Explain synchronization in CDM's PersistentDataStore (cad2f559) · Commits · e / os / android_frameworks_base

services/companion/java/com/android/server/companion/PersistentDataStore.java

+39 −21

Original line number	Diff line number	Diff line
		@@ -67,17 +67,20 @@ import java.util.concurrent.ConcurrentMap;
		* The class responsible for persisting Association records and other related information (such as
		* previously used IDs) to a disk, and reading the data back from the disk.
		*
		* Before Android T the data was stored to `companion_device_manager_associations.xml` file in
		* {@link Environment#getUserSystemDirectory(int)}
		* (eg. `/data/system/users/0/companion_device_manager_associations.xml`)
		* @see #getBaseLegacyStorageFileForUser(int)
		* <p>
		* Before Android T the data was stored in "companion_device_manager_associations.xml" file in
		* {@link Environment#getUserSystemDirectory(int) /data/system/user/}.
		*
		* Before Android T the data was stored using the v0 schema.
		* See {@link #getBaseLegacyStorageFileForUser(int) getBaseLegacyStorageFileForUser()}.
		*
		* @see #readAssociationsV0(TypedXmlPullParser, int, Collection)
		* @see #readAssociationV0(TypedXmlPullParser, int, int, Collection)
		* <p>
		* Before Android T the data was stored using the v0 schema. See:
		* <ul>
		* <li>{@link #readAssociationsV0(TypedXmlPullParser, int, Collection) readAssociationsV0()}.
		* <li>{@link #readAssociationV0(TypedXmlPullParser, int, int, Collection) readAssociationV0()}.
		* </ul>
		*
		* The following snippet is a sample of a the file that is using v0 schema.
		* The following snippet is a sample of a file that is using v0 schema.
		* <pre>{@code
		* <associations>
		* <association
		@@ -93,24 +96,28 @@ import java.util.concurrent.ConcurrentMap;
		* </associations>
		* }</pre>
		*
		* <p>
		* Since Android T the data is stored to "companion_device_manager.xml" file in
		* {@link Environment#getDataSystemDeDirectory(int) /data/system_de/}.
		*
		* Since Android T the data is stored to `companion_device_manager.xml` file in
		* {@link Environment#getDataSystemDeDirectory(int)}.
		* (eg. `/data/system_de/0/companion_device_manager.xml`)
		* @see #getBaseStorageFileForUser(int)

		* See {@link #getBaseStorageFileForUser(int) getBaseStorageFileForUser()}
		*
		* <p>
		* Since Android T the data is stored using the v1 schema.
		* In the v1 schema, a list of the previously used IDs is storead along with the association
		*
		* In the v1 schema, a list of the previously used IDs is stored along with the association
		* records.
		* V1 schema adds a new optional `display_name` attribute, and makes the `mac_address` attribute
		* optional.
		*
		* @see #CURRENT_PERSISTENCE_VERSION
		* @see #readAssociationsV1(TypedXmlPullParser, int, Collection)
		* @see #readAssociationV1(TypedXmlPullParser, int, Collection)
		* @see #readPreviouslyUsedIdsV1(TypedXmlPullParser, Map)
		* V1 schema adds a new optional "display_name" attribute, and makes the "mac_address" attribute
		* optional.
		* <ul>
		* <li> {@link #CURRENT_PERSISTENCE_VERSION}
		* <li> {@link #readAssociationsV1(TypedXmlPullParser, int, Collection) readAssociationsV1()}
		* <li> {@link #readAssociationV1(TypedXmlPullParser, int, Collection) readAssociationV1()}
		* <li> {@link #readPreviouslyUsedIdsV1(TypedXmlPullParser, Map) readPreviouslyUsedIdsV1()}
		* </ul>
		*
		* The following snippet is a sample of a the file that is using v0 schema.
		* The following snippet is a sample of a file that is using v0 schema.
		* <pre>{@code
		* <state persistence-version="1">
		* <associations>
		@@ -206,6 +213,8 @@ final class PersistentDataStore {
		final AtomicFile file = getStorageFileForUser(userId);
		if (DEBUG) Slog.d(LOG_TAG, " > File=" + file.getBaseFile().getPath());

		// getStorageFileForUser() ALWAYS returns the SAME OBJECT, which allows us to synchronize
		// accesses to the file on the file system using this AtomicFile object.
		synchronized (file) {
		File legacyBaseFile = null;
		final AtomicFile readFrom;
		@@ -269,6 +278,8 @@ final class PersistentDataStore {

		final AtomicFile file = getStorageFileForUser(userId);
		if (DEBUG) Slog.d(LOG_TAG, " > File=" + file.getBaseFile().getPath());
		// getStorageFileForUser() ALWAYS returns the SAME OBJECT, which allows us to synchronize
		// accesses to the file on the file system using this AtomicFile object.
		synchronized (file) {
		persistStateToFileLocked(file, associations, previouslyUsedIdsPerPackage);
		}
		@@ -329,6 +340,13 @@ final class PersistentDataStore {
		});
		}

		/**
		* Creates and caches {@link AtomicFile} object that represents the back-up file for the given
		* user.
		*
		* IMPORTANT: the method will ALWAYS return the same {@link AtomicFile} object, which makes it
		* possible to synchronize reads and writes to the file using the returned object.
		*/
		private @NonNull AtomicFile getStorageFileForUser(@UserIdInt int userId) {
		return mUserIdToStorageFile.computeIfAbsent(userId,
		u -> new AtomicFile(getBaseStorageFileForUser(userId)));