LaTeX errors do not properly propagate

[TGlas]

Sorry for opening so many issues is a short time, but it seems that some things still need fixes. This one is about error reporting in a broken LaTeX document.

LaTeX document containing bugs

\documentclass[11pt]{article}

\usepackage{amsmath}

\begin{document}
Hello World in \LaTeX!

\buggy}   %% should result in an error message!

$$ a^2 + b^2 = c^2 $$

\end{document}

local compilation with pdflatex on Ubuntu

$ pdflatex --no-shell-escape --interaction=nonstopmode --halt-on-error my.tex
This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023/Debian) (preloaded format=pdflatex)
entering extended mode
(./my.tex
LaTeX2e <2023-11-01> patch level 1
L3 programming layer <2024-01-22>
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2023/05/17 v1.4n Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size11.clo))
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsmath.sty
For additional information on amsmath, use the `?' option.
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amstext.sty
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsgen.sty))
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsbsy.sty)
(/usr/share/texlive/texmf-dist/tex/latex/amsmath/amsopn.sty))
(/usr/share/texlive/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def)
No file my.aux.
! Undefined control sequence.
l.8 \buggy
          }   %% should result in an error message!
!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on my.log.

compilation with playground.html

Compiling with pdflatex...
[...log shortened...]
=== Compilation Started ===
deferredBundleNames: ["extra-cjk","extra-misc"]
Single-pass mode: no cross-references detected
WASM ready in 25ms
VFS: Lazy loading enabled
Mounted 480 files from bundle core
Mounted 1 files from bundle fmt-pdflatex
Mounted 38 files from bundle l3
Mounted 581 files from bundle fonts-cm
Mounted 749 files from bundle fonts-misc
Mounted 596 files from bundle fonts-lm-tfm
Mounted 184 files from bundle fonts-lm-type1
Mounted 55 files from bundle dvips
Mounted 15 files from bundle extra-maps
Mounted 15 files from bundle fonts-cmextra
Mounted 15 files from bundle fonts-symbols
Mounted 29 files from bundle graphics
Mounted 30 files from bundle amsmath
Mounted 422 files from bundle cm-super
Registered 3 deferred files from bundle extra-cjk
Deferred bundle extra-cjk: mounted 3 file markers
Registered 29 deferred files from bundle extra-misc
Deferred bundle extra-misc: mounted 29 file markers
Rewrote pdftex.map: 2440 font paths resolved
VFS finalized: 3241 files
[TeX ERR] program exited (with status: 1), but keepRuntimeAlive() is set (counter=0) due to an async operation, so halting execution but not exiting the runtime or preventing further async execution (you can use emscripten_force_exit, if you want to force a true shutdown)
[TeX ERR] 
Checking pending deferred bundles: none
[RETRY] enableCtan=true
[RETRY] missingFiles=none
[RETRY] Found undefined commands: buggy[0]
Retry #1...
WASM ready in 8ms
VFS: Lazy loading enabled
Mounted 480 files from bundle core
Mounted 1 files from bundle fmt-pdflatex
Mounted 38 files from bundle l3
Mounted 581 files from bundle fonts-cm
Mounted 749 files from bundle fonts-misc
Mounted 596 files from bundle fonts-lm-tfm
Mounted 184 files from bundle fonts-lm-type1
Mounted 55 files from bundle dvips
Mounted 15 files from bundle extra-maps
Mounted 15 files from bundle fonts-cmextra
Mounted 15 files from bundle fonts-symbols
Mounted 29 files from bundle graphics
Mounted 30 files from bundle amsmath
Mounted 422 files from bundle cm-super
Registered 3 deferred files from bundle extra-cjk
Deferred bundle extra-cjk: mounted 3 file markers
Registered 29 deferred files from bundle extra-misc
Deferred bundle extra-misc: mounted 29 file markers
Rewrote pdftex.map: 2440 font paths resolved
VFS finalized: 3241 files
Auto-shimming 1 commands: buggy[0]
[TeX ERR] program exited (with status: 1), but keepRuntimeAlive() is set (counter=0) due to an async operation, so halting execution but not exiting the runtime or preventing further async execution (you can use emscripten_force_exit, if you want to force a true shutdown)
[TeX ERR] 
Checking pending deferred bundles: none
[RETRY] enableCtan=true
[RETRY] missingFiles=none
Total time: 346ms
Compilation failed: Unknown error

Observations

Comparing the error messages, the pdflatex error

! Undefined control sequence.
l.8 \buggy
          }   %% should result in an error message!
!  ==> Fatal error occurred, no output PDF file produced!

is rather clear. However, the report of the playground ends with

Compilation failed: Unknown error

which is not helpful at all. In particular, there is no file name name and no line number. The only hint is found in the line

[RETRY] Found undefined commands: buggy[0]

Things that look odd to me:

• Why don't I get the same error as in my Ubuntu system?
• Why retry compilation in the first place? There are neither any files missing nor any references to resolve. I think that in this case the decision to retry is wrong (and I don't claim to have an ultimate logic on when retrying helps and when not).
• Why is there no concrete error in the second compiler run? When running pdflatex in bash I get the same error output each time, irrespective of whether an aux file exists or not.