xdg-mime silently ignores invalid .desktop file

[Duncan Bayne]

I created an invalid ~/.local/share/applications/mailto.desktop file (note the quotes in Exec):

#!/usr/bin/env xdg-open
[Desktop Entry]
Version=1.0
Type=Application
Terminal=false
Name[en_AU]=mailto
Exec="mailto %f"
Name=mailto

I then set the default, and queried it:

$ xdg-mime default mailto.desktop 'x-scheme-handler/mailto'
$ echo $?
0
$ xdg-mime query default 'x-scheme-handler/mailto'
thunderbird.desktop
$ echo $?
0

As you can see, no error codes are returned, but the system default application is returned.

If I fix the mailto.desktop file by removing the quotes, the query performs as expected:

$ xdg-mime query default 'x-scheme-handler/mailto'
mailto.desktop

Running in verbose mode doesn't give any further information, i.e., that the .desktop file was passed over in search because it was invalid:

$ # bad
$ XDG_UTILS_DEBUG_LEVEL=9999 xdg-mime query default 'x-scheme-handler/mailto' --verbose
Checking /home/duncan/.config/mimeapps.list
Checking /home/duncan/.local/share/applications/mimeapps.list
Checking /home/duncan/.local/share/applications/defaults.list and /home/duncan/.local/share/applications/mimeinfo.cache
Checking /home/duncan/.local/share/applications/defaults.list and /home/duncan/.local/share/applications/mimeinfo.cache
Checking /usr/share/stumpwm/applications/defaults.list and /usr/share/stumpwm/applications/mimeinfo.cache
Checking /usr/share/stumpwm/applications/defaults.list and /usr/share/stumpwm/applications/mimeinfo.cache
Checking /usr/local/share//applications/defaults.list and /usr/local/share//applications/mimeinfo.cache
Checking /usr/local/share//applications/defaults.list and /usr/local/share//applications/mimeinfo.cache
Checking /usr/share//applications/defaults.list and /usr/share//applications/mimeinfo.cache
thunderbird.desktop

$ # good
$ XDG_UTILS_DEBUG_LEVEL=9999 xdg-mime query default 'x-scheme-handler/mailto' --verbose
Checking /home/duncan/.config/mimeapps.list
mailto.desktop

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: xdg-utils 1.1.3-2ubuntu1.20.10.2
ProcVersionSignature: Ubuntu 5.8.0-50.56-generic 5.8.18
Uname: Linux 5.8.0-50-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu50.5
Architecture: amd64
CasperMD5CheckResult: skip
Date: Wed Apr 28 12:39:08 2021
InstallationDate: Installed on 2020-11-03 (175 days ago)
InstallationMedia: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
PackageArchitecture: all
SourcePackage: xdg-utils
UpgradeStatus: No upgrade log present (probably fresh install)