fix(decorations): wrong key name in unknown decoration option error

Introduced by previous fix #1083.

Signed-off-by: Yukai Chou <muzimuzhi@gmail.com>

doc/generic/pgf/CHANGELOG.md

@@ -20,6 +20,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Resolve missing `gnuplot` plots in manual #1238
 - Treat varargs for `min()` and `max()` in `luamath` pgf library pgf-tikz/pgfplots#492 #1359
 - Fixed support for the `\tikz` command in the `dvisvgm4ht` driver for TeX4ht
+- Wrong key name in unknown decoration option error #1082
 
 ### Changed
 

testfiles/gh-1082.lvt

@@ -0,0 +1,15 @@
+\documentclass{minimal}
+\input{pgf-regression-test}
+
+\usepackage{tikz}
+\usetikzlibrary{decorations}
+
+\START
+
+\BEGINTEST{Unknown decoration option}
+\tikzset{
+  decoration={segment length=5pt, red}
+}
+\ENDTEST
+
+\END

testfiles/gh-1082.tlg

@@ -0,0 +1,15 @@
+This is a generated file for the l3build validation system.
+Don't change this file in any respect.
+============================================================
+TEST 1: Unknown decoration option
+============================================================
+! Package pgfkeys Error: I do not know the key '/pgf/decoration/red' and I am going to ignore it. Perhaps you misspelled it.
+See the pgfkeys package documentation for explanation.
+Type  H <return>  for immediate help.
+ ...                                              
+l. ...}
+This error message was generated by an \errmessage
+command, so I can't give any explicit help.
+Pretend that you're Hercule Poirot: Examine all clues,
+and deduce the truth by order and method.
+============================================================

tex/generic/pgf/frontendlayer/tikz/libraries/tikzlibrarydecorations.code.tex

@@ -14,13 +14,15 @@
 
 \tikzset{decorate/.is if=tikz@decoratepath,
   /pgf/decoration/name/.store in=\tikz@decoration@name,
-  /pgf/decoration/.unknown/.code=%
-    \pgfifdecoration{\pgfkeyscurrentname}{\edef\tikz@decoration@name{\pgfkeyscurrentname}}
-    {\pgfifmetadecoration{\pgfkeyscurrentname}{\edef\tikz@decoration@name{\pgfkeyscurrentname}}
-    % Fully expand `\pgfkeyscurrentname' before being used in first-arg of
-    % `/errors/unknown key'.
-    {\pgfkeys{/errors/unknown
-        key/.expanded={/pgf/decoration/\pgfkeyscurrentname}{\pgfutil@unexpanded{#1}}}}},%
+  /pgf/decoration/.unknown/.code={%
+    \pgfifdecoration{\pgfkeyscurrentname}
+      {\edef\tikz@decoration@name{\pgfkeyscurrentname}}
+      {\pgfifmetadecoration{\pgfkeyscurrentname}
+        {\edef\tikz@decoration@name{\pgfkeyscurrentname}}
+        % expand \pgfkeyscurrentname _before_ it's updated by nested \pgfkeys
+        {\pgfutil@expanded{\noexpand\pgfkeys{/errors/unknown key=
+          {/pgf/decoration/\pgfkeyscurrentname}{\pgfutil@unexpanded{#1}}}}}}%
+  },
   /pgf/decoration/raise/.code={\def\tikz@dec@shift{\pgftransformyshift{#1}}\tikz@dec@trans},
   /pgf/decoration/mirror/.code={%
     \csname if#1\endcsname