Variable assignment in subshell affects parent.

[cassioneri]

This is a SSCCE:

$ cat bug.sh
#!/bin/ksh93

unset a b
c=0

function set_ac { a=1; c=1; }
function set_abc { ( set_ac ; b=1 ) }

set_abc
echo "a=$a b=$b c=$c"
$ ./bug.sh
a=1 b= c=0

Hence, although set_abc calls set_ac in a subshell, the assignment a=1 propagates to the parent shell. There are a few known workarounds [1]. Replace the definition of set_abc with either

function set_abc { ( set_ac ; b=1 ) | echo -n; }

or

function set_abc { ( ulimit -t unlimited ; set_ac ; b=1 ) }

Here is a third workaround that shows how bad subshells step on each other's toes. By setting a in a second subshell, it prevents the first subshell to set a in the parent shell.

function set_abc { ( set_ac ; b=1 ) }
( a=1 )

With either of the three above we get

$ ./bug.sh
a= b= c=0

References:
[1] att#73 (comment)
[2] https://stackoverflow.com/q/62385690/1137388

[McDutchie]

For the acronym-deficient: SSCCE. Take your pick, all of them might apply... ;)

Meanwhile, I've found that if we change the second function's form to that of a POSIX subshell function definition:

unset a b
c=0

function set_ac { a=1; c=1; }
set_abc() ( set_ac ; b=1 )

set_abc
echo "a=$a b=$b c=$c"

…the bug vanishes. Which suggests to me that it is probably a bug in the parser, and not so much in the subshell mechanism itself. It appears that the subshell in your original SSCCE is wrongly optimised out.

[McDutchie]

My suspicion that it was a parser issue looks to be wrong. If you change set_abc() back into a ksh-style function and defeat any possible optimisation by adding an extra command:

function set_abc { ( set_ac ; b=1 ); echo end; }

…then the bug still occurs.

[JohnoKing]

This bug was originally reported in att#7. It is caused by the following section of code, which does not save the state of variables in a newer scope:

ksh/src/cmd/ksh93/sh/subshell.c

Lines 254 to 256 in ef1621c

	/* don't bother to save if in newer scope */
	if(sp->var!=shp->var_tree && sp->var!=shp->var_base && shp->last_root==shp->var_tree)
	return(np);

I have made a pull request (#16) to fix this bug.

[McDutchie]

Fixed by #16. Many thanks @JohnoKing!