path: root/chromium/docs/updater/design_doc.md

# Chromium Updater Design Document

This is the design document for [Chromium Updater](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/).

[TOC]

## Overview
Chromium updater is an program that keeps itself and other programs up to date
by downloading new versions of software over the Internet. The updater is an
open-source drop-in replacement for Google Update/Omaha and can be customized
by 3rd party embedders for updating non-Google client software.

The updater is responsible for:
*   Installing applications.
*   Silently keeping those applications (and itself) up-to-date.
*   Providing those applications with services to manage their installation.
*   Providing update servers with anonymous telemetry about those applications.
*   Detecting the uninstallation of those applications, and automatically
    removing itself when all other applications have been uninstalled.

The updater is written for Windows, macOS, Linux. The behavior of the updater
is mostly platform-independent. However, platform-specific modules exist to
translate cross-platform concepts (such as IPC interfaces) to platform-specific
technologies (such as COM or XPC). Additionally, some updater behavior related
to installs and uninstalls are tailored to platform conventions.

The updater is layered atop //components/update\_client, which implements a
cross-platform mechanism to interact with an Omaha server for the purpose of
updating CRXs. //components/update\_client is also used by the component and
extension updaters in Chrome.

To keep applications up-to-date, the updater periodically polls an Omaha server,
communicating the state of installed applications, and receiving update
instructions. It applies updates the server instructs it to, and then reports
the results of the operation back to Omaha servers.

## Updater Design
Once installed, the updater operates as a collection of processes that are
launched on-demand and orchestrate their operations over IPC. The *server
process* hosts an engine that conducts most of the work of updating software,
and it is driven by *client processes* that issue commands to it and potentially
display UI to the user.

![Updater process architecture diagram](images/architecture.svg)

The updater may be installed *per-user* or *system-wide*. If installed per-user,
the updater can only update applications owned by that user, whereas a system-
wide updater can update applications owned by any entity on the system. In
multi-user systems, it is efficient for software such as the browser to be
installed system-wide, owned by root (or the system user) and run by individual
users, but this requires the updater to maintain root privileges in order to
update it. Therefore, in a system-wide installation, the server process runs as
root (or at high integrity). One system-wide installation of the updater and any
number of per-user installations of the updater can coexist and operate
independently on the same system.

Different versions of the updater can coexist even within the same installation
of the updater, but only one such instance is *active*. Inactive versions of the
updater periodically attempt to *qualify* themselves by running self-tests and
(if they pass) take over as the active updater; or uninstall themselves as out
of date once a newer version of the updater has activated.

The functionality of the server process is split into two interfaces:
*   UpdateService, which is served only by the active instance and may be called
    by any client. When an instance activates, it replaces IPC registrations so
    that UpdateService routes to it.
*   UpdateServiceInternal, which is served by all instances but may only be
    called by updater clients of the same version.

Where necessary, the updater acquires a single cross-process lock to prevent
races between activations and ongoing update operations.

### Installing the Updater
Instances of the updater are installed in one of the following ways:
*   A user runs an installer for the updater with the intent of installing an
    application. The updater will install itself, and then use itself to install
    the application. (This is the primary flow on Windows.)
*   An application installs the updater as part of its own installation,
    first-run experience, or repair/recovery. (This is the primary flow on
    macOS.)
*   The updater downloads an update for itself and installs it.

#### Updater States
![Updater state flowchart](images/updater_states.svg)

Only one instance per installation is `Active` at any time. Only the active
instance is allowed to update applications or modify any of the updater's state.
The state of the updater (including the list of applications registered for
updates, the time of the last update check, and more) is stored in a "global
prefs" file. Access to the global prefs file is protected by a global
(cross-process) lock.

Each instance of the updater also has its own separate local prefs file. Local
prefs store information specific to the instance that owns them.

##### Qualification
Instances are installed in the `Unqualified` state. Unqualified instances of
the updater perform a self-test before activating. Each instance registers a
"qualification app" with itself (using local prefs) and checks the Internet for
updates to that application. The Internet responds with an update, which the
updater downloads and applies. If this all succeeds, the updater transitions to
the `Qualified` state and exits.

The qualification app installer is a no-op installer that simply exits with no
error, but could be customized in the future to do additional checks.

Qualification is skipped (along with any other pre-active states) if there is
no active instance of the updater.

##### Activation
When an instance transitions to the active state, it acquires the updater's
lock on the global prefs file, writes and flushes the "swapping" bit to that
prefs file, and then replaces any non-side-by-side elements of the installation
(such as COM or launchd registrations, Omaha 3 or Keystone shims, and more)
with its own. Then, it clears the "swapping" bit, flushes the prefs file again,
and starts listening for instructions on the RPC channels. The swapping bit
ensures that the an updater will recover and restart activation if the program
is interrupted mid-activation.

##### Deactivation
When a newer instance of the updater activates, formerly active instances will
discover that they have been replaced when they start up. These instances
uninstall themselves.

##### Locking
Read and write access to the global prefs file is controlled by a lock. The lock
must also be held while the updater performs any non-side-by-side operation
(such as installing an update for a registered application). The lock is per-
process. The updater uses a polling strategy to acquire the lock, sleeping for
a short time between polls.

### Installing Applications
Applications are installed by the active updater instance primarily by
downloading application-specific installer executables from the Internet and
running them. The updater passes the installer certain arguments or environment
variables depending on the instructions received from the Internet and reads the
installer's exit code as an indicator of success or failure. On some platforms,
more sophisticated APIs to communicate installer progress or error codes are
available.

Fetching the application installer from the Internet ensures that the user gets
the latest version of the application. But in some cases it is better to install
applications without a dependency on the Internet. Therefore, some applications
offer "offline" installers. These offline installers are wrapped in an updater
installer. They install the updater, place the application installer on the
disk in a secure location, and then act as a client and instruct the updater
server to run the application installer.

During the installation process, the updater displays a UI that communicates
the installation progress to the user. The language of the UI is based on the
system's primary langague.

In some scenarios, the application has already been installed on the system by
another mechanism, but needs to register itself with the updater. The updater
exposes a registration API that allow applications to register themselves as
present on the system with the updater.

Regardless of how the application is installed or registered, at the end of the
process the updater knows the application's identity, current version, brand
code (if any), and has a means to check whether the application has been
uninstalled (an existence checker).

### Updating Applications
Each instance of the updater has its own timed scheduler that invokes the
updater to check for updates. The scheduler creates a `--wake` client every
hour, which invokes the `RunPeriodicTasks` RPC on the corresponding server.

If the server is inactive, it qualifies, activates, or uninstalls, as
described above. If the server is active, it checks that registered applications
are still installed, updates enterprise policies, and potentially checks for
updates. The updater will not check for updates if the last check was less than
five hours ago. (This period can be altered by enterprise policy and is
subject to some randomization to avoid synchronized load on the Internet.)

#### Telemetry
To keep the updater simple, the decision about whether to apply an update or not
is made remotely. Updaters contact Internet servers and provide them with
information about the registered applications, operating system, hardware
capabilities, and more, and trust the server's response about whether to apply
an update, using the [Omaha Protocol](protocol_3_1.md).

Applications may signal the updater when they are actively used using a
platform-specific "Actives API". Updaters will transmit to the server whether
or not each application has been actively used since the previous communication
in order to enable the server to compute aggregate and anonymous counts of the
population of users.

Updaters also report the outcome of any installations or updates, including
error codes, download attempts, and timing to the server. Servers may use this
information to detect problems with updates, cease serving bad installers, or
balance load among multiple download sources.

Some telemetry is guarded behind usage-stats opt-in. If the user has opted-in
to transmitting crash reports and usage stats for any software registered with
the updater, the updater will transmit its own usage stats and crash reports as
well.

### Uninstalling Applications
The updater detects the uninstallation of applications by using existence
checkers registered with the updater at installation or registration. When an
application is uninstalled, the updater transmits an uninstall ping to the
Internet recording the uninstallation.

When all managed applications are uninstalled, the updater will uninstall
itself, leaving behind only a log file. Applications may trigger immediate
uninstallation of the updater by running an updater `--wake` client.

Additionally, the updater can be uninstalled by hand using `--uninstall`, even
if registered applications are still present.

## Advanced Topics

### Recovering the Updater
To avoid a single point of failure in the update system, some applications have
a backup updater build into the application that is capable of reinstalling the
updater. Google Chrome contains a "Recovery Component" that is capable of
downloading and reinstalling a new version of the updater if a problem with the
updater is detected or if the version of Chrome becomes too old.

For system-wide installations, recovery must be performed at high integrity.
Google Chrome installs an
[elevator service](../../chrome/elevation_service/README.md) for this purpose.

### UI Strings & Localization
The strings for the metainstaller live in the //chrome/app/chromium_strings.grd
and //chrome/app/google_chrome_strings.grd files. This allows the updater
strings to utilize the Chromium repo's translation process instead of generating
its own. Having it in existing grd files also eliminates the need to onboard
updater specific grd files.

During the build process, the updater strings are embedded directly into the
metainstaller binary via `generate_embedded_i18n`. `generate_embedded_i18n` also
allows an `extractor_datafile`, which can define specific strings to pick out
from the originating grd file. This way, the metainstaller only has the strings
specific to the updater and not any of the other strings within the grd file.
When the `generate_embedded_i18n` is complete, it generates an
`updater_installer_strings.h` header, which contains macro definitions of the
message ids and the offsets. The strings are mapped with their var name appended
with `_BASE`. Then the  `_BASE` appended macros are defined to be the first
localization id in the list, in which case it is `_AF`.

An example from the `updater_installer_strings.h`
```
#define IDS_BUNDLE_INSTALLED_SUCCESSFULLY_AF 1600
#define IDS_BUNDLE_INSTALLED_SUCCESSFULLY_AM 1601

...

#define IDS_BUNDLE_INSTALLED_SUCCESSFULLY_BASE IDS_BUNDLE_INSTALLED_SUCCESSFULLY_AF

...

#define DO_STRING_MAPPING \
  HANDLE_STRING(IDS_BUNDLE_INSTALLED_SUCCESSFULLY_BASE, IDS_BUNDLE_INSTALLED_SUCCESSFULLY) \
```

Within the metainstaller, an l10_util.h/cc has three functions to get localized
strings.
```
GetLocalizedString(int base_message_id)
GetLocalizedStringF(int base_message_id, const std::wstring& replacement)
GetLocalizedStringF(int base_message_id, std::vector<std::wstring> replacements)
```

One function for getting the literal string and two functions to get formatted
strings. `GetLocalizedString()` uses the base id plus the offset based on the
language to look through the binary's string table to get the correct, localized
string. The formatted strings utilize GetLocalizedString() to get the string and
then uses `base::ReplaceStringPlaceholders()` to remove the `$i` placeholders
within the string. With regards to picking the correct language to utilize for
the localized string, `base::win::i18n::GetUserPreferredUILanguageList()` is
used to get the preferred UI languages from MUI. If there are multiple languages
in the list, the first language in the list is picked.

## Testing
In addition to unit testing individual sections of code, the updater provides
integration tests that install and manipulate the updater on the system running
the test.

Testing is possible by programmatically controlling certain parameters of the
updater. These parameters are hardcoded in the production builds but they can
be modified in test builds. For this reason, the updater build scripts define
two separate set of targets. One target produces binaries suitable to
releasing. The other target produces test binaries, which are suffixed by
`_test`. The test binaries link the test hooks code, which slightly alters the
behavior of the updater to facilitate testing it.

Some tests require two versions of the updater. For this reason, an older
version of the updater is checked into `//third_party/updater`. Updaters for
each architecture and platform, Google-branded and Chromium-branded, are
available there. These versions of the updater are sourced from Chromium or
Google's official build output, brought in through CIPD and 3pp.

The updater has its own
[console page](https://ci.chromium.org/p/chromium/g/chromium.updater/console),
which describes the set of continuous integration builders.

## Release
The updater is built by Chrome's official builders and uploaded to the
unsigned builds bucket along with Chrome. It is versioned along with Chrome. The
primary build output is `updater.zip`, which contains the updater executables,
the signing scripts, and other updater-related artifacts.

Google's signing infrastructure unpacks the updater.zip and operates on it to
generate signed executables, which are then released through Omaha similar to
Chrome.

TODO(crbug.com/1035895): Decide on and document branching & versioning.

TODO(crbug.com/1035895): Document A/B release qualification process.

## Tools
TODO(crbug.com/1035895): Document tagger.

## Windows-Specific Notes

### Installer

#### Updater Installer Types
Windows updater installers come in two types:
*   Online Installers
*   Offline Installers

##### Online Installers
An online installer installs the updater and then acts as a client process that
commands the server process to install the application the user desired.

Online installers are composed of three parts: a *tag*, a *metainstaller*, and
an *updater resource*. The metainstaller is a lightweight executable that
uncompresses its updater resource into a safe temporary directory and then
launches the updater's setup client process (`--install`). It passes along the
tag, an unsigned (and untrusted) piece of data that is embedded in the
executable which communicates the installation parameters for the software the
user is trying to install.

Online installers are typically run by the end user after being downloaded from
the Internet. When the installer is executed, the OS launches it at
[medium integrity](https://docs.microsoft.com/en-us/windows/win32/secauthz/mandatory-integrity-control).

However, depending on the tag, the updater and application may need to be
installed system-wide. To achieve this, the metainstaller must re-launch itself
at high integrity. This will result in an
[UAC prompt](https://docs.microsoft.com/en-us/windows/security/identity-protection/user-account-control/how-user-account-control-works)
on Windows.

The metainstaller determines whether to relaunch before unpacking the updater
resources, since unpacking to a user-writable temp directory is not secure for
a system-wide installation.

The metainstaller appends the `--expect-elevated` switch to the relaunch command
line, to allow the relaunched process to exit with an error if it is not running
at the correct integrity level.

##### Offline Installers
Offline install performs the installation with no update check or file download
against the server in the process. All data is read from the files in the
specified directory instead.

An example offline install command line on Windows platform:

```
updater.exe /handoff "&appguid={8A69D345-D564-463C-AFF1-A69D9E530F96}&appname=MyApp&needsadmin=True&installdataindex =verboselog"
           /installsource offline
           /sessionid "{E85204C6-6F2F-40BF-9E6C-4952208BB977}"
           /offlinedir "C:\Users\chrome-bot\AppData\Local\ForgedPath"]
```
DOS style command line switches are also supported for backward compatibility.

##### Manifest file
The offline install process looks for `OfflineManifest.gup` in the
offline directory, and falls back to *`<app-id>`*`.gup` if needed.
The `.gup` file contains the update check response in XML format. An
example of the XML file:
```
<?xml version="1.0" encoding="UTF-8"?>
<response protocol="3.0">
  <app appid="{CDABE316-39CD-43BA-8440-6D1E0547AEE6}" status="ok">
    <updatecheck status="ok">
      <urls>
        <url codebase="http://dl.google.com/foo/install/1.2.3.4/"/>
      </urls>
      <manifest version="1.2.3.4">
        <packages>
          <package hash="abcdef" hash_sha256="sha256hash_foobar"
           name="my_installer.exe" required="true" size="12345678"/>
        </packages>
        <actions>
          <action event="install" needsadmin="false" run="my_installer.exe"
           arguments="--baz"/>
          <action event="postinstall" onsuccess="exitsilentlyonlaunchcmd"/>
        </actions>
      </manifest>
    </updatecheck>
    <!-- "system_level" is not included in any of the definitions. needsadmin
          controls this. -->
    <data index="verboselog" name="install" status="ok">
      {
        "distribution": {
          "verbose_logging": true
        }
      }
    </data>
  </app>
</response>
```
The manifest file is parsed to extract the installer command. The above
manifest leads to installation command:
 ```
 C:\Users\chrome-bot\AppData\Local\ForgedPath\my_installer.exe --baz
 ```

The example handoff install command line also specifies
`installdataindex=verboselog`, and the manifest has a matching install
data. That means the install data is extracted and passed to the app
installer. See [installdataindex](#installdataindex) below for details.

#### Dynamic Install Parameters

##### Steps to create a tagged metainstaller

A tagged metainstaller can be created using the signing tool
[sign.py](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/win/signing/sign.py)
and the metainstaller tagging tool
[tag.py](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/tools/tag.py).

Here are the steps to create a tagged metainstaller for the following tag:
`--tag="appguid=FOO_BAR_APP_ID&appname=SomeName&needsadmin=prefers"`

The source file is the untagged metainstaller `out\Default\UpdaterSetup.exe`,
and the final tagged file will be `out\Default\Tagged_UpdaterSetup.signed.exe`.

* One-time step: from an elevated powershell prompt:
```
New-SelfSignedCertificate -DnsName id@domain.tld -Type CodeSigning
 -CertStoreLocation cert:\CurrentUser\My
```
* Note: all the steps below are run from a medium cmd prompt.
* One-time step: `python3 -m pip install pypiwin32`
* One-time step:
`set PYTHONPATH=C:\src\chromium\src\chrome\tools\build\win`
*
```
python3 C:\src\chromium\src\chrome\updater\win\signing\sign.py --in_file
 C:\src\chromium\src\out\Default\UpdaterSetup.exe
 --out_file C:\src\chromium\src\out\Default\UpdaterSetup.signed.exe
 --lzma_7z "C:\Program Files\7-Zip\7z.exe"
 --signtool c:\windows_sdk_10\files\bin\10.0.22000.0\x64\signtool.exe
 --identity id@domain.tld
 --certificate_tag C:\src\chromium\src\out\Default\certificate_tag.exe
```
*
```
python3 C:\src\chromium\src\chrome\updater\tools\tag.py
 --certificate_tag=C:\src\chromium\src\out\Default\certificate_tag.exe
 --in_file=C:\src\chromium\src\out\Default\UpdaterSetup.signed.exe
 --out_file=out\Default\Tagged_UpdaterSetup.signed.exe
 --tag="appguid=FOO_BAR_APP_ID&appname=SomeName&needsadmin=prefers"
```

##### `needsadmin`

`needsadmin` is one of the install parameters that can be specified for
first installs via the
[metainstaller tag](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/tools/tag.py).
`needsadmin` is used to indicate whether the application needs admin rights to
install.

For example, here is a command line for the Updater on Windows that includes:
```
UpdaterSetup.exe --install --tag="appguid=YourAppID&needsadmin=False"
```

In this case, the updater client understands that the application installer
needs to install the application on a per-user basis for the current user.

`needsadmin` has the following supported values:
* `true`: the application supports being installed systemwide and once
installed, is available to all users on the system.
* `false`: the application supports only user installs.
* `prefers`: the application installation is first attempted systemwide. If the
user refuses the
[UAC prompt](https://docs.microsoft.com/en-us/windows/security/identity-protection/user-account-control/how-user-account-control-works)
however, the application is then only installed for the current user. The
application installer needs to be able to support the installation as system, or
per-user, or both modes.

When `UpdaterSetup` encounters `needsadmin`, it does the following, based on the
`needsadmin` value:
* `true`: runs the application installer as `system`, with any
[elevation prompts](https://docs.microsoft.com/en-us/windows/security/identity-protection/user-account-control/how-user-account-control-works)
that may be required to run as system first.
* `false`: runs the application installer as the current user.
* `prefers`: attempts to run the application installation as system first. If
elevation fails however, runs the application installer as the current user.

##### `installdataindex`

`installdataindex` is one of the install parameters that can be specified for
first installs on the command line or via the
[metainstaller tag](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/tools/tag.py).

For example, here is a typical command line for the Updater on Windows:
```
UpdaterSetup.exe /install "appguid=YourAppID&appname=YourAppName&needsadmin=False&lang=en&installdataindex =verboselog"
```

In this case, the updater client sends the `installdataindex` of `verboselog` to
the update server.

This involves:
* [Parsing the tag](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/tag.h)
into individual tag components, including install_data_index.
* Creating a CrxComponent with the install_data_index and providing it to the
[update_client](https://source.chromium.org/chromium/chromium/src/+/main:components/update_client/update_client.h).
* update_client sends the install_data_index to the update server.

This is how a [JSON](https://www.json.org/) request from update_client may look
like:

```
{
   "request":{
      "@os":"win",
      "@updater":"updater",
      "acceptformat":"crx3",
      "app":[
         {
            "appid":"YourAppID",
            "data":[
               {
                  "index":"verboselog",
                  "name":"install"
               }
            ],
            "enabled":true,
            "installsource":"ondemand",
            "ping":{
               "r":-2
            },
            "updatecheck":{
               "sameversionupdate":true
            },
            "version":"0.1"
         }
      ],
      "arch":"x86",
      "dedup":"cr",
      "domainjoined":true,
      "hw":{
         "avx":true,
         "physmemory":32,
         "sse":true,
         "sse2":true,
         "sse3":true,
         "sse41":true,
         "sse42":true,
         "ssse3":true
      },
      "ismachine":false,
      "lang":"en-US",
      "nacl_arch":"x86-64",
      "os":{
         "arch":"x86_64",
         "platform":"Windows",
         "version":"10.0.19042.1586"
      },
      "prodversion":"101.0.4949.0",
      "protocol":"3.1",
      "requestid":"{6b417770-1f68-4d52-8843-356760c84d33}",
      "sessionid":"{37775211-4487-48d5-845d-35a1d71b03bc}",
      "updaterversion":"101.0.4949.0",
      "wow64":true
   }
}
```

The server retrieves the data corresponding to `installdataindex=verboselog`
and returns it back to update_client.

This is how a JSON response from the update server may look like:

```
  "response":{
   "protocol":"3.1",
   "app":[
    {"appid":"12345",
     "data":[{
      "status":"ok",
      "name":"install",
      "index":"verboselog",
      "#text":"{\"logging\":{\"verbose\":true}}"
     }],
     "updatecheck":{
     "status":"ok",
     "urls":{"url":[{"codebase":"http://example.com/"},
                    {"codebasediff":"http://diff.example.com/"}]},
     "manifest":{
      "version":"1.2.3.4",
      "prodversionmin":"2.0.143.0",
      "run":"UpdaterSetup.exe",
      "arguments":"--arg1 --arg2",
      "packages":{"package":[{"name":"extension_1_2_3_4.crx"}]}}
     }
    }
   ]
  }
```

update_client provides this response data back to the Updater.

The updater client writes this data to a temporary file in the same directory as
the application installer. This is for security reasons, since writing the data
to the temp directory could potentially allow a man-in-the-middle attack.

The updater client provides the temporary file as a parameter to the application
installer.

Let's say, as shown above, that the update server responds with these example
file contents:
```
{"logging":{"verbose":true}}
```

The updater client will now create a temporary file, say `c:\my
path\temporaryfile.dat` (assuming the application installer is running from
`c:\my path\YesExe.exe`), with the following file contents:
```
\xEF\xBB\xBF{"logging":{"verbose":true}}
```

and then provide the file as a parameter to the application installer:
```
"c:\my path\YesExe.exe" --installerdata="c:\my path\temporaryfile.dat"
```

* Notice above that the temp file contents are prefixed with an UTF-8 Byte Order
Mark of `EF BB BF`.
* For MSI installers, a property will passed to the installer:
`INSTALLERDATA="pathtofile"`.
* For exe-based installers, as shown above, a command line parameter will be
passed to the installer: `--installerdata="pathtofile"`.
* For Mac installers, an environment variable will be set:
`INSTALLERDATA="pathtofile"`.
* Ownership of the temp file is the responsibility of the application installer.
The updater will not delete this file.
* This installerdata is not persisted anywhere else, and it is not sent as a
part of pings to the update server.


### Locking
On Windows, the global prefs lock is implemented as a kernel mutex.

### Application Commands

The Application Command feature allows installed Updater-managed applications to
pre-register and then later run command lines. This provides applications in a
system-wide installation a mechanism to silently elevate functionality. The
command lines can also include replaceable parameters substituted at runtime.

For legacy reasons:
* the interface is called `IAppCommandWeb` because it was meant to be used from
an ActiveX control on a webpage at one point, and
* it is derived from `IDispatch` for the same reason, to be used from script.

For more information, please see the
[functional spec](functional_spec.md#Application-Commands).

### Existence Checking
On Windows, programs create a key at
`(HKLM|HKCU)\\Software\\Company\\Update\\Clients\\AppID` during installation.
They remove this key when their uninstallers are run. If the key has been
removed, the updater considers the application to have been uninstalled.

### Periodic Task Scheduling
Windows utilizes the Task Scheduler and the Windows Task Scheduler API to create
a scheduler.

### Legacy State
TODO(crbug.com/1035895): Document usage of O3 registry on Windows.

### COM on Windows

The COM client and server code are implemented in terms of the Windows Runtime
C++ (WRL) library provided by Windows SDK.

For system installs, the updater installs a COM service. The COM service is
started by the SCM when CoCreate is called on one of several CLSIDs that the
service supports. This is used as:
* The Server for the UI when installing Machine applications.
* The On-Demand COM Server for Machine applications.
* COM Server for launching processes at System Integrity, as an Elevator. See
[process launcher](functional_spec.md#process-launcher) and
[application commands](functional_spec.md#application-commands-applicable-to-the-windows-version-of-the-updater)
in the functional spec.

#### COM Security

The legacy COM classes in
[updater_legacy_idl.template](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/app/server/win/updater_legacy_idl.template)
such as on-demand, application commands, and process launcher, allow non-admin
callers because the interfaces expose functionality that non-admin callers need
to use. These interfaces therefore only provide restricted functionality.

The new COM classes in
[updater_internal_idl.template](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/app/server/win/updater_internal_idl.template)
and
[updater_idl.template](https://source.chromium.org/chromium/chromium/src/+/main:chrome/updater/app/server/win/updater_idl.template)
require the callers to be admin (enforced in `RuntimeClassInitialize()`). This
is because the corresponding interfaces allow for unrestricted functionality,
such as installing any app that the updater supports. For non-admins, COM
creation will fail with E_ACCESSDENIED.

#### COM System Service
The updater installs an `internal` SxS system service for the new version
alongside the existing `internal` service for the current version. This internal
service is named in the following format:

{ProductName}{InternalService/Service}{UpdaterVersion}.
For instance: ChromiumUpdaterInternalService92.0.0.1.

The internal service supports the SxS COM interfaces, but not the common COM
interfaces. The latter interfaces are hosted only by the active version in a
service that is named in the format ChromiumUpdaterService91.0.0.1.

Once the new version is deemed operational, it will install the new active
service, ChromiumUpdaterService92.0.0.1, and this service will take over the
common COM interfaces. The old version will now uninstall itself, including its
internal and active services.

Service installation uses the `InstallServiceWorkItem` for installing and
rolling back the service.

### Network
On Windows, the updater uses WinHTTP to implement the network.

## macOS-Specific Notes

### Installer
On macOS, the updater is most commonly bundled within an application's .app
bundle and installed on the first run of the application.

### Locking
On macOS, the global prefs lock is implemented using `bootstrap_check_in()`,
interpreting ownership of receive rights on a Mach service name as ownership of
a lock.

### Existence Checking
On POSIX, the most common means of uninstalling a program is to delete the
program's application bundle from disk. When a program registers itself with
the updater, it provides the path to the application bundle. If the bundle has
been removed (or is owned by a user different from the updater), the updater
considers it uninstalled and ceases attempting to update it.

### Periodic Task Scheduling
On Mac, the scheduler is implemented via LaunchAgents (for user-level installs)
and LaunchDaemons (for system-level installs). The scheduled task is defined by
the `org.chromium.ChromiumUpdater.wake.1.2.3.4.plist`, which contains a Label
corresponding to the name of the plist, program arguments, which contains the
path to the executable and the arguments it'll run with, and a StartInterval,
which denotes interval for when launchctl should invoke the program. An example:

```
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>AbandonProcessGroup</key>
	<true/>
	<key>Label</key>
	<string>org.chromium.ChromiumUpdater.wake.1.2.3.4</string>
	<key>LimitLoadToSessionType</key>
	<string>Aqua</string>
	<key>ProgramArguments</key>
	<array>
		<string>/Users/user/Library/Chromium/ChromiumUpdater/1.2.3.4/ChromiumUpdater.app/Contents/MacOS/ChromiumUpdater</string>
		<string>--wake</string>
		<string>--vmodule=*/chrome/updater/*=2,*/components/update_client/*=2</string>
		<string>--enable-logging</string>
	</array>
	<key>StartInterval</key>
	<integer>3600</integer>
</dict>
</plist>
```

### Legacy State
TODO(crbug.com/1035895): Document usage of Keystone tickets on macOS.

### IPC
On Mac, the IPC utilized for the updater is XPC, which is the macOS native IPC
system controlled by launchd. The main portion to utilize XPC is to create
launch agent plists under `${HOME}/Library/LaunchAgents` for user level installs
and launch daemon plists under `/Library/LaunchDaemons` for system level
installs.

The updater uses multiple layers of XPC to start-up separate processes
that are responsible for different actions. There are two XPC plists that the
updater creates for the server. The first is the `.internal` plist, which goes
through some control tasks first, and then there's the `.service` plist, which
actually performs the update check and the update itself.

The XPC interface is also utilized to communicate between the browser and the
updater for on-demand updates. The protocol utilized by the browser has to be in
sync with the updater's protocol, or else the XPC call will fail. The same
`.service` launchd plist is utilized to communicate between the browser and the
updater for the on-demand updates.

Lastly, the XPC interface connects the browser and updater to promote the
updater to a system-level updater. Promotion in this context means that the
browser will install a system-level updater. This process entails the browser
showing UI to the user on the About Page to prompt the user to promote. When the
user accepts, the browser will then ask the user to elevate to allow root
privileges so that a system-level updater can be installed. Inside of the
browser, there exists a Privileged Helper tool executable. This is installed
from the browser if promotion is selected via SMJobBless. When the Privileged
Helper tool is installed, the browser can make an XPC connection to it and
invoke a call to start the system-level updater installation process.

### Network
On macOS, the updater uses NSURLSession to implement the network.