Fundamental flaw in forking checks for shared-state command substitution

[McDutchie]

The code contains various checks to see if a subshell needs to fork, like this one in the ulimit builtin:

ksh/src/cmd/ksh93/bltins/ulimit.c

Lines 139 to 140 in 090b65e

	if(shp->subshell && !shp->subshare)
	sh_subfork();

All checks of this form are fatally broken, as each one of them causes shared-state command substitutions to ignore parent virtual subshells.

Taking ulimit as an example again:

$ ulimit -t
unlimited
$ (ulimit -t 1); ulimit -t         
unlimited
$ (dummy=${ ulimit -t 1; }); ulimit -t
1

The last command wrongly imposed a 1-second CPU time limit on the main shell.

So all these checks need updating. They need a fix similar to the one applied to sh_assignok() in 911d6b0. That fix should be applied to sh_subfork(), but it will be complicated.

The places to look are:

$ grep -rwn sh_subfork src/cmd/ksh93/                                                                                              
src/cmd/ksh93//include/shell.h:229:extern void		sh_subfork(void);
src/cmd/ksh93//bltins/typeset.c:188:		sh_subfork();			/* avoid affecting main shell's alias table */
src/cmd/ksh93//bltins/typeset.c:709:						sh_subfork();
src/cmd/ksh93//bltins/typeset.c:1148:			sh_subfork();
src/cmd/ksh93//bltins/typeset.c:1255:		sh_subfork();
src/cmd/ksh93//bltins/ulimit.c:140:				sh_subfork();
src/cmd/ksh93//bltins/cd_pwd.c:121:			sh_subfork();
src/cmd/ksh93//bltins/misc.c:142:			sh_subfork();
src/cmd/ksh93//sh/io.c:1157:				sh_subfork();
src/cmd/ksh93//sh/xec.c:1603:					sh_subfork();
src/cmd/ksh93//sh/xec.c:2437:					sh_subfork();
src/cmd/ksh93//sh/fault.c:108:		sh_subfork();
src/cmd/ksh93//sh/fault.c:553:				sh_subfork();
src/cmd/ksh93//sh/subshell.c:188:void sh_subfork(void)
src/cmd/ksh93//sh/subshell.c:679:				sh_subfork();			/* ...we have to fork, as we cannot fchdir back to it. */
src/cmd/ksh93//sh/subshell.c:689:					sh_subfork();

[McDutchie]

After some experimentation I think it's not currently feasible to fix virtual subshells to coexist correctly with shared-state command substitutions.

An easy fix/workaround to ensure correct behaviour is to fork a virtual subshell before executing a shared-state command substitution within it. That is what I'll do for the time being.

In the long term I think shared-state command substitutions should probably be redesigned to disassociate them completely from the virtual subshell mechanism.