additional changes to the 404 mechanism based on feedback:

- removed config output_404
 - ensure serve overrides the site url, and hosts the correct 404 file
 - refactor 404 rendering into separate fn
 - formatting

book-example/book.toml

@@ -9,7 +9,7 @@ edition = "2018"
 
 [output.html]
 mathjax-support = true
-site-url = "/"
+site-url = "/mdBook/"
 
 [output.html.playpen]
 editable = true

src/cmd/serve.rs

@@ -6,6 +6,7 @@ use futures_util::sink::SinkExt;
 use futures_util::StreamExt;
 use mdbook::errors::*;
 use mdbook::utils;
+use mdbook::utils::fs::get_404_output_file;
 use mdbook::MDBook;
 use std::net::{SocketAddr, ToSocketAddrs};
 use std::path::PathBuf;
@@ -68,6 +69,8 @@ pub fn execute(args: &ArgMatches) -> Result<()> {
     if let Some(dest_dir) = args.value_of("dest-dir") {
         book.config.build.build_dir = dest_dir.into();
     }
+    // Override site-url for local serving of the 404 file
+    book.config.set("output.html.site-url", "/")?;
 
     book.build()?;
 
@@ -76,13 +79,19 @@ pub fn execute(args: &ArgMatches) -> Result<()> {
         .next()
         .ok_or_else(|| anyhow::anyhow!("no address found for {}", address))?;
     let build_dir = book.build_dir_for("html");
+    let input_404 = book
+        .config
+        .get("output.html.input-404")
+        .map(toml::Value::as_str)
+        .flatten();
+    let file_404 = get_404_output_file(input_404);
 
     // A channel used to broadcast to any websockets to reload when a file changes.
     let (tx, _rx) = tokio::sync::broadcast::channel::<Message>(100);
 
     let reload_tx = tx.clone();
     let thread_handle = std::thread::spawn(move || {
-        serve(build_dir, sockaddr, reload_tx);
+        serve(build_dir, sockaddr, reload_tx, &file_404);
     });
 
     let serving_url = format!("http://{}", address);
@@ -120,7 +129,12 @@ pub fn execute(args: &ArgMatches) -> Result<()> {
 }
 
 #[tokio::main]
-async fn serve(build_dir: PathBuf, address: SocketAddr, reload_tx: broadcast::Sender<Message>) {
+async fn serve(
+    build_dir: PathBuf,
+    address: SocketAddr,
+    reload_tx: broadcast::Sender<Message>,
+    file_404: &str,
+) {
     // A warp Filter which captures `reload_tx` and provides an `rx` copy to
     // receive reload messages.
     let sender = warp::any().map(move || reload_tx.subscribe());
@@ -144,8 +158,7 @@ async fn serve(build_dir: PathBuf, address: SocketAddr, reload_tx: broadcast::Se
     // A warp Filter that serves from the filesystem.
     let book_route = warp::fs::dir(build_dir.clone());
     // The fallback route for 404 errors
-    let fallback_file = "404.html";
-    let fallback_route = warp::fs::file(build_dir.join(fallback_file))
+    let fallback_route = warp::fs::file(build_dir.join(file_404))
         .map(|reply| warp::reply::with_status(reply, warp::http::StatusCode::NOT_FOUND));
     let routes = livereload.or(book_route).or(fallback_route);
     warp::serve(routes).run(address).await;

src/config.rs

@@ -507,10 +507,8 @@ pub struct HtmlConfig {
     /// FontAwesome icon class to use for the Git repository link.
     /// Defaults to `fa-github` if `None`.
     pub git_repository_icon: Option<String>,
-    /// Input path for the 404 file, defaults to 404.md
+    /// Input path for the 404 file, defaults to 404.md, set to "" to disable 404 file output
     pub input_404: Option<String>,
-    /// Output path for 404.html file, defaults to 404.html, set to "" to disable 404 file output
-    pub output_404: Option<String>,
     /// Absolute url to site, used to emit correct paths for the 404 page, which might be accessed in a deeply nested directory
     pub site_url: Option<String>,
     /// This is used as a bit of a workaround for the `mdbook serve` command.
@@ -545,7 +543,6 @@ impl Default for HtmlConfig {
             git_repository_url: None,
             git_repository_icon: None,
             input_404: None,
-            output_404: None,
             site_url: None,
             livereload_url: None,
             redirect: HashMap::new(),
@@ -679,6 +676,7 @@ impl<'de, T> Updateable<'de> for T where T: Serialize + Deserialize<'de> {}
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::utils::fs::get_404_output_file;
 
     const COMPLEX_CONFIG: &str = r#"
         [book]
@@ -1024,7 +1022,10 @@ mod tests {
         let got = Config::from_str(src).unwrap();
         let html_config = got.html_config().unwrap();
         assert_eq!(html_config.input_404, None);
-        assert_eq!(html_config.output_404, None);
+        assert_eq!(
+            &get_404_output_file(html_config.input_404.as_deref()),
+            "404.html"
+        );
     }
 
     #[test]
@@ -1038,6 +1039,9 @@ mod tests {
         let got = Config::from_str(src).unwrap();
         let html_config = got.html_config().unwrap();
         assert_eq!(html_config.input_404, Some("missing.md".to_string()));
-        assert_eq!(html_config.output_404, Some("missing.html".to_string()));
+        assert_eq!(
+            &get_404_output_file(html_config.input_404.as_deref()),
+            "missing.html"
+        );
     }
 }

src/renderer/html_handlebars/hbs_renderer.rs

@@ -12,6 +12,7 @@ use std::collections::HashMap;
 use std::fs::{self, File};
 use std::path::{Path, PathBuf};
 
+use crate::utils::fs::get_404_output_file;
 use handlebars::Handlebars;
 use regex::{Captures, Regex};
 
@@ -105,6 +106,58 @@ impl HtmlHandlebars {
         Ok(())
     }
 
+    fn render_404(
+        &self,
+        ctx: &RenderContext,
+        html_config: &HtmlConfig,
+        src_dir: &PathBuf,
+        handlebars: &mut Handlebars<'_>,
+        data: &mut serde_json::Map<String, serde_json::Value>,
+    ) -> Result<()> {
+        let destination = &ctx.destination;
+        let content_404 = if let Some(ref filename) = html_config.input_404 {
+            let path = src_dir.join(filename);
+            std::fs::read_to_string(&path)
+                .with_context(|| format!("unable to open 404 input file {:?}", path))?
+        } else {
+            // 404 input not explicitly configured try the default file 404.md
+            let default_404_location = src_dir.join("404.md");
+            if default_404_location.exists() {
+                std::fs::read_to_string(&default_404_location).with_context(|| {
+                    format!("unable to open 404 input file {:?}", default_404_location)
+                })?
+            } else {
+                "# Document not found (404)\n\nThis URL is invalid, sorry. Please use the \
+                navigation bar or search to continue."
+                    .to_string()
+            }
+        };
+        let html_content_404 = utils::render_markdown(&content_404, html_config.curly_quotes);
+
+        let mut data_404 = data.clone();
+        let base_url = if let Some(site_url) = &html_config.site_url {
+            site_url
+        } else {
+            debug!(
+                "HTML 'site-url' parameter not set, defaulting to '/'. Please configure \
+                this to ensure the 404 page work correctly, especially if your site is hosted in a \
+                subdirectory on the HTTP server."
+            );
+            "/"
+        };
+        data_404.insert("base_url".to_owned(), json!(base_url));
+        // Set a dummy path to ensure other paths (e.g. in the TOC) are generated correctly
+        data_404.insert("path".to_owned(), json!("404.md"));
+        data_404.insert("content".to_owned(), json!(html_content_404));
+        let rendered = handlebars.render("index", &data_404)?;
+
+        let rendered = self.post_process(rendered, &html_config.playpen, ctx.config.rust.edition);
+        let output_file = get_404_output_file(html_config.input_404.as_deref());
+        utils::fs::write_file(&destination, output_file, rendered.as_bytes())?;
+        debug!("Creating 404.html ✓");
+        Ok(())
+    }
+
     #[cfg_attr(feature = "cargo-clippy", allow(clippy::let_and_return))]
     fn post_process(
         &self,
@@ -438,51 +491,8 @@ impl Renderer for HtmlHandlebars {
         }
 
         // Render 404 page
-        if html_config.output_404 != Some("".to_string()) {
-            let default_404_location = src_dir.join("404.md");
-            let content_404 = if let Some(ref filename) = html_config.input_404 {
-                let path = src_dir.join(filename);
-                std::fs::read_to_string(&path).map_err(|failure| {
-                    std::io::Error::new(
-                        failure.kind(),
-                        format!("Unable to open 404 input file {:?}", &path),
-                    )
-                })?
-            } else if default_404_location.exists() {
-                std::fs::read_to_string(&default_404_location).map_err(|failure| {
-                    std::io::Error::new(
-                        failure.kind(),
-                        format!(
-                            "Unable to open default 404 input file {:?}",
-                            &default_404_location
-                        ),
-                    )
-                })?
-            } else {
-                "# 404 - Document not found\n\nUnfortunately, this URL is no longer valid, please use the navigation bar or search to continue.".to_string()
-            };
-            let html_content_404 = utils::render_markdown(&content_404, html_config.curly_quotes);
-
-            let mut data_404 = data.clone();
-            let base_url = if let Some(site_url) = &html_config.site_url {
-                site_url
-            } else {
-                warn!("HTML 'site-url' parameter not set, defaulting to '/'. Please configure this to ensure the 404 page work correctly, especially if your site is hosted in a subdirectory on the HTTP server.");
-                "/"
-            };
-            data_404.insert("base_url".to_owned(), json!(base_url));
-            data_404.insert("path".to_owned(), json!("404.md"));
-            data_404.insert("content".to_owned(), json!(html_content_404));
-            let rendered = handlebars.render("index", &data_404)?;
-
-            let rendered =
-                self.post_process(rendered, &html_config.playpen, ctx.config.rust.edition);
-            let output_file = match &html_config.output_404 {
-                None => "404.html",
-                Some(file) => &file,
-            };
-            utils::fs::write_file(&destination, output_file, rendered.as_bytes())?;
-            debug!("Creating 404.html ✓");
+        if html_config.input_404 != Some("".to_string()) {
+            self.render_404(ctx, &html_config, &src_dir, &mut handlebars, &mut data)?;
         }
 
         // Print version

src/utils/fs.rs

@@ -177,6 +177,10 @@ pub fn copy_files_except_ext(
     Ok(())
 }
 
+pub fn get_404_output_file(input_404: Option<&str>) -> String {
+    input_404.unwrap_or("404.md").replace(".md", ".html")
+}
+
 #[cfg(test)]
 mod tests {
     use super::copy_files_except_ext;