Re: Enable compgen even when programmable completions are not available?

[Kerin Millar]

On Wed, 28 Jun 2023 12:42:16 +0200
Fabien Orjollet <of1+bugbash@disroot.org> wrote:

> On 28/06/2023 00:40, Kerin Millar wrote:
> > On Tue, 27 Jun 2023 21:52:53 +0200
> > of1 <of1@disroot.org> wrote:
> > 
> >> On 27/06/2023 21:05, Kerin Millar wrote:
> >>> It doesn't work at all for >=5.2. The reason for this is interesting and 
> >>> I may make a separate post about it.
> >>>
> >>> Prior to 5.2, it can easily be tricked into printing names that do not 
> >>> exist.
> >>>
> >>> $ VAR=$'\nNONEXISTENT=' ./declare-P | grep ^NONEXISTENT
> >>> NONEXISTENT
> >>>
> >> Thank you. I was just reading the discussion in Gentoo forum and
> >> realizing that I've been to quickly: it doesn't pass the
> >> FOO=$'\nBAR BAZ QUUX=' test. But what about this? (I also quickly
> >> tested with Bash 5.2.15).
> >>
> >>
> >> FOO=$'\nBAR BAZ QUUX='
> >> VAR=$'\nNONEXISTENT='
> >>
> >> declare-P() {
> >>      local curVar
> >>      declare -a curVars
> >>
> >>      readarray -t curVars <<<"$1"
> >>      curVars=( "${curVars[@]%%=*}" )
> >>      curVars=( "${curVars[@]##* }" )
> >>      for curVar in "${curVars[@]}"; do
> >>         ### we can use [[ -v "$curVar" ]] at some point!
> >>         [[ "${curVar//[a-zA-Z0-9_]}" || \
> >>            "${curVar:0:1}" == [0-9] || \
> >>            ! -v "$curVar" || \
> >>            ! "$curVar" =~ $2 ]] || printf '%s\n' "$curVar"
> >>      done
> >> }
> >>
> >> declare-P "$(declare -p)"
> >> echo "##################"
> >> declare-P "$(declare -p)" "QU|^NON|VAR"r
> >>
> > 
> > This use of test -v is probably sufficient to address that particular 
> > issue. It is just as well that the test occurs after determining that the 
> > string is a valid identifier because test -v has the power to facilitate 
> > arbitrary code execution.
> > 
> > My assertion that your code is broken in 5.2 was incorrect. Rather, the 
> > problem seems to be that regcomp(3) behaviour differs across platforms, 
> > where given an empty expression to compile. To address that, you could 
> > determine whether a second positional parameter was given and is non-empty 
> > before proceeding to use it.
> 
> Thank you, good to know. I think that ${2:-.} should be correct.
> 
> > 
> > Curiously, repeated invocation may lead to obvious issues of resource 
> > consumption.
> > 
> > # This is excruciatingly expensive
> > time for i in {1..100}; do
> >     declare-P "$(declare -p)"       
> > done
> > 
> > # This is not so expensive (!)
> > time for i in {1..100}; do
> >     declare-P "$(declare -p)"
> >     :
> > done
> 
> I made some testing by adding
> echo -n "$(( ++COUNT ))>$(wc -c <<<"$1") "
> echo "$1" >"/tmp/declare-P$COUNT"
> to the function:
> 
> 1>4651 2>9785 3>15283 4>21504 5>29173 6>39739 7>56095 8>84036
> 9>135145 10>232592 11>422710 12>798174 13>1544326 14>3031854
> 15>6002134 16>11937918 ^C
> 
> ls -rtnh /tmp/declare-P*
> -rw-r--r-- 1 1000 1000 4,6K 28 juin  11:09 /tmp/declare-P1
> -rw-r--r-- 1 1000 1000 9,6K 28 juin  11:09 /tmp/declare-P2
> -rw-r--r-- 1 1000 1000  15K 28 juin  11:09 /tmp/declare-P3
> -rw-r--r-- 1 1000 1000  21K 28 juin  11:09 /tmp/declare-P4
> -rw-r--r-- 1 1000 1000  29K 28 juin  11:09 /tmp/declare-P5
> -rw-r--r-- 1 1000 1000  39K 28 juin  11:09 /tmp/declare-P6
> -rw-r--r-- 1 1000 1000  55K 28 juin  11:09 /tmp/declare-P7
> -rw-r--r-- 1 1000 1000  83K 28 juin  11:09 /tmp/declare-P8
> -rw-r--r-- 1 1000 1000 132K 28 juin  11:09 /tmp/declare-P9
> -rw-r--r-- 1 1000 1000 228K 28 juin  11:09 /tmp/declare-P10
> -rw-r--r-- 1 1000 1000 413K 28 juin  11:09 /tmp/declare-P11
> -rw-r--r-- 1 1000 1000 780K 28 juin  11:09 /tmp/declare-P12
> -rw-r--r-- 1 1000 1000 1,5M 28 juin  11:09 /tmp/declare-P13
> -rw-r--r-- 1 1000 1000 2,9M 28 juin  11:09 /tmp/declare-P14
> -rw-r--r-- 1 1000 1000 5,8M 28 juin  11:09 /tmp/declare-P15
> -rw-r--r-- 1 1000 1000  12M 28 juin  11:09 /tmp/declare-P16
> 
> But by adding set -p to the subshell:

Your best option would probably be to run declare -p within the function while 
avoiding the use of command substitution altogether, so as not to 'pollute' the 
_ variable.

-- 
Kerin Millar