TikZ: Make stealth decoration follow arc

Question

In this MWE

\documentclass[border=5mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}
\begin{tikzpicture}[path/.append style={
   decoration={
   markings,
     mark=at position 0.5 with {\arrow [scale=1.5]{stealth}},
   },
   postaction=decorate,
   thick,
 }]

   \draw[path] (-2, 0) -- (-0.5, 0);
   \draw[path] (-0.5, 0) arc [start angle=180, delta angle=-180, radius=0.5];
   \draw[path] (0.5, 0) -- (2, 0);
   \draw[path] (2, 0) arc [start angle=0, end angle=180, radius=2];
\end{tikzpicture}
\end{document}

The position of the arrows along the arc doesn't quite look right. Also, the arrows do not seem to be placed in the middle of the paths (wrong anchor).

How can I fix this?

Answer 1

It looks like you can add an option to \arrow to shift it.

\documentclass[border=5mm,png]{standalone}    
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}
\begin{tikzpicture}[path/.append style={
   decoration={
       markings,
       mark=at position 0.5 with {\arrow[xshift=2.5\pgflinewidth,red,>=stealth]{>}}
   },
   postaction=decorate,
   thick,
 }]
   \draw[path] (-2, 0) -- (-0.5, 0);
   \draw[path] (-0.5, 0) arc [start angle=180, delta angle=-180, radius=0.5];
   \draw[path] (0.5, 0) -- (2, 0);
   \draw[path] (2, 0) arc [start angle=0, end angle=180, radius=2];
   \draw[help lines] (0,0) --++ (0,2) (1.25,0) --++ (0,2) (-1.25,0) --++ (0,02);
   \draw[help lines] (-2,0) --++ (4,0) (-2,0.5) --++ (4,0) (-2,2) --++ (4,0);
\end{tikzpicture}
\end{document}

The magic number 2.5\pgflinewidth is half of the length of the stealth arrow tip, from point to the vertex of the cavity, as can be found by digging into pgfcorearrows.code.tex. This is about as good as I can do at the moment.