Different Behavior Of Virtual Packages In Snapcraft Stage-Packages Across Architectures And Time

Jul 21, 2025 by gitftunila 97 views

Understanding the Varied Behavior of Virtual Packages in Snapcraft Stage-Packages Across Architectures and Time

In the realm of Snapcraft, developers often encounter scenarios where virtual packages behave inconsistently across different architectures and over time. This article delves into the intricacies of this issue, focusing on the varying installation behaviors of virtual packages within the stage-packages section of a snapcraft.yaml file. We'll examine a real-world example, the libasound2 package in the Checkbox project, to illustrate these inconsistencies and provide insights into potential causes and solutions. Understanding these nuances is crucial for developers aiming to create robust and reliable snaps.

The Anomaly: Different Package Installation Outcomes

When defining dependencies in a Snapcraft project, the stage-packages directive in the snapcraft.yaml file plays a pivotal role. It specifies the Debian packages required for the snap to function correctly. However, the behavior of virtual packages listed under stage-packages can be perplexing. A virtual package is a placeholder package that doesn't exist in its own right but is provided by one or more actual packages. For instance, libasound2 is a virtual package in Ubuntu Noble, which can be satisfied by libasound2t64 or liboss4-salsa-asound2. The expected behavior is that Snapcraft should consistently resolve and install the appropriate provider package.

Case Study: libasound2 in Checkbox

The Checkbox project, a comprehensive testing framework, utilizes Snapcraft for its packaging. The snapcraft.yaml file for Checkbox includes libasound2 in its stage-packages. However, build logs reveal inconsistent behavior across different architectures and build times.

Architectural Discrepancies

On ARM64 architectures, both libasound2t64 and liboss4-salsa-asound2 were installed, a situation that is incorrect since these packages conflict with each other. Conversely, on AMD64 architectures, only libasound2t64 was installed, which is a more aligned behavior but still raises questions about the resolution process.

Temporal Variations

Adding to the complexity, historical builds on AMD64 showed that both libasound2t64 and liboss4-salsa-asound2 were installed on May 24th, a stark contrast to the current behavior where only libasound2t64 is installed. This temporal inconsistency highlights the dynamic nature of package resolution during the Snapcraft build process.

Deep Dive into Virtual Packages and Snapcraft

To unravel these inconsistencies, it's essential to understand how Snapcraft handles virtual packages. When a virtual package is listed in stage-packages, Snapcraft's underlying mechanism, typically APT, attempts to resolve it to a concrete package. This resolution process can be influenced by several factors:

Package Availability: The packages available in the configured repositories at the time of the build.
Architecture: The architecture for which the snap is being built (e.g., AMD64, ARM64).
APT Configuration: The APT configuration used during the build, including preferences and priorities.
Timing: Changes in package availability or repository configurations over time.

Understanding APT's Role

APT (Advanced Package Tool) is the package management system used by Debian and Ubuntu. When Snapcraft encounters a virtual package, it relies on APT to determine the appropriate provider package. APT uses a sophisticated algorithm that considers dependencies, version constraints, and priorities to select the best candidate. However, this process is not always deterministic, especially when multiple providers exist or when package states change over time.

Potential Causes for Inconsistent Behavior

Several factors might contribute to the observed inconsistencies:

Conflicting Dependencies: If multiple packages providing the virtual package have conflicting dependencies, APT might make different choices based on the architecture or the presence of other packages.
Repository Mirrors: Different mirror servers might have slightly different package versions or availability, leading to variations in resolution.
APT Configuration Changes: Changes in the APT configuration within the build environment can alter the resolution behavior.
Package Updates: Updates to the available packages in the repositories can influence which provider is selected.
Snapcraft's Internal Logic: Snapcraft's internal logic for handling virtual packages might have subtle variations or edge cases that lead to inconsistencies.

Reproducing the Issue: A Step-by-Step Guide

To better understand and address the issue, reproducing it in a controlled environment is crucial. Here’s a step-by-step guide based on the provided information:

Clone the Repository:

git clone https://github.com/canonical/checkbox.git

Navigate to the Project Directory:
```
cd checkbox/checkbox-core-snap/
```
Prepare the Environment:
```
./prepare.sh 24
```
Enter the Series Directory:
```
cd series24
```
Build the Snap:
```
snapcraft
```

By following these steps, you can attempt to reproduce the inconsistent behavior and examine the build logs to pinpoint the exact packages being installed on different architectures.

Environment Details: Ubuntu 24.04 and LXD

The environment in which the issue was observed consists of Ubuntu 24.04 running within LXD containers. This setup is common for Snapcraft builds, as LXD provides a clean and isolated environment. However, it also introduces potential complexities, such as the configuration of APT within the container and the interaction with the host system's package repositories.

The Role of LXD Containers

LXD containers offer a lightweight virtualization solution, allowing Snapcraft to build snaps in a consistent and reproducible environment. Each container has its own filesystem and process namespace, isolating the build process from the host system. This isolation is beneficial for ensuring that the build is not affected by the host's configuration. However, it also means that the container's APT configuration and package repositories must be carefully managed.

Ubuntu 24.04 (Noble Numbat)

Ubuntu 24.04, codenamed Noble Numbat, is the latest LTS (Long Term Support) release of Ubuntu. It introduces several changes and updates to the underlying system, including package versions and APT configurations. These changes can potentially affect the behavior of Snapcraft builds, especially when dealing with virtual packages.

Examining the snapcraft.yaml Configuration

The snapcraft.yaml file is the heart of a Snapcraft project, defining the snap's metadata, dependencies, and build process. Analyzing the snapcraft.yaml for Checkbox can provide valuable clues about the virtual package issue.

Key Sections of the snapcraft.yaml

The provided snapcraft.yaml file includes several key sections:

name, summary, description: Metadata about the snap.
grade, confinement: Snap confinement and stability level.
base: The base snap to use (core24 in this case).
slots: Interface slots for content sharing.
package-repositories: Additional APT repositories to use.
parts: The core of the snap definition, specifying build steps and dependencies for each component.

The `parts` Section: A Closer Look

The parts section is where the snap's components are defined. Each part specifies a plugin, source code, build steps, and dependencies. The stage-packages directive within a part lists the Debian packages required for that component. The inconsistency with libasound2 arises within the checkbox-provider-base part.

The `checkbox-provider-base` Part

The checkbox-provider-base part includes libasound2 in its stage-packages. This part is responsible for building the base provider component of the Checkbox snap. The relevant snippet from the snapcraft.yaml is:

checkbox-provider-base:
 plugin: dump
 source: providers/base
 source-type: local
 build-environment:
 - PYTHONPYCACHEPREFIX: "/tmp"
 override-build: |
 export PYTHONPATH=$CRAFT_STAGE/lib/python3.12/site-packages:$SNAPCRAFT_STAGE/usr/lib/python3/dist-packages
 for path in $(find "$CRAFT_STAGE/providers/" -mindepth 1 -maxdepth 1 -type d); do export PROVIDERPATH=$path${PROVIDERPATH:+:$PROVIDERPATH}; done
 python3 manage.py validate
 python3 manage.py build
 python3 manage.py install --layout=relocatable --prefix=/providers/checkbox-provider-base --root="$CRAFT_PART_INSTALL"
 override-prime: |
 # needed because else this will trigger the store autoreviewer and be autorejected
 # cleaning this bit should not cause any issue but if it does do not only remove this line
 # contact also the store/update the package because else this will always trigger
 # a manual review
 for libamd_path in $(find .. -name "*.so.*"); do
 echo "Clearing execstack on $libamd_path";
 # the || true is because there are some  "*.so*.py" in gdblib
 execstack --clear-execstack "$libamd_path" || true
 done;
 craftctl default
 stage-packages:
 - python3-opencv
 - bc
 - bonnie++
 - bpftrace
 - cryptsetup-bin
 - dbus
 - debsums
 - dmidecode
 - dmsetup
 - efibootmgr
 - ethtool
 - freeipmi-tools
 - fswebcam
 - gir1.2-cheese-3.0
 - gir1.2-clutter-1.0
 - gir1.2-gst-plugins-base-1.0
 - gir1.2-gudev-1.0
 - glmark2
 - glmark2-es2
 - glmark2-wayland
 - glmark2-es2-wayland
 - gnome-screenshot
 - gstreamer1.0-tools
 - gstreamer1.0-plugins-bad
 - gstreamer1.0-pulseaudio
 - hdparm
 - i2c-tools
 - ipmitool
 - iperf
 - iperf3
 - iw
 - jq
 - kmod
 - libasound2
 - libcap2-bin
 - libfdt1
 - libsvm3
 - lsb-release
 - lshw
 - mesa-utils
 - mokutil
 - net-tools
 - nmap
 - nux-tools
 - nvme-cli
 - obexftp
 - parted
 - pciutils
 - pulseaudio-utils
 - pyotherside-doc
 - pyotherside-tests
 - qml-module-io-thp-pyotherside
 - qml6-module-io-thp-pyotherside
 - python3-dbus
 - python3-evdev
 - python3-gi
 - python3-natsort
 - python3-pil
 - python3-psutil
 - python3-pyqrcode
 - python3-serial
 - python3-yaml
 - python3-zbar
 - qml-module-qtquick-controls
 - qml-module-qtquick-layouts
 - qmlscene
 - smartmontools
 - usbutils
 - util-linux
 - uuid-runtime
 - wget
 - wmctrl
 - xz-utils
 - rt-tests # For realtime performance test
 - mdadm
 - on armhf:
 - python3-rpi.gpio # only in focal
 - on arm64:
 - python3-rpi.gpio # only in focal
 build-packages:
 - execstack # needed to clear the execstack
 - libasound2-dev
 - libcap-dev
 organize:
 usr/lib/lib*.so*: usr/lib/$CRAFT_ARCH_TRIPLET_BUILD_FOR/
 after: [checkbox-provider-resource]

Conditional Dependencies

Notice the use of conditional dependencies (on armhf: and on arm64:) for python3-rpi.gpio. This indicates that Snapcraft supports architecture-specific dependencies. However, the main issue with libasound2 is not addressed with such conditions, as the problem is the selection of the provider package rather than the package's availability on a specific architecture.

Analyzing Relevant Log Output

The provided log snippets offer direct evidence of the inconsistent behavior:

ARM64 Build Log:
```
Downloading package: liboss4-salsa-asound2
Downloading package: libasound2t64
```
This log shows that both potential providers for libasound2 were installed on ARM64, which is incorrect.
AMD64 Build Log:
```
Downloading package: libasound2t64
```
This log shows that only libasound2t64 was installed on AMD64, which is a more reasonable outcome but still doesn't explain the temporal variations.

Interpreting the Logs

The logs clearly demonstrate that the package resolution for libasound2 is not consistent across architectures. The ARM64 build installs both provider packages, indicating a potential misconfiguration or a bug in the resolution process. The AMD64 build installs only one provider, but the historical data suggests that this behavior has changed over time.

Potential Solutions and Workarounds

Addressing the inconsistent behavior of virtual packages requires a multi-faceted approach. Here are several potential solutions and workarounds:

Specify Provider Package: The most direct solution is to explicitly specify the provider package instead of the virtual package. For example, replace libasound2 with libasound2t64 in the stage-packages.
```
stage-packages:
- libasound2t64
# Remove libasound2 from the list
```
This approach ensures that the correct package is installed consistently across architectures and over time. However, it might introduce dependencies that are not present in all environments.
Use Architecture-Specific Dependencies: If different architectures require different providers, use the on <architecture>: syntax to specify architecture-specific dependencies.
```
stage-packages:
- on amd64:
- libasound2t64
- on arm64:
- libasound2t64 # Or liboss4-salsa-asound2 if needed
```
This approach provides fine-grained control over package selection but requires careful consideration of the target architectures.
Pin Package Versions: Pinning package versions can help ensure consistency over time. This can be achieved by specifying the exact version of the provider package in the stage-packages.
```
stage-packages:
- libasound2t64=1.2.11-1ubuntu0.1
```
However, this approach can lead to dependency conflicts if other packages require different versions.
Investigate APT Configuration: Examine the APT configuration within the Snapcraft build environment. Ensure that the repositories are correctly configured and that there are no conflicting preferences or priorities.
Report the Issue: If the behavior appears to be a bug in Snapcraft or APT, report the issue to the Snapcraft team or the relevant package maintainers. Providing detailed information and reproduction steps can help them identify and fix the problem.

Additional Context and Considerations

In addition to the technical aspects, it's essential to consider the broader context of the issue.

No Response in Additional Context

The original issue report indicates "No response" in the "Additional context" section. This suggests that there might not be any specific additional information or constraints that influence the issue. However, it's always good to gather as much context as possible to ensure a comprehensive understanding.

The Importance of Reproducible Builds

One of the core principles of Snapcraft is to create reproducible builds. This means that building the same snap from the same source code should always produce the same result, regardless of the build environment or the time of the build. The inconsistent behavior of virtual packages undermines this principle and can lead to unexpected issues in production.

Conclusion

The inconsistent behavior of virtual packages in Snapcraft stage-packages across architectures and time is a complex issue that requires careful attention. By understanding the underlying mechanisms of APT and Snapcraft, developers can diagnose and address these inconsistencies. Specifying provider packages explicitly, using architecture-specific dependencies, pinning package versions, and investigating APT configurations are all potential solutions. Ultimately, ensuring reproducible builds is crucial for the reliability and stability of snaps.

By delving deep into this issue, we hope to empower developers with the knowledge and tools necessary to navigate the complexities of Snapcraft package resolution and create robust and consistent snaps. This detailed exploration not only addresses the immediate problem but also enhances the broader understanding of Snapcraft's inner workings, fostering a more reliable and predictable snap development experience.