Provide a convenient workaround for pgf-tikz#508 (also pgf-tikz#915)

doc/generic/pgf/text-en/pgfmanual-en-math-algorithms.tex

@@ -205,3 +205,27 @@ \section{Customizing the Mathematical Engine}
 \end{codeexample}
     %
 \end{key}
+
+\begin{key}{/pgf/declare function/execute at begin function=\meta{tokens}}
+    These \meta{tokens} are inserted just before |\pgfmathdeclarefunction|
+    scans the body of the function definition.  This is a rather low-level
+    option, so you should read the implementation to figure out where the
+    \meta{tokens} are inserted.
+\end{key}
+
+\begin{key}{/pgf/declare function/execute at end function=\meta{tokens}}
+    These \meta{tokens} are inserted just after |\pgfmathdeclarefunction| has
+    finished scanning the body of the function definition.  This is a rather
+    low-level option, so you should read the implementation to figure out where
+    the \meta{tokens} are inserted.
+\end{key}
+
+\begin{key}{/pgf/declare function/ignore spaces=\meta{boolean}}
+    Uses the two keys |/pgf/declare function/execute at begin function| and
+    |/pgf/declare function/execute at end function| to install catcodes such
+    that spaces inside the body of the function definition of
+    |\pgfmathdeclarefunction| are ignored.  The usual \TeX\ tokenization rules
+    apply, so if the body of the function had already been tokenized by other
+    means this will become ineffective.  If you want to use a space you can use
+    |~| in the function body which has its catcode set to 10 (space).
+\end{key}

tex/generic/pgf/math/pgfmathfunctions.code.tex

@@ -52,6 +52,7 @@
         {\expandafter\noexpand\csname pgfmath#1@\endcsname}%
         {\csname pgfmath@operation@#1@arity\endcsname}%
     }%
+    \pgfkeysvalueof{/pgf/declare function/execute at begin function}%
     \pgfmath@marshal%
 }
 
@@ -61,6 +62,7 @@
 % #4 - code for function <name>.
 %
 \def\pgfmath@@declarefunction#1#2#3#4{% Function of arity 0 - 9.
+    \pgfkeysvalueof{/pgf/declare function/execute at end function}%
     \ifcase#3\relax%
         \def#1{#2}%
         \def\pgfmath@@head{\def#2}%
@@ -109,7 +111,27 @@
 \pgfkeys{%
     /pgf/declare function/.code={%
         \pgfmath@local@functions#1@=@;%
-    }
+    },
+    /pgf/declare function/execute at begin function/.initial={},
+    /pgf/declare function/execute at end function/.initial={},
+    /pgf/declare function/ignore spaces/.is choice,
+    /pgf/declare function/ignore spaces/.default=true,
+    /pgf/declare function/ignore spaces/true/.style={%
+        /pgf/declare function/execute at begin function={%
+            \begingroup
+            \catcode`\^^I=9\relax
+            \catcode`\ =9\relax
+            \catcode`\~=10\relax
+            \endlinechar=`\ \relax
+        },
+        /pgf/declare function/execute at end function={%
+            \endgroup
+        },
+    },
+    /pgf/declare function/ignore spaces/false/.style={%
+        /pgf/declare function/execute at begin function={},
+        /pgf/declare function/execute at end function={},
+    },
 }
 
 \def\pgfmath@local@gobbleone#1{}