Fix crash when configured font is missing

Fixes #3942.

CHANGELOG.md

@@ -69,6 +69,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Cursor color escape ignored when its color is set to inverted in the config
 - Fontconfig's `autohint` and `hinting` options being ignored
 - Ingoring of default FreeType properties
+- Alacritty crashing at startup when the configured font does not exist
 
 ## 0.4.3
 

alacritty/src/renderer/mod.rs

@@ -10,7 +10,7 @@ use std::time::Duration;
 
 use fnv::FnvHasher;
 use font::{
-    self, BitmapBuffer, FontDesc, FontKey, GlyphKey, Rasterize, RasterizedGlyph, Rasterizer,
+    self, BitmapBuffer, FontDesc, FontKey, GlyphKey, Rasterize, RasterizedGlyph, Rasterizer, Size,
 };
 use log::{error, info};
 use notify::{watcher, DebouncedEvent, RecursiveMode, Watcher};
@@ -224,7 +224,7 @@ impl GlyphCache {
         let regular_desc =
             Self::make_desc(&font.normal(), font::Slant::Normal, font::Weight::Normal);
 
-        let regular = rasterizer.load_font(&regular_desc, size)?;
+        let regular = Self::load_regular_font(rasterizer, &regular_desc, size)?;
 
         // Helper to load a description if it is not the `regular_desc`.
         let mut load_or_regular = |desc: FontDesc| {
@@ -255,6 +255,26 @@ impl GlyphCache {
         Ok((regular, bold, italic, bold_italic))
     }
 
+    fn load_regular_font(
+        rasterizer: &mut Rasterizer,
+        description: &FontDesc,
+        size: Size,
+    ) -> Result<FontKey, font::Error> {
+        match rasterizer.load_font(description, size) {
+            Ok(font) => Ok(font),
+            Err(err) => {
+                error!("{}", err);
+
+                let fallback_desc = Self::make_desc(
+                    &Font::default().normal(),
+                    font::Slant::Normal,
+                    font::Weight::Normal,
+                );
+                rasterizer.load_font(&fallback_desc, size)
+            },
+        }
+    }
+
     fn make_desc(
         desc: &config::FontDescription,
         slant: font::Slant,
@@ -343,7 +363,7 @@ impl GlyphCache {
         let mut rasterizer = font::Rasterizer::new(dpr as f32, font.use_thin_strokes())?;
         let regular_desc =
             GlyphCache::make_desc(&font.normal(), font::Slant::Normal, font::Weight::Normal);
-        let regular = rasterizer.load_font(&regular_desc, font.size)?;
+        let regular = Self::load_regular_font(&mut rasterizer, &regular_desc, font.size)?;
         rasterizer.get_glyph(GlyphKey { font_key: regular, c: 'm', size: font.size })?;
 
         rasterizer.metrics(regular, font.size)