Do not always create cache

If exit status is not zero, do not create cache, but allow user to
override the default match.

src/cached.sh.skel

     # Usage:
     #
     #     CACHED__ROOT=$HOME/.cache/myapp
-    #     cached [-m] [-w] [-a ATTR] CMD [ARG]
+    #     cached [-m] [-w] [-a ATTR] [-e ES_EXPR] CMD [ARG]
     #
     # Look up CMD with any ARGs in local cache and return result on hit.
-    # In case of miss, run command to create the cache first.
+    # In case of miss, run command and if exit status is zero (can be changed
+    # by -e argument; see below), create the cache and return it.
     #
     # Cache objects are identified by computing a MD5 hash from combination
     # of several attributes.  By default, only CMD and ARGs are included;
     # All cache objects are queried or created under directory specified by
     # global variable $CACHED__ROOT, which must be specified beforehand.
     #
+    # Exit status has to match ES_EXPR in order to be matched.  ES_EXPR has
+    # form of comma-separated list of exit statuses or simple exit status
+    # ranges. Valid ranges are: `N` which matches exactly `N` or `M-N`,
+    # which matches any status from `M` to `N` inclusively.
+    #
+    # For example, following commands do or do not create cache (assuming
+    # no cache hit):
+    #
+    #     cached false              # no: by default ES_EXPR is only 0
+    #     cached -e 0,1 false       # yes: 1 matches `0,1`
+    #     cached -e 0-3,9 exit_4    # no: 4 does not match `0-3,9`
+    #     cached -e 0-3,9 exit_3    # yes: 3 matches `0-3,9`
+    #
     # NOTE: Caching of commands that process standard input is not supported.
     # (I.e. cached() will close standard input immediately.)
     #
     local ObjPath           # cache object path
     local MatchWD=false     # does workdir matter?
     local Attr              # custom attribute
+    local EsExpr=0          # allowed exit status
     while true; do case $1 in
         --)     shift; break ;;
         -a)     Attr=$2; shift 2 || return 2 ;;
+        -e)     EsExpr=$2; shift 2 || return 2 ;;
         -w)     MatchWD=true; shift ;;
         -m)     Miss=true; shift ;;
         -*)     warn "bad argument: $1"; return 2 ;;
     test -n "$Attr" && echo "Attr=$Attr"
 }
 
+__cached__es_match() {
+    #
+    # True if exit status $1 matches expression $EsExpr
+    #
+    local es=$1
+    local part
+    for part in ${EsExpr//,/ }; do
+        test -n "$part" || continue
+        #FIXME: a rather funny implementation (works, though...)
+        eval "echo {${part/-/..}}" | grep -qwF "$es" && return 0
+    done
+    return 1
+}
+
 __cached__hit() {
     #
     # True if $Command has cache hit
 
 __cached__run() {
     #
-    # Run command, creating cache object
+    # Run command, creating cache object if exit status matches $EsExpr
     #
     local es        # command exit status
     rm -rf "$ObjPath"
     mkdir -p "$ObjPath"
     __cached__describe >"$ObjPath/desc"
     eval "$Command" \
-        >"$ObjPath/out"\
-        2>"$ObjPath/err"; es=$?
-    echo $es>"$ObjPath/es"
+        >"$ObjPath/out.tmp"\
+        2>"$ObjPath/err.tmp"; es=$?
+    echo $es>"$ObjPath/es.tmp"
+    if __cached__es_match $es; then
+        mv "$ObjPath/out.tmp" "$ObjPath/out"
+        mv "$ObjPath/err.tmp" "$ObjPath/err"
+        mv "$ObjPath/es.tmp" "$ObjPath/es"
+    else
+        warn "bad exit status; skipping cache creation: $es does not match $EsExpr"
+        cat "$ObjPath/out.tmp"
+        cat "$ObjPath/err.tmp" >&2
+        rm "$ObjPath/out.tmp"
+        rm "$ObjPath/err.tmp"
+        rm "$ObjPath/es.tmp"
+    fi
     return $es
 }