From d1b5a8f982dc695b19a21e5519ef9ca5a3467e08 Mon Sep 17 00:00:00 2001
From: Michael Bryan <michaelfbryan@gmail.com>
Date: Sun, 21 Jul 2019 02:35:18 +0800
Subject: [PATCH 1/4] The MDBook::build() method no longer cleans the
 renderer's build directory

---
 src/book/mod.rs | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/src/book/mod.rs b/src/book/mod.rs
index 830bf9db..c8cab192 100644
--- a/src/book/mod.rs
+++ b/src/book/mod.rs
@@ -192,16 +192,6 @@ impl MDBook {
 
         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) {

From e56c41a1c2c9ebe9f861ec09212bdc8fee4e2204 Mon Sep 17 00:00:00 2001
From: Michael Bryan <michaelfbryan@gmail.com>
Date: Sun, 21 Jul 2019 02:37:09 +0800
Subject: [PATCH 2/4] The HTML renderer now cleans its own build directory

---
 src/renderer/html_handlebars/hbs_renderer.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/renderer/html_handlebars/hbs_renderer.rs b/src/renderer/html_handlebars/hbs_renderer.rs
index 1490f8af..16c884e1 100644
--- a/src/renderer/html_handlebars/hbs_renderer.rs
+++ b/src/renderer/html_handlebars/hbs_renderer.rs
@@ -282,6 +282,9 @@ impl Renderer for HtmlHandlebars {
         let destination = &ctx.destination;
         let book = &ctx.book;
 
+        utils::remove_dir_contents(destination)
+            .chain_err(|| "Unable to remove stale HTML output")?
+
         trace!("render");
         let mut handlebars = Handlebars::new();
 

From 5b0a23ebabcf5473f36f9e7666182e2b55b2f683 Mon Sep 17 00:00:00 2001
From: Michael Bryan <michaelfbryan@gmail.com>
Date: Sun, 21 Jul 2019 02:40:58 +0800
Subject: [PATCH 3/4] Updated the documentation

---
 book-example/src/for_developers/backends.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/book-example/src/for_developers/backends.md b/book-example/src/for_developers/backends.md
index f8ddb0f3..ec7ecd79 100644
--- a/book-example/src/for_developers/backends.md
+++ b/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

From bb412edf5370823797dc7ab94f9aade65f5681c9 Mon Sep 17 00:00:00 2001
From: Michael Bryan <michaelfbryan@gmail.com>
Date: Sun, 21 Jul 2019 04:32:28 +0800
Subject: [PATCH 4/4] Made sure the tests pass

---
 src/book/mod.rs                              | 3 ---
 src/renderer/html_handlebars/hbs_renderer.rs | 6 ++++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/book/mod.rs b/src/book/mod.rs
index c8cab192..f464b496 100644
--- a/src/book/mod.rs
+++ b/src/book/mod.rs
@@ -190,9 +190,6 @@ impl MDBook {
             renderer.name().to_string(),
         );
 
-        let name = renderer.name();
-        let build_dir = self.build_dir_for(name);
-
         for preprocessor in &self.preprocessors {
             if preprocessor_should_run(&**preprocessor, renderer, &self.config) {
                 debug!("Running the {} preprocessor.", preprocessor.name());
diff --git a/src/renderer/html_handlebars/hbs_renderer.rs b/src/renderer/html_handlebars/hbs_renderer.rs
index 16c884e1..7a7f0ef6 100644
--- a/src/renderer/html_handlebars/hbs_renderer.rs
+++ b/src/renderer/html_handlebars/hbs_renderer.rs
@@ -282,8 +282,10 @@ impl Renderer for HtmlHandlebars {
         let destination = &ctx.destination;
         let book = &ctx.book;
 
-        utils::remove_dir_contents(destination)
-            .chain_err(|| "Unable to remove stale HTML output")?
+        if destination.exists() {
+            utils::fs::remove_dir_content(destination)
+                .chain_err(|| "Unable to remove stale HTML output")?;
+        }
 
         trace!("render");
         let mut handlebars = Handlebars::new();