From 3ab0e57e29f476590570221a0a6c793d5c1636b6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Ma=C4=87kowski?= <m4tx@m4tx.pl>
Date: Wed, 25 Sep 2024 14:06:44 +0100
Subject: [PATCH] feat: add simple Error page (#34)

The error page is displayed when a handler returns a Result::Err.

A nice-to-have thing would be to catch panics as well.
---
 flareon/src/error_page.rs    | 135 +++++++++++++++++++++++++++++++++++
 flareon/src/lib.rs           |  12 ++--
 flareon/src/router.rs        |  32 +++++++++
 flareon/templates/error.css  | 101 ++++++++++++++++++++++++++
 flareon/templates/error.html |  66 +++++++++++++++++
 flareon/templates/fail.html  |  17 +++++
 6 files changed, 357 insertions(+), 6 deletions(-)
 create mode 100644 flareon/src/error_page.rs
 create mode 100644 flareon/templates/error.css
 create mode 100644 flareon/templates/error.html
 create mode 100644 flareon/templates/fail.html

diff --git a/flareon/src/error_page.rs b/flareon/src/error_page.rs
new file mode 100644
index 0000000..5dc272d
--- /dev/null
+++ b/flareon/src/error_page.rs
@@ -0,0 +1,135 @@
+use std::sync::Arc;
+
+use askama::Template;
+use log::error;
+
+use crate::router::Router;
+use crate::{Error, Result, StatusCode};
+
+#[derive(Debug)]
+pub(super) struct FlareonDiagnostics {
+    router: Arc<Router>,
+}
+
+impl FlareonDiagnostics {
+    #[must_use]
+    pub(super) fn new(router: Arc<Router>) -> Self {
+        Self { router }
+    }
+}
+
+#[derive(Debug, Template)]
+#[template(path = "error.html")]
+struct ErrorPageTemplate {
+    error_data: Vec<ErrorData>,
+    route_data: Vec<RouteData>,
+}
+
+#[derive(Debug)]
+struct ErrorData {
+    description: String,
+    debug_str: String,
+    is_flareon_error: bool,
+}
+
+#[derive(Debug)]
+struct RouteData {
+    index: String,
+    path: String,
+    kind: String,
+    name: String,
+}
+
+#[must_use]
+pub(super) fn handle_response_error(
+    error: Error,
+    diagnostics: FlareonDiagnostics,
+) -> axum::response::Response {
+    let response = build_error_response(error, diagnostics);
+
+    match response {
+        Ok(error_str) => axum::response::Response::builder()
+            .status(StatusCode::INTERNAL_SERVER_ERROR)
+            .body(axum::body::Body::new(error_str))
+            .unwrap_or_else(|_| build_flareon_failure_page()),
+        Err(error) => {
+            error!("Failed to render error page: {}", error);
+            build_flareon_failure_page()
+        }
+    }
+}
+
+fn build_error_response(error: Error, diagnostics: FlareonDiagnostics) -> Result<String> {
+    let mut error_data = Vec::new();
+    build_error_data(&mut error_data, &error);
+    let mut route_data = Vec::new();
+    build_route_data(&mut route_data, &diagnostics.router, "", "");
+
+    let template = ErrorPageTemplate {
+        error_data,
+        route_data,
+    };
+    Ok(template.render()?)
+}
+
+fn build_route_data(
+    route_data: &mut Vec<RouteData>,
+    router: &Router,
+    url_prefix: &str,
+    index_prefix: &str,
+) {
+    for (index, route) in router.routes().iter().enumerate() {
+        route_data.push(RouteData {
+            index: format!("{index_prefix}{index}"),
+            path: route.url(),
+            kind: match route.kind() {
+                crate::router::RouteKind::Router => if route_data.is_empty() {
+                    "RootRouter"
+                } else {
+                    "Router"
+                }
+                .to_owned(),
+                crate::router::RouteKind::Handler => "View".to_owned(),
+            },
+            name: route.name().unwrap_or_default().to_owned(),
+        });
+
+        if let Some(inner_router) = route.router() {
+            build_route_data(
+                route_data,
+                inner_router,
+                &format!("{}{}", url_prefix, route.url()),
+                &format!("{index_prefix}{index}."),
+            );
+        }
+    }
+}
+
+fn build_error_data(vec: &mut Vec<ErrorData>, error: &(dyn std::error::Error + 'static)) {
+    let data = ErrorData {
+        description: error.to_string(),
+        debug_str: format!("{error:#?}"),
+        is_flareon_error: error.is::<Error>(),
+    };
+    vec.push(data);
+
+    if let Some(source) = error.source() {
+        build_error_data(vec, source);
+    }
+}
+
+const FAILURE_PAGE: &[u8] = include_bytes!("../templates/fail.html");
+
+/// A last-resort error page.
+///
+/// This page is displayed when an error occurs that prevents Flareon from
+/// rendering a proper error page. This page is very simple and should only be
+/// displayed in the event of a catastrophic failure, likely caused by a bug in
+/// Flareon itself.
+#[must_use]
+fn build_flareon_failure_page() -> axum::response::Response {
+    axum::response::Response::builder()
+        .status(StatusCode::INTERNAL_SERVER_ERROR)
+        .body(axum::body::Body::from(FAILURE_PAGE))
+        .expect("Building the Flareon failure page should not fail")
+}
diff --git a/flareon/src/lib.rs b/flareon/src/lib.rs
index 79f3a82..4a3ce13 100644
--- a/flareon/src/lib.rs
+++ b/flareon/src/lib.rs
@@ -20,6 +20,7 @@ mod headers;
 #[doc(hidden)]
 #[path = "private.rs"]
 pub mod __private;
+mod error_page;
 pub mod middleware;
 pub mod request;
 pub mod response;
@@ -46,6 +47,7 @@ use request::Request;
 use router::{Route, Router};
 use tower::Service;
 
+use crate::error_page::FlareonDiagnostics;
 use crate::response::Response;
 
 /// A type alias for a result that can return a `flareon::Error`.
@@ -450,7 +452,10 @@ where
 
         pass_to_axum(request, &mut handler)
             .await
-            .unwrap_or_else(handle_response_error)
+            .unwrap_or_else(|error| {
+                let diagnostics = FlareonDiagnostics::new(router.clone());
+                error_page::handle_response_error(error, diagnostics)
+            })
     };
 
     eprintln!(
@@ -514,11 +519,6 @@ impl Html {
     }
 }
 
-#[allow(clippy::needless_pass_by_value)]
-fn handle_response_error(_error: Error) -> axum::response::Response {
-    todo!("500 error handler is not implemented yet")
-}
-
 #[cfg(test)]
 mod tests {
     use std::pin::Pin;
diff --git a/flareon/src/router.rs b/flareon/src/router.rs
index ade7295..52a4e32 100644
--- a/flareon/src/router.rs
+++ b/flareon/src/router.rs
@@ -198,6 +198,32 @@ impl Route {
             name: None,
         }
     }
+
+    #[must_use]
+    pub fn url(&self) -> String {
+        self.url.to_string()
+    }
+
+    #[must_use]
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_deref()
+    }
+
+    #[must_use]
+    pub(crate) fn kind(&self) -> RouteKind {
+        match &self.view {
+            RouteInner::Handler(_) => RouteKind::Handler,
+            RouteInner::Router(_) => RouteKind::Router,
+        }
+    }
+
+    #[must_use]
+    pub(crate) fn router(&self) -> Option<&Router> {
+        match &self.view {
+            RouteInner::Router(router) => Some(router),
+            _ => None,
+        }
+    }
 }
 
 fn handle_not_found() -> Response {
@@ -207,6 +233,12 @@ fn handle_not_found() -> Response {
     )
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub(crate) enum RouteKind {
+    Handler,
+    Router,
+}
+
 #[derive(Clone)]
 enum RouteInner {
     Handler(Arc<Box<dyn RequestHandler + Send + Sync>>),
diff --git a/flareon/templates/error.css b/flareon/templates/error.css
new file mode 100644
index 0000000..789971d
--- /dev/null
+++ b/flareon/templates/error.css
@@ -0,0 +1,101 @@
+*, *::before, *::after {
+    box-sizing: border-box;
+}
+
+* {
+    margin: 0;
+    padding: 0;
+    border: 0;
+    font-size: 100%;
+    font: inherit;
+    font-family: "Open Sans", sans-serif;
+    vertical-align: baseline;
+}
+
+body {
+    line-height: 1.5;
+}
+
+img, picture, video, canvas, svg {
+    display: block;
+    max-width: 100%;
+}
+
+input, button, textarea, select {
+    font: inherit;
+}
+
+p, h1, h2, h3, h4, h5, h6 {
+    overflow-wrap: break-word;
+}
+
+em {
+    font-style: italic;
+}
+
+body {
+    background-color: #e8edf1;
+    padding: 1rem;
+}
+
+h1 {
+    margin: -1rem -1rem 1rem -1rem;
+    padding: 0.5rem;
+    font-size: 2rem;
+    font-weight: lighter;
+
+    background-color: rgb(255, 63, 79);
+    color: white;
+}
+
+h2 {
+    font-size: 1.5rem;
+    margin-top: .75rem;
+}
+
+h3 {
+    font-size: 1.25rem;
+    margin-top: .5rem;
+}
+
+table, th, td {
+    border: 1px solid #d9dde1;
+    border-collapse: collapse;
+}
+
+table > thead {
+    vertical-align: bottom;
+}
+
+table th {
+    font-weight: bold;
+}
+
+th, td {
+    padding: .5rem;
+    border-bottom-color: #d9dde1;
+    border-bottom-width: 1px;
+}
+
+pre {
+    white-space: pre-wrap;
+    word-wrap: break-word;
+
+    font-size: .75rem;
+    font-family: "Noto Sans Mono", monospace;
+}
+
+.badge {
+    color: #fff;
+    background-color: #1876ff;
+
+    display: inline-block;
+    font-size: 0.7em;
+    padding: 0.25em 0.5em;
+    border-radius: 1em;
+    line-height: 1;
+
+    text-align: center;
+    white-space: nowrap;
+    vertical-align: baseline;
+}
diff --git a/flareon/templates/error.html b/flareon/templates/error.html
new file mode 100644
index 0000000..1f7fb76
--- /dev/null
+++ b/flareon/templates/error.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Flareon failure</title>
+
+    <style>
+        {% include "error.css" %}
+    </style>
+</head>
+<body>
+<h1>Flareon failure</h1>
+<p>An error occurred when handling a request.</p>
+
+<h2>Errors</h2>
+
+<table>
+    <thead>
+    <tr>
+        <th scope="col">#</th>
+        <th scope="col">Description</th>
+        <th scope="col">Structure</th>
+    </tr>
+    </thead>
+    <tbody>
+    {% for error in error_data %}
+    <tr>
+        <th scope="row">{{ loop.index0 }}</th>
+        <td>{{ error.description }}</td>
+        <td>
+            {% if error.is_flareon_error %}<code class="badge">flareon::Error</code>{% endif %}
+            <pre>{{ error.debug_str }}</pre>
+        </td>
+    </tr>
+    {% endfor %}
+    </tbody>
+</table>
+
+<h2>Diagnostics</h2>
+
+<h3>Routes</h3>
+
+<table>
+    <thead>
+    <tr>
+        <th scope="col">#</th>
+        <th scope="col">URL</th>
+        <th scope="col">Type</th>
+        <th scope="col">Name</th>
+    </tr>
+    </thead>
+    <tbody>
+    {% for route in route_data %}
+    <tr>
+        <th scope="row">{{ route.index }}</th>
+        <td>{% if route.path.is_empty() %}<em>&lt;empty&gt;</em>{% else %}{{ route.path }}{% endif %}</td>
+        <td>{{ route.kind }}</td>
+        <td>{% if route.name.is_empty() %}<em>&lt;none&gt;</em>{% else %}{{ route.name }}{% endif %}</td>
+    </tr>
+    {% endfor %}
+    </tbody>
+</table>
+
+</body>
+</html>
diff --git a/flareon/templates/fail.html b/flareon/templates/fail.html
new file mode 100644
index 0000000..b86155e
--- /dev/null
+++ b/flareon/templates/fail.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Flareon failure</title>
+</head>
+<body>
+<h1>Flareon failure</h1>
+<p>An error occurred when trying to build Flareon error page.</p>
+<p>
+    If you are a user, please report this to the website administrator. If you are a website administrator, this is
+    likely a Flareon bug. Please report it on the <a href="https://github.com/flareon-rs/flareon/">Flareon bug
+    tracker</a>.
+</p>
+</body>
+</html>