[PATCH] Remove NOERREXIT_UNTIL_EXEC

[Philippe Altherr <philippe.altherr@gmail.com>]

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The noerrexit NOERREXIT_UNTIL_EXEC bit is never set and can therefore be
removed. In other words the patch only removes dead code.

I have been intrigued by the NOERREXIT_UNTIL_EXEC bit for a while. I didn't
understand its purpose nor why the NOERREXIT_EXIT and NOERREXIT_RETURN bits
wouldn't be enough to solve the problem at hand. After finally having a
closer look, I must admit that I still don't understand. However, I
discovered that the bit is never set and can safely be removed. More on
that below.

The bit was introduced in commit d6a32dd
<https://github.com/zsh-users/zsh/commit/d6a32ddeed914434f5b56b013c9d03b28781d065>,
where the noerrexit value 2 corresponds to today's NOERREXIT_UNTIL_EXEC and
the value 1 to NOERREXIT_EXIT. There was no distinction yet between
NOERREXIT_EXIT and NOERREXIT_RETURN. At that time, there was also no
this_noerrexit variable. The distinction between NOERREXIT_EXIT and
NOERREXIT_RETURN was introduced in commit 97d4bdb
<https://github.com/zsh-users/zsh/commit/97d4bdbc7e86e6e8da0d4a059b118ffab289d3a9>,
which also introduced today's noerrexit bits.

One thing that distinguishes the NOERREXIT_UNTIL_EXEC bit from the other
noerrexit bits is that it seems intended for the caller(s) of the function
that sets it, while the other bits are intended for the callee(s). This is
similar to today's this_noerrexit variable. I wonder whether this bit was a
first attempt at solving the problem that is solved in today's code by the
this_noerrexit variable. This would explain why everything is still fine
even though the bit is no longer set.

In today's code, the NOERREXIT_UNTIL_EXEC bit is only ever set in loop.c at
line 578
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L578>.
This line can only be reached if run != 0 && run !=2 && olderrexit == 0 &&
lastval == 0. The loop above
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L551>
can
only be exited at one of the following lines:

- line 551
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L551>=>
run == 0
- line 557
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L557>
=> run == 0 || run == 2
- line 565
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L565>
=> run == 1 && lastval != 0
- line 568
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L568>
=> run == 0 && lastval == 0

None of these cases lead to line 578. Thus the NOERREXIT_UNTIL_EXEC bit is
never set and can safely be removed.

The code "noerrexit &= ~ (NOERREXIT_EXIT | NOERREXIT_RETURN)" at line 580
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/loop.c#L580>
can
be simplified into "noerrexit = olderrexit" because NOERREXIT_UNTIL_EXEC
was the only noerrexit bit that was flowing from callees to their caller.

Obviously all tests still pass. And I checked that the tests
introduced by commit
d6a32dd
<https://github.com/zsh-users/zsh/commit/d6a32ddeed914434f5b56b013c9d03b28781d065>
are
still present.

I have built this patch on top of my patch
patch-06-fix-eval-and-source-statements.txt but I don't think that it
depends on it nor on any of my previous patches (but I haven't tried to
confirm it).

QUESTION: Should NOERREXIT_SIGNAL's value
<https://github.com/zsh-users/zsh/blob/41b402d36d0aeac594cf424a9e46b5edb20c815d/Src/zsh.h#L2226>
be changed to 4 (from 8) since the value 4 is no longer used
by NOERREXIT_UNTIL_EXEC?

Philippe

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