Figure with empty caption and cross-ref label rendered to HTML crashes main.lua

[jbrick55]

Bug description

There appears to be something in Lua filter causing errors when I do not have a caption for a figure but I do have it labeled for cross-reference. I believe this is related to #9675

Steps to reproduce

The following code is for a graph (this also happens with images with no captions) with no fig-cap but has a label. This will produce a Lua error (as shown below).
If I remove the label inside the r code and use :::{#fig-Rgraph} before the code and remove the #|label from the code chunk it will render properly. All of my previously working graphs and images now throw this error when trying to render.

---
title: "Reproducible Quarto Document"
format: html
engine: knitr
---

```{r message=FALSE, warning=FALSE, error=FALSE, out.width="70%"}
#| label: fig-Rgraph
#| fig-alt: "Simple plot"
#| eval: true
#| echo: false
#| lightbox: true

x <- 1:10
y <- x^2

plot(x, y, 
     type = "o",        
     col = "blue",      
     pch = 16,         
     lty = 2,           
     main = "Simple Plot",   
     xlab = "X axis",  
     ylab = "Y axis"    
     )
```

Expected behavior

The code should render to html as a graph.

Actual behavior

Lua Error:

Error running filter /Applications/quarto/share/filters/main.lua:
/Applications/quarto/share/filters/main.lua:11765: attempt to index a nil value (field 'caption_long')
stack traceback:
	[C]: in ?
	[C]: in method 'walk'
	/Applications/quarto/share/filters/main.lua:557: in function </Applications/quarto/share/filters/main.lua:546>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:11762: in local 'filter_fn'
	/Applications/quarto/share/filters/main.lua:635: in function </Applications/quarto/share/filters/main.lua:625>
	(...tail calls...)
	[C]: in ?
	[C]: in method 'walk'
	/Applications/quarto/share/filters/main.lua:557: in function </Applications/quarto/share/filters/main.lua:546>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:1334: in local 'callback'
	/Applications/quarto/share/filters/main.lua:1352: in upvalue 'run_emulated_filter_chain'
	/Applications/quarto/share/filters/main.lua:1388: in function </Applications/quarto/share/filters/main.lua:1385>
stack traceback:
	/Applications/quarto/share/filters/main.lua:557: in function </Applications/quarto/share/filters/main.lua:546>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:11762: in local 'filter_fn'
	/Applications/quarto/share/filters/main.lua:635: in function </Applications/quarto/share/filters/main.lua:625>
	(...tail calls...)
	[C]: in ?
	[C]: in method 'walk'
	/Applications/quarto/share/filters/main.lua:557: in function </Applications/quarto/share/filters/main.lua:546>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:1334: in local 'callback'
	/Applications/quarto/share/filters/main.lua:1352: in upvalue 'run_emulated_filter_chain'
	/Applications/quarto/share/filters/main.lua:1388: in function </Applications/quarto/share/filters/main.lua:1385>
stack traceback:
	/Applications/quarto/share/filters/main.lua:557: in function </Applications/quarto/share/filters/main.lua:546>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:1334: in local 'callback'
	/Applications/quarto/share/filters/main.lua:1352: in upvalue 'run_emulated_filter_chain'
	/Applications/quarto/share/filters/main.lua:1388: in function </Applications/quarto/share/filters/main.lua:1385>

Your environment

• IDE: RStudio Version 2024.04.2+764 (2024.04.2+764)
• MacOS: 14.5 (23F79)

Quarto check output

Quarto 1.5.53
[✓] Checking versions of quarto binary dependencies...
Pandoc version 3.2.0: OK
Dart Sass version 1.70.0: OK
Deno version 1.41.0: OK
Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
Version: 1.5.53
Path: /Applications/quarto/bin

[✓] Checking tools....................OK
TinyTeX: (external install)
Chromium: (not installed)

[✓] Checking LaTeX....................OK
Using: TinyTex
Path: /Users/jmh491/Library/TinyTeX/bin/universal-darwin
Version: 2023

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
Version: 3.12.3
Path: /Library/Frameworks/Python.framework/Versions/3.12/bin/python3
Jupyter: 5.7.2
Kernels: sas, python3

(/) Checking Jupyter engine render....Traceback (most recent call last):
File "/Applications/quarto/share/jupyter/jupyter.py", line 21, in
from notebook import notebook_execute, RestartKernel
File "/Applications/quarto/share/jupyter/notebook.py", line 15, in
from yaml import safe_load as parse_string
ModuleNotFoundError: No module named 'yaml'
[✓] Checking Jupyter engine render....OK

[cderv]

I can reproduce using 1.5.54 but it does not happen with 1.6.1 nor 1.4.555.

Thanks for the report !

[cderv]

This is an issue with lightbox: true when no caption is set.

---
title: "Reproducible Quarto Document"
format: html
engine: knitr
keep-md: true
---

```{r}
#| label: fig-Rgraph
#| lightbox: true
x <- 1:10
y <- x^2
plot(x, y)
```

[cderv]

This is related to

• "attempt to index a nil value (field 'caption_long')" error thrown when trying to preview a page with both a lightbox image and an embedded scanpy figure #10196

fixed in

• lua - protect against nil caption_long fields in floats #10198 (e6190a4)

So using latest pre-release will have the fix

@cscheid did you plan to backport ? Or is there a reason not to ? I see the hot-fix milestone in #10196 so I would have expected #10198 to have been backported to 1.5 branch too for next patch release

Thanks

[cscheid]

@cderv Thanks, I missed this. I've now backported to v1.5. It'll be out on our next stable release.

[cderv]

Perfect - thanks ! I would have done it, but I prefered checking with you first.

For any future readers, this will be fixed in v1.5.55.