Client side

Introduction

The client tool is used to calculate the update, download the files, validate them and then set the needed flags for the upgrader to pick them up and apply.

Requirements

• Secure download of the indexes (HTTPS + GPG)

• Support for everything described in the GPG spec (revocation list, multiple keyrings, ...)

• Resolution of the best upgrade path based on different policies (total download size, least number of reboots, ...)
• Download and validation of the files
• Flexible implementation to allow different upgrader setups with minimal changes required
• Support for suspend/resume of downloads

Nice to have

• Bandwidth limiting is a nice to have and might be tricky to implement.

Implementation

The current implementation is a command line tool, however it's expected to be turned into a DBus service that the Touch UI can drive.

Step-by-step example for an update

Whenever called the client does the following:

1. Grab https://server/channels.json and lookup the index for the current channel. If present, also grab the device GPG keyring.

2. Grab https://server/<channel>/<model>/index.json

3. Read the current version number of the device (ubuntu-build file)
4. Look for the most recent version available
5. Resolve an upgrade path to it, minimizing download size and number of reboots
6. Download any file needed up until the next reboot
7. Validate all the files
8. Write them to the cache partition
9. Write the list of updates for the upgrader to use
10. Reboot into the upgrader

Those steps don't include all of the specific GPG validation bits required to ensure the authenticity of all files. Those are detailed in the separate GPG wiki page.

Security (e.g. what to download over https/http) is outlined in the server security section.

DBus API

The client will export a DBus API on the system bus which will allow for a u/i in the System Settings to query, begin, cancel, and apply a system update. This service starts via DBus activation and exits automatically after a configurable amount of time (i.e. it does not run forever). It maintains state such that the client can exit and get restarted to continue the update, even across reboots. The client uses the download service to manage all file downloads. Here is the DBus API specification:

Signals

UpdateAvailableStatus(is_available, downloading, available_version, update_size, last_update_date, descriptions, error_reason)
Sent in response to CheckForUpdate() async calls, this signal provides information about the state of the update.

• is_available - boolean which indicates whether an update is available or not. This will be false if the device's build number is equal to or greater than any candidate build on the server (IOW, there is no candidate available). This flag will be true when there is an update available.

• downloading - boolean indicating whether a download is in progress. This doesn't include any preliminary downloads needed to determine whether a candidate is available or not (e.g. keyrings, blacklists, channels.json, and index.json files). This flag will be false if a download is paused.

• available_version - integer specifying the candidate build number we plan to update to.

• update_size - integer providing total size in bytes for an available upgrade. This does not include any preliminary files needed to determine whether an update is available or not.

• last_update_date - the ISO 8601 format UTC date (to the second) that the last update was applied to this device. This will be the empty string if no update has been previously applied.

• descriptions - array of mappings containing the descriptions for all updates that will be downloaded. Each element of the array is a mapping of language-specific description keys to the UTF-8 description for that language. See the server documentation for details.

• error_reason - A UTF-8 English human readable string indicating why the download did not start. Only useful if the second argument (downloading) is false, otherwise ignore this value.

Depending on the state of the system, as described below, some of the arguments of this signal may be ignored.

UpdateProgress(percentage, eta)
Sent periodically, while a download is in progress. This signal is not sent when an upgrade is paused.

• percentage - integer between 0 and 100 indicating how much of the download (not including preliminary files) have been currently downloaded. This may be 0 if we do not yet know what percentage has been downloaded.

• eta - estimated time remaining to complete the download, in float seconds. This may be 0 if we don't have a reasonable estimate.

UpdatePaused(percentage)
Sent whenever a download is paused as detected via the download service.

• percentage - integer between 0 and 100 indicating how much of the download (not including preliminary files) have been currently downloaded. May be 0 if this information cannot be obtained.

UpdateDownloaded()
Sent when the currently in progress update has been completely and successfully downloaded. When this signal is received, it means that the device is ready to have the update applied via ApplyUpdate().

UpdateFailed(consecutive_failure_count, last_reason)
Sent when the update failed for any reason (including cancellation). The client will remain in the failure state until the next CheckForUpdate() call.

• consecutive_failure_count - an integer specifying the number of times in a row that a CheckForUpdate() has resulted in an update failure. This increments until an update completes successfully (i.e. until the next UpdateDownloaded() signal is issued).

• last_reason - a UTF-8 string containing the English reason for why this updated failed.

Methods

CheckForUpdate()
Asynchronous call instructing the client to check for an available update. If a check is already in progress, it continues. If the client is in auto-download mode (see below), then this call will both check the server for an available update, and automatically initiate the download of an available update. If the client is in manual-download mode, then the downloading of any available update will not occur automatically (use DownloadUpdate() explicitly to start the download). It is also possible for the download to only occur automatically if other criteria are met, e.g. we are not on a wifi, or the device has a low battery. Note that CheckForUpdate() will never resume a download.

In all cases, an UpdateAvailableStatus signal will be issued once an update candidate is determined to be available; the arguments can have the following values:

• UpdateAvailableStatus(true, true, build_number, size, "YYYY-MM-DDTHH:MM:SS", descriptions, "") - This means that an update is available and is currently downloading. The build number of the candidate update is given, as is its total size in bytes, and the descriptions of the updates in all available languages.

• UpdateAvailableStatus(true, false, build_number, size, "YYYY-MM-DDTHH:MM:SS", descriptions, "paused") - This means that an update is available, but it is not yet downloading, possibly because the client is in manual-update mode, or because the download is currently paused. The reason is given in the last argument, and the build number, size, and descriptions are given as above.

• UpdateAvailableStatus(false, ?, ?, ?, "YYYY-MM-DDTHH:MM:SS", ?, ?) - There is no update available. The ISO 8601 date of the last applied update is given, but all other arguments should be ignored.

If we are in auto-download mode, an UpdateProgress() signal is sent as soon as the download is started, usually with a percentage of 0 and an eta of 0.

DownloadUpdate()
Asynchronous method to download an available update. No-ops if there is no update to download, an automatic download is already in progress, CheckForUpdate() was not called first, or the update status is in an error condition. If a previous download was paused, this resumes the download. As with auto-download mode, an UpdateProgress() signal is sent as soon as the download is started.

ApplyUpdate()
Synchronous method to apply the update and initiate the reboot. No-ops if no new update has been downloaded. Because the success path leads almost immediately to a reboot, the return value of this method is not meaningful (but will in practice be the empty string). In the failure case, the return value is a UTF-8 English string indicating the cause of the failure.

CancelUpdate()
Synchronous method to cancel any update check or download (even finished or paused) in progress. The empty string is returned unless an error occurred, in which case the error message is returned as a UTF-8 English string.

PauseDownload()
Synchronous method to pause the current download. The empty string is returned unless an error occurred, in which case the error message is returned as a UTF-8 English string.

SetSetting(key, value)
Synchronous method to write or update a setting. key and value are UTF-8 strings. While any key/value pair may be set, only the following currently have useful semantics:

• min_battery - The minimum battery strength which will allow downloads to proceed. The value is the string representation of a number between 0 and 100 percent. If the battery level is less than this, an available download will not be automatically initiated, although an explicit DownloadUpdate() call can force this.

• auto_download - A tri-state (currently) value indicating whether downloads should normally proceed automatically if an update is available or not. The value is the string representation of the following integer values.

  • 0 - Never download automatically (i.e. an explicit DownloadUpdate() call is required to start the download)

  • 1 - Only auto-download if the device is connected via wifi (the default)
  • 2 - Always download the update automatically

• failures_before_warning - Unused by the client, but stored here for use by the user interface.

For the values with semantics, any invalid value is ignored (i.e. not set or stored).

GetSetting(key)
Synchronous method to read a setting. If key is unknown, the empty string is returned.

Exit()
Synchronous method causing the client process to exit immediately. There is no return value. If this is not called, the client will still exit normally after some configurable amount of time. DBus activation will restart it.

Code

• https://code.launchpad.net/~ubuntu-system-image/ubuntu-system-image/client

Mock scenarios

Normal update in auto mode

option: --testing=update-auto-success

1. CheckForUpdate() is received

2. wait for 3 seconds

3. UpdateAvailableStatus(is_available: true, downloading: true, available_version: 42, update_size: 1337*1024*1024, last_update_date: 1983-09-13T12:13:14, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "") is sent by the daemon

4. immediately the daemon sends UpdateProgress(0, 50)

5. then every half a second, send UpdateProgress(+1, -0.5)

6. daemon sends UpdateDownloaded()

7. client sends ApplyUpdate(), daemon gives back an empty string (success)

Note that while download is in progress, calling:

• PauseDownload will pause the emission of UpdateProgress(). It's a noop otherwise (even if called when not download).

• DownloadUpdate will resume the emission of this signal. It's a noop otherwise (even if called when not download).

• CancelUpdate will cancel the update. It's a noop otherwise (even if called when not download). Next CheckForUpdate will restart from 1.

• CheckForUpdate will immediately resends UpdateSignal and UpdateProgress(where is it) as per spec.

If not Downloading:

• CheckForUpdate called once UpdateDownloaded() has been sent is a noop. (if CancelUpdate wasn't called yet)

General:

• GetSetting("auto_download") returns "1"

• SetSetting("auto_download", int) will set the value in memory and then GetSetting() will return it

• GetSetting("foo") returns ""

Normal update in manual mode

--testing=update-manual-success

1. CheckForUpdate() is received

2. wait for 3 seconds

3. UpdateAvailableStatus(is_available: true, downloading: false, available_version: 42, update_size: 1337*1024*1024, last_update_date: 83-09-13, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "") is sent by the daemon

4. clients sends DownloadUpdate

5. immediately the daemon sends UpdateProgress(0, 50)

6. then every half a second, send UpdateProgress(+1, -0.5)

7. daemon sends UpdateDownloaded()

8. client sends ApplyUpdate, daemon gives back an empty string (success)

Failing update

--testing=update-failed The daemon is already in failure mode

1. the client will send CheckForUpdate

2. the daemon answers with UpdateAvailableStatus(is_available: true, downloading: false, available_version: 42, update_size: 1337*1024*1024, last_update_date: 83-09-13, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "You need some network for downloading.")

3. the daemon then send UpdateFailed(9, "You need some network for downloading.")

Failing apply

option: --testing=fail-apply the daemon is in UpdateDownloaded mode

1. the client will send CheckForUpdate

2. the daemon answers with UpdateAvailableStatus(is_available: true, downloading: false, available_version: 42, update_size: 1337*1024*1024, last_update_date: 83-09-13, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "")

3. then the daemon sends the UpdateDownloaded signal

4. client sends ApplyUpdate and receive "Not enough battery, you need to plug your phone." string

Failing Resume / Manual download

--testing=fail-resume the daemon is paused, 42% of download is done

1. the client will send CheckForUpdate

2. the daemon answers with UpdateAvailableStatus(is_available: true, downloading: false, available_version: 42, update_size: 1337*1024*1024, last_update_date: 83-09-13, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "")

3. the daemons sends the UpdatePaused(42) signal

4. client tries to call DownloadUpdate() and receive an UpdateFailed(9, "You need some network for downloading.")

Failing pause

--testing=failing-pause the daemon is downloading, initially, 10% is downloaded, there is no known ETA.

1. the client will send CheckForUpdate

2. the daemon answers with UpdateAvailableStatus(is_available: true, downloading: true, available_version: 42, update_size: 1337*1024*1024, last_update_date: 83-09-13, descriptions: [{'description': "Ubuntu Edge support", 'description-en_GB': 'change the background colour', 'description-fr': "Support d'Ubuntu Edge"}, {'description': "Flipped container with 200% boot speed improvement"}], error_reason: "")

3. immediately the daemon sends UpdateProgress(10, 0) (note! 0 for no ETA)

4. the clients then sends PauseDownload() and will receive "no no, not now" as a string from the daemon.