Issue #3248976: Add API documentation for Package Manager

package_manager/package_manager.api.php

+<?php
+
+/**
+ * @file
+ * Documentation related to Package Manager.
+ */
+
+/**
+ * Package Manager is an API-only module which provides the scaffolding and
+ * functionality needed for Drupal to make changes to its own running code base
+ * via Composer. It doesn't provide any user interface.
+ *
+ * At the center of Package Manager is the concept of a staging area. A staging
+ * area is a complete copy of the active Drupal code base, which is created in a
+ * separate location that isn't accessible over the web, such as a temporary
+ * directory. The staging area doesn't include site-specific assets that aren't
+ * managed by Composer, such as settings.php, uploaded files, or SQLite
+ * databases.
+ *
+ * Package Manager can run Composer commands in the staging area to require or
+ * update packages in it, and then copy those changes back into the live,
+ * running code base (which is referred to as the "active directory"). When
+ * that's done, the staging area can be safely deleted. These four distinct
+ * operations -- create, require, apply, and destroy -- comprise the "stage life
+ * cycle".
+ *
+ * Package Manager's PHP API is based on \Drupal\package_manager\Stage, which
+ * controls the stage life cycle. This class may be extended to implement custom
+ * behavior, but in most situations, custom code should use the event system
+ * to interact with the stage.
+ *
+ * Only one staging area can exist at any given time, and it is "owned" by the
+ * user or session that originally created it. Only the owner can perform
+ * operations on the staging area, and only using the same class (i.e.,
+ * \Drupal\package_manager\Stage or a subclass) they used to create it.
+ *
+ * Events are dispatched before and after each operation in the stage life
+ * cycle. There are two types of events: pre-operation and post-operation.
+ * Pre-operation events can analyze the state of the staging area, or the system
+ * as a whole, and flag errors if they detect any problems. If errors are
+ * flagged, the operation is prevented. Therefore, pre-operation events are
+ * helpful to ensure that the staging area is in a valid state. Post-operation
+ * events are simple triggers allowing custom code to react when an operation
+ * is complete. They cannot flag errors that block the stage life cycle from
+ * continuing, although they can use the core messenger and logging systems as
+ * needed.
+ *
+ * All stage events extend \Drupal\package_manager\Event\StageEvent, and all
+ * pre-operation events extend
+ * \Drupal\package_manager\Event\PreOperationStageEvent. All events have a
+ * getStage() method which allows access to the stage object itself.
+ *
+ * The stage dispatches the following events during its life cycle:
+ *
+ * - \Drupal\package_manager\Event\PreCreateEvent
+ *   Dispatched before the staging area is created. At this point, the stage
+ *   will have recorded which user or session owns it, so another staging area
+ *   cannot be created until the current one is destroyed. If subscribers flag
+ *   errors during this event, the stage will release its ownership. This is
+ *   the earliest possible time to detect problems that might prevent the
+ *   stage from completing its life cycle successfully. Only dispatched once
+ *   during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PostCreateEvent
+ *   Dispatched after the staging area is created, which means that all of the
+ *   running Drupal code base has been copied into a separate, temporary
+ *   location. Only dispatched once during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PreRequireEvent
+ *   Dispatched before one or more Composer packages are required into the
+ *   staging area. May be dispatched multiple times during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PostRequireEvent
+ *   Dispatched after one or more Composer packages have been added to the
+ *   staging area. May be dispatched multiple times during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PreApplyEvent
+ *   Dispatched before changes in the staging area (i.e., new or updated
+ *   packages) are copied to the active directory (the running Drupal code
+ *   base). This is the final opportunity for event subscribers to flag errors
+ *   before the active directory is modified. Once the active directory has
+ *   been modified, the changes cannot be undone. May be dispatched multiple
+ *   times during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PostApplyEvent
+ *   Dispatched after changes in the staging area have been copied to the active
+ *   directory. May be dispatched multiple times during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PreDestroyEvent
+ *   Dispatched before the staging area is destroyed, during which the temporary
+ *   directory will be completely deleted and the stage will release its
+ *   ownership. Only dispatched once during a stage's life cycle.
+ *
+ * - \Drupal\package_manager\Event\PostDestroy
+ *   Dispatched after the staging area is destroyed and the stage has released
+ *   its ownership. Only dispatched once during a stage's life cycle.
+ *
+ * The public API of any stage consists of the following methods:
+ *
+ * - \Drupal\package_manager\Stage::create()
+ *   Creates the staging area, records ownership, and dispatches pre- and
+ *   post-create events. Returns a unique token which calling code must use to
+ *   verify stage ownership before performing operations on the staging area
+ *   in subsequent requests (when the staging area is created, its ownership
+ *   is automatically verified for the duration of the current request). See
+ *   \Drupal\package_manager\Stage::claim() for more information.
+ *
+ * - \Drupal\package_manager\Stage::require()
+ *   Adds packages to the staging area and dispatches pre- and post-require
+ *   events.
+ *
+ * - \Drupal\package_manager\Stage::apply()
+ *   Copies changes from the staging area into the active directory, and
+ *   dispatches pre- and post-apply events.
+ *
+ * - \Drupal\package_manager\Stage::destroy()
+ *   Destroys the staging area, releases ownership, and dispatches pre- and
+ *   post-destroy events.
+ *
+ * - \Drupal\package_manager\Stage::getActiveComposer()
+ *   \Drupal\package_manager\Stage::getStageComposer()
+ *   These methods initialize an instance of Composer's API in the active
+ *   directory and staging area, respectively, and return an object which can
+ *   be used by event subscribers to inspect the directory and get relevant
+ *   information from Composer's API, such as what packages are installed and
+ *   where.
+ *
+ * Package Manager automatically enforces certain constraints at various points
+ * of the stage life cycle, to ensure that both the active directory and staging
+ * area are kept in a safe, consistent state:
+ *
+ * - No Composer operation which changes the lock file may be executed in the
+ *   active directory while a staging area exists, either via Package Manager's
+ *   API or externally.
+ * - Composer must use HTTPS to download packages and metadata (i.e., Composer's
+ *   secure-http configuration option must be enabled). This is the default
+ *   behavior.
+ * - The Drupal root, and vendor directory, must be writable.
+ * - A supported version of the Composer executable must be accessible by PHP.
+ *   Its path can be explicitly set in the package_manager.settings config, or
+ *   it will be auto-detected.
+ * - Certain files are never copied into the staging area because they are
+ *   irrelevant to Composer or Package Manager. Examples include settings.php
+ *   and related files, public and private files, SQLite databases, and git
+ *   repositories. Custom code can use
+ *   \Drupal\package_manager\Event\PreCreateEvent::excludePath() to exclude a
+ *   specific path from being copied from the active directory into the staging
+ *   area, or \Drupal\package_manager\Event\PreApplyEvent::excludePath() to
+ *   exclude a specific path from being copied from the staginga area back into
+ *   the active directory.
+ */