[shell] Avoid side-effects during eval (#3459)

Take two.

* Avoid eval if the prefix contains `:=`
    * This is not to evaluate variable assignment. e.g. ${FOO:=BAR}
* [zsh] Prevent `>(...)` form
* Suppress error message from prefix evaluation
* Stop completion when prefix evaluation failed

Thanks to @calestyo

shell/completion.bash

@@ -170,9 +170,9 @@ __fzf_generic_path_completion() {
   COMPREPLY=()
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
   cur="${COMP_WORDS[COMP_CWORD]}"
-  if [[ "$cur" == *"$trigger" ]] && [[ $cur != *'$('* ]] && [[ $cur != *'<('* ]] && [[ $cur != *'`'* ]]; then
+  if [[ "$cur" == *"$trigger" ]] && [[ $cur != *'$('* ]] && [[ $cur != *':='* ]] && [[ $cur != *'`'* ]]; then
     base=${cur:0:${#cur}-${#trigger}}
-    eval "base=$base"
+    eval "base=$base" 2> /dev/null || return
 
     dir=
     [[ $base = *"/"* ]] && dir="$base"
@@ -235,7 +235,7 @@ _fzf_complete() {
   cmd="${COMP_WORDS[0]//[^A-Za-z0-9_=]/_}"
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
   cur="${COMP_WORDS[COMP_CWORD]}"
-  if [[ "$cur" == *"$trigger" ]] && [[ $cur != *'$('* ]] && [[ $cur != *'<('* ]] && [[ $cur != *'`'* ]]; then
+  if [[ "$cur" == *"$trigger" ]] && [[ $cur != *'$('* ]] && [[ $cur != *':='* ]] && [[ $cur != *'`'* ]]; then
     cur=${cur:0:${#cur}-${#trigger}}
 
     selected=$(FZF_DEFAULT_OPTS="--height ${FZF_TMUX_HEIGHT:-40%} --reverse --bind=ctrl-z:ignore ${FZF_DEFAULT_OPTS-} ${FZF_COMPLETION_OPTS-} $str_arg" __fzf_comprun "${rest[0]}" "${args[@]}" -q "$cur" | $post | tr '\n' ' ')

shell/completion.zsh

@@ -137,10 +137,10 @@ __fzf_generic_path_completion() {
   tail=$6
 
   setopt localoptions nonomatch
-  if [[ $base = *'$('* ]] || [[ $base = *'<('* ]] || [[ $base = *'`'* ]]; then
+  if [[ $base = *'$('* ]] || [[ $base = *'<('* ]] || [[ $base = *'>('* ]] || [[ $base = *':='* ]] || [[ $base = *'`'* ]]; then
     return
   fi
-  eval "base=$base"
+  eval "base=$base" 2> /dev/null || return
   [[ $base = *"/"* ]] && dir="$base"
   while [ 1 ]; do
     if [[ -z "$dir" || -d ${dir} ]]; then
@@ -307,7 +307,7 @@ fzf-completion() {
     d_cmds=(${=FZF_COMPLETION_DIR_COMMANDS:-cd pushd rmdir})
 
     [ -z "$trigger"      ] && prefix=${tokens[-1]} || prefix=${tokens[-1]:0:-${#trigger}}
-    if [[ $prefix = *'$('* ]] || [[ $prefix = *'<('* ]] || [[ $prefix = *'`'* ]]; then
+    if [[ $prefix = *'$('* ]] || [[ $prefix = *'<('* ]] || [[ $prefix = *'>('* ]] || [[ $prefix = *':='* ]] || [[ $prefix = *'`'* ]]; then
       return
     fi
     [ -n "${tokens[-1]}" ] && lbuf=${lbuf:0:-${#tokens[-1]}}