code with print_unaligned_vertical(), but I can't

see why

need_recordsep = true;
is a no-op here (scan-build dislikes only need_recordsep = false;).
I suspect that removing that line will change the behaviour in cases

when

need_recordsep = false after the loop "print cells" and the loop
"for (footers)" is executed.

As I understand cont->cells is supoused to have all cont->ncolumns *

cont->nrows

entries filled so the loop "print cells" always assigns need_recordsep =

true,

except when there are no cells at all or cancel_pressed == true.
If cancel_pressed == true then footers are not printed. So to have
need_recordsep == false before the loop "for (footers)" table should be

empty,

and need_recordsep should be false before the loop "print cells". It can

only

be false there when cont->opt->start_table == true and opt_tuples_only

== true

so that headers are not printed. But when opt_tuples_only == true

footers are

not printed either.

So technically removing "need_recordsep = true;" won't change the

outcome. But

it's not obvious, so I also have doubts about removing this line. If

someday

print options are changed, for example to support printing footers and

not

printing headers, or anything else changes in this function, the output

might

be unexpected with this line removed.

Hi Alexander,

I think that the question that should be answered before moving forward
here is: what this discussion is expected to result in?

I hope to make the function more readable and maintainable.

If the goal is to avoid an unused value to make Coverity/clang`s scan-build
a little happier, then maybe just don't touch other line, that is not
recognized as dead (at least by scan-build;

I wonder what Coverity says
about that line).

if (cont->opt->stop_table)
477 {
478 printTableFooter *footers = footers_with_default(cont);
479
480 if (!opt_tuples_only && footers != NULL && !
cancel_pressed)
481 {
482 printTableFooter *f;
483
484 for (f = footers; f; f = f->next)
485 {
486 if (need_recordsep)
487 {
488 print_separator(cont->opt->
recordSep, fout);

CID 1512766 (#1 of 1): Unused value (UNUSED_VALUE)assigned_value: Assigning
value false to need_recordsep here, but that stored value is overwritten
before it can be used.
489 need_recordsep = false;
490 }
491 fputs(f->data, fout);

value_overwrite: Overwriting previous write to need_recordsep with value
true.
492 need_recordsep = true;
493 }
494 }
495

Otherwise, if the goal is to do review the code and make it cleaner, then
why not get rid of "if (need_recordsep)" there?

I don't agree with removing this line, as it is essential to print the
separators, in the footers.

best regards,
Ranier Vilela

But
it's not obvious, so I also have doubts about removing this line. If
someday
print options are changed, for example to support printing footers and not
printing headers, or anything else changes in this function, the output
might
be unexpected with this line removed.

That part I didn't understand.
How are we going to make this function less readable by removing the
complicating part.

My point is, technically right now you won't see any difference in output
if you remove the line. Because if we get to that line the need_recordsep
is already true. However, understanding why it is true is complicated.
That's
why if you remove the line people who read the code will wonder why we don't
need a separator after "fputs"ing a footer. So keeping that line will make
the code more readable.
Moreover, removing the line will possibly complicate the future maintenance.
As I wrote in the part you just quoted, if the function changes in the way
that need_recordsep is not true right before printing footers any more, then
output will be unexpected.

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/

On Thu, Jul 6, 2023 at 5:37 PM Karina Litskevich
<litskevichkarina@gmail.com> wrote:

My point is, technically right now you won't see any difference in output
if you remove the line. Because if we get to that line the need_recordsep
is already true. However, understanding why it is true is complicated. That's
why if you remove the line people who read the code will wonder why we don't
need a separator after "fputs"ing a footer. So keeping that line will make
the code more readable.
Moreover, removing the line will possibly complicate the future maintenance.
As I wrote in the part you just quoted, if the function changes in the way
that need_recordsep is not true right before printing footers any more, then
output will be unexpected.

I agree with Karina here. Either this patch should keep the
"need_recordsep = true;" line, thus removing the no-op assignment to
false and making the code slightly less unreadable; or the entire
function should be refactored for readability.

.m

Em ter., 11 de jul. de 2023 às 19:34, Marko Tiikkaja <marko@joh.to>
escreveu:

On Thu, Jul 6, 2023 at 5:37 PM Karina Litskevich
<litskevichkarina@gmail.com> wrote:

My point is, technically right now you won't see any difference in output
if you remove the line. Because if we get to that line the need_recordsep
is already true. However, understanding why it is true is complicated.

That's

why if you remove the line people who read the code will wonder why we

don't

need a separator after "fputs"ing a footer. So keeping that line will

make

the code more readable.
Moreover, removing the line will possibly complicate the future

maintenance.

As I wrote in the part you just quoted, if the function changes in the

way

that need_recordsep is not true right before printing footers any more,

then

output will be unexpected.

I agree with Karina here. Either this patch should keep the
"need_recordsep = true;" line, thus removing the no-op assignment to
false and making the code slightly less unreadable; or the entire
function should be refactored for readability.

As there is consensus to keep the no-op assignment,
I will go ahead and reject the patch.

regards,
Ranier Vilela

On Wed, Jul 12, 2023 at 1:46 AM Ranier Vilela <ranier.vf@gmail.com> wrote:

As there is consensus to keep the no-op assignment,
I will go ahead and reject the patch.

In your patch you suggest removing two assignments, and we only have
consensus to keep one of them. The other one is an obvious no-op.

I attached a patch that removes only one assignment. Could you please try
it and check whether Coverity is still complaining about need_recordsep
variable?

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/

Em sex., 21 de jul. de 2023 às 09:13, Karina Litskevich <
litskevichkarina@gmail.com> escreveu:

On Wed, Jul 12, 2023 at 1:46 AM Ranier Vilela <ranier.vf@gmail.com> wrote:

As there is consensus to keep the no-op assignment,
I will go ahead and reject the patch.

In your patch you suggest removing two assignments, and we only have
consensus to keep one of them. The other one is an obvious no-op.

I attached a patch that removes only one assignment. Could you please try
it and check whether Coverity is still complaining about need_recordsep
variable?

Yeah.
Checked today, Coverity does not complain about need_recordsep.

best regards,
Ranier Vilela

On Tue, Jul 25, 2023 at 1:04 AM Ranier Vilela <ranier.vf@gmail.com> wrote:

Checked today, Coverity does not complain about need_recordsep.

Great! Thanks.
So v2 patch makes Coverity happy, and as for me doesn't make the code
less readable. Does anyone have any objections?

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/