Allow backends to cache previous results

book-example/src/for_developers/backends.md

@@ -261,6 +261,10 @@ in [`RenderContext`].
 > **Note:** There is no guarantee that the destination directory exists or is
 > empty (`mdbook` may leave the previous contents to let backends do caching),
 > so it's always a good idea to create it with `fs::create_dir_all()`.
+>
+> If the destination directory already exists, don't assume it will be empty.
+> To allow backends to cache the results from previous runs, `mdbook` may leave
+> old content in the directory.
 
 There's always the possibility that an error will occur while processing a book
 (just look at all the `unwrap()`'s we've written already), so `mdbook` will

src/book/mod.rs

@@ -190,19 +190,6 @@ impl MDBook {
             renderer.name().to_string(),
         );
 
-        let name = renderer.name();
-        let build_dir = self.build_dir_for(name);
-        if build_dir.exists() {
-            debug!(
-                "Cleaning build dir for the \"{}\" renderer ({})",
-                name,
-                build_dir.display()
-            );
-
-            utils::fs::remove_dir_content(&build_dir)
-                .chain_err(|| "Unable to clear output directory")?;
-        }
-
         for preprocessor in &self.preprocessors {
             if preprocessor_should_run(&**preprocessor, renderer, &self.config) {
                 debug!("Running the {} preprocessor.", preprocessor.name());

src/renderer/html_handlebars/hbs_renderer.rs

@@ -282,6 +282,11 @@ impl Renderer for HtmlHandlebars {
         let destination = &ctx.destination;
         let book = &ctx.book;
 
+        if destination.exists() {
+            utils::fs::remove_dir_content(destination)
+                .chain_err(|| "Unable to remove stale HTML output")?;
+        }
+
         trace!("render");
         let mut handlebars = Handlebars::new();