How to debug Mir snaps¶

Getting a useful stack trace or debugging session with snaps requires some set up, as they don’t contain debugging information. This document describes the steps required to supply the debugger with the necessary data.

Enable `debuginfod`¶

First, we’ll make sure you have debuginfod enabled. It’s a service that hosts the debug info stripped from Ubuntu packages, which debuggers can retrieve by matching the build-id embedded in the executables and libraries. In modern Ubuntu it’s installed by default, but enable it in GDB by default with:

sudo apt install libdebuginfod1t64
# If not already there, this will be added to your environment on next login
export DEBUGINFOD_URLS=https://debuginfod.ubuntu.com
echo "set debuginfod enabled on" >> ~/.gdbinfo

To learn more, head to About debuginfod on Ubuntu Server documentation pages.

Install the snap¶

Next, install the snap you want to debug. You have two options:

Mir CI snaps¶

If debugging an issue in a Mir Pull Request, you can install the snap from Mir Continuous Integration pipelines. These are published in the store for as long as the pull request is active - install with, for example:

snap install mir-test-tools --channel "edge/mir-pr<number>"

These are Debug builds, so you can go straight to Running Mir snaps under GDB below.

PPA-based snaps¶

The snaps available outside of the mir-pr<number> branches come from PPA builds, which strip debug info into separate packages, making it a bit more involved to help GDB find them.

best create a folder in which you’ll hold the debug files and point GDB at it

DEBUG_DIR=$HOME/.local/debug
echo "set debug-file-directory $DEBUG_DIR/usr/lib/debug:/usr/lib/debug/" >> ~/.gdbinit
mkdir --parents $DEBUG_DIR
cd $DEBUG_DIR

next, add the PPA source for the base of the snap you’re looking at:

PPA=dev
RELEASE=noble
sudo add-apt-repository --no-update ppa:mir-team/$PPA
sed \
   -e "s|^Suites:.*|Suites: $RELEASE|" \
   -e "s|^Components:.*|Components: main/debug|" \
   /etc/apt/sources.list.d/mir-team-ubuntu-$PPA-$( lsb_release -sc ).sources \
   | sudo tee /etc/apt/sources.list.d/mir-team-ubuntu-$PPA-$RELEASE.sources
sudo apt-get update

then configure the sources’ priority down, so they don’t get considered for installation

tee <<EOF > /etc/apt/preferences.d/mir-debug
Package: *
Pin: release n=$RELEASE, o=LP-PPA-mir-team-$PPA
Pin-Priority: -50
EOF

and download all the dbgsym packages into a folder of choice
```
apt download {lib,}mir*-dbgsym/noble
```

finally, unpack the downloaded packages

for pkg in *deb; do dpkg-deb --extract $pkg .; done

Running Mir snaps under GDB¶

Next, to get into a GDB session, run the snap with snap run --gdbserver <snap> and follow the steps to open a gdb prompt. To avoid breaking out, ignore the SIGSTOPs being sent on Mir startup:

handle SIGSTOP nostop nopass
break miral::MirRunner::run_with
continue
# when it breaks out, look at the printed path and point at your local source
set substitute-path /usr/src/mir-<version>/ .

You can find out more about debugging snaps in general in Snapcraft’s Debug with GDB.