From 0c917f72b248a5de58ac6439abcb7697d9ae7660 Mon Sep 17 00:00:00 2001
From: Christophe Dervieux <christophe.dervieux@gmail.com>
Date: Fri, 29 May 2026 18:26:03 +0200
Subject: [PATCH] Detect luaotfload font-fallback crash and surface an
 actionable error

On recent TeX Live (luaotfload v3.29), setting a font fallback
(mainfontfallback/monofontfallback/...) crashes LuaLaTeX before a PDF is
produced: luaotfload's fallback resolver dereferences a nil font. Quarto only
showed a generic "compilation failed" with no usable signal, because the crash
is a Lua runtime error with no standard "! ...Here is how much" block for the
log parser to key off.

Detect the crash signature in findLatexError and return a message that names
the fallback options, explains it is a known upstream luaotfload bug on this
TeX Live, and points to the single-font workaround. Upstream report:
https://github.com/latex3/luaotfload/issues/331

See #14553.
---
 news/changelog-1.10.md                    |  1 +
 src/command/render/latexmk/parse-error.ts | 14 ++++++++++++
 tests/unit/latexmk/parse-error.test.ts    | 28 ++++++++++++++++++++++-
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/news/changelog-1.10.md b/news/changelog-1.10.md
index e59c72deb4f..41f2c1fe058 100644
--- a/news/changelog-1.10.md
+++ b/news/changelog-1.10.md
@@ -25,6 +25,7 @@ All changes included in 1.10:
 ### `pdf`
 
 - ([#13588](https://github.com/quarto-dev/quarto-cli/issues/13588)): Fix Lua error when rendering PDF with `reference-location: margin` and a footnote alongside a figure with `fig-cap`. (author: @mcanouil)
+- ([#14553](https://github.com/quarto-dev/quarto-cli/issues/14553)): Report a clear, actionable error when a font fallback (`mainfontfallback`, `monofontfallback`, ...) crashes LuaLaTeX on recent TeX Live, pointing to a single-font workaround, instead of a generic compilation failure.
 
 ### `typst`
 
diff --git a/src/command/render/latexmk/parse-error.ts b/src/command/render/latexmk/parse-error.ts
index 09cb24949a3..0e7002ed97f 100644
--- a/src/command/render/latexmk/parse-error.ts
+++ b/src/command/render/latexmk/parse-error.ts
@@ -205,10 +205,24 @@ export function findMissingHyphenationFiles(logText: string) {
 const kErrorRegex = /^\!\s([\s\S]+)?Here is how much/m;
 const kEmptyRegex = /(No pages of output)\./;
 
+// luaotfload's font-fallback resolver crashes on recent TeX Live (luaotfload
+// v3.29): when a fallback is set (mainfontfallback / monofontfallback / ...),
+// define_font of the internal `<font>;-fallback` name returns nil and
+// luaotfload-fallback.lua dereferences it. The crash is a Lua runtime error
+// with no `! ...Here is how much` block, so the generic extraction below finds
+// nothing — detect it directly and give actionable guidance.
+// Upstream: https://github.com/latex3/luaotfload/issues/331
+const kLuaotfloadFallbackCrash =
+  /luaotfload-fallback\.lua:\d+: attempt to index a nil value/;
+
 export function findLatexError(
   logText: string,
   stderr?: string,
 ): string | undefined {
+  if (kLuaotfloadFallbackCrash.test(logText)) {
+    return "A font fallback (e.g. 'mainfontfallback' or 'monofontfallback') triggered a known luaotfload bug on this TeX Live version, which crashes LuaLaTeX before a PDF is produced. Until it is fixed upstream, set a single font that covers the required glyphs (e.g. 'monofont: JuliaMono') instead of a fallback list. See https://github.com/latex3/luaotfload/issues/331";
+  }
+
   const errors: string[] = [];
 
   const match = logText.match(kErrorRegex);
diff --git a/tests/unit/latexmk/parse-error.test.ts b/tests/unit/latexmk/parse-error.test.ts
index 4771a6d7b40..288362f90d3 100644
--- a/tests/unit/latexmk/parse-error.test.ts
+++ b/tests/unit/latexmk/parse-error.test.ts
@@ -5,7 +5,7 @@
 *
 */
 
-import { findMissingFontsAndPackages, findMissingHyphenationFiles } from "../../../src/command/render/latexmk/parse-error.ts"
+import { findLatexError, findMissingFontsAndPackages, findMissingHyphenationFiles } from "../../../src/command/render/latexmk/parse-error.ts"
 import { unitTest } from "../../test.ts";
 import { assert } from "testing/asserts";
 
@@ -79,6 +79,32 @@ unitTest("Detect missing files with `findMissingFontsAndPackages`", async () =>
   cwd: () => "unit/latexmk/"
 })
 
+// deno-lint-ignore require-await
+unitTest("Detect luaotfload font fallback crash and surface an actionable hint", async () => {
+  // The luaotfload font-fallback resolver crashes on recent TeX Live when a
+  // fallback font is set (mainfontfallback/monofontfallback): define_font of the
+  // internal `<font>;-fallback` name returns nil and luaotfload-fallback.lua
+  // dereferences it. There is no `! ...Here is how much` block, so the generic
+  // error extraction finds nothing — we want a specific, actionable message.
+  // Upstream: https://github.com/latex3/luaotfload/issues/331
+  const fallbackCrashLog =
+    `luaotfload | db : Reload initiated (formats: otf,ttf,ttc); reason: Font "Latin Modern Roman;-fallback" not found.
+luaotfload | resolve : sequence of 3 lookups yielded nothing appropriate....texmf-dist/tex/luatex/luaotfload/luaotfload-fallback.lua:50: attempt to index a nil value (local 'f').
+!  ==> Fatal error occurred, no output PDF file produced!`;
+
+  const message = findLatexError(fallbackCrashLog) ?? "";
+  assert(
+    message.includes("font fallback"),
+    `Expected a font-fallback hint, got: "${message}"`,
+  );
+  assert(
+    message.includes("luaotfload/issues/331"),
+    `Expected the upstream issue link, got: "${message}"`,
+  );
+}, {
+  cwd: () => "unit/latexmk/"
+})
+
 unitTest("Detect missing hyphenation with babel warnings", async () => {
   // Test backtick-quote format (old format)
   const logWithBacktick = `Package babel Warning: No hyphenation patterns were preloaded for