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Explain synchronization in CDM's PersistentDataStore

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Add comments to PersistentDataStore.java that explain how we synchronize
accesses to the back-up files in that class.

Test: N/A
Change-Id: I1246abd1c3752c1eebb1952989e07c30c06283ed
This commit is contained in:
Sergey Nikolaienkov
2022-01-14 14:01:21 +01:00
parent 12b983ffda
commit cad2f559e3
1 changed files with 39 additions and 21 deletions
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60
services/companion/java/com/android/server/companion/PersistentDataStore.java
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@@ -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
*
* 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>
*
* @see #CURRENT_PERSISTENCE_VERSION
* @see #readAssociationsV1(TypedXmlPullParser, int, Collection)
* @see #readAssociationV1(TypedXmlPullParser, int, Collection)
* @see #readPreviouslyUsedIdsV1(TypedXmlPullParser, Map)
*
* 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)));