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zeus
2025-01-22 14:09:10 +08:00
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iOS-SwiftUI-Code/GRDB.swift-6.29.3/GRDB/Core/DatabasePool.swift
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import Dispatch
import Foundation
#if os(iOS)
import UIKit
#endif
public final class DatabasePool {
private let writer: SerializedDatabase
/// The pool of reader connections.
/// It is constant, until close() sets it to nil.
private var readerPool: Pool<SerializedDatabase>?
@LockedBox var databaseSnapshotCount = 0
/// If Database Suspension is enabled, this array contains the necessary `NotificationCenter` observers.
private var suspensionObservers: [NSObjectProtocol] = []
// MARK: - Database Information
public var configuration: Configuration {
writer.configuration
}
/// The path to the database.
public var path: String {
writer.path
}
// MARK: - Initializer
/// Opens or creates an SQLite database.
///
/// For example:
///
/// ```swift
/// let dbPool = try DatabasePool(path: "/path/to/database.sqlite")
/// ```
///
/// The SQLite connections are closed when the database pool
/// gets deallocated.
///
/// - parameters:
/// - path: The path to the database file.
/// - configuration: A configuration.
/// - throws: A ``DatabaseError`` whenever an SQLite error occurs.
public init(path: String, configuration: Configuration = Configuration()) throws {
GRDBPrecondition(configuration.maximumReaderCount > 0, "configuration.maximumReaderCount must be at least 1")
// Writer
writer = try SerializedDatabase(
path: path,
configuration: configuration,
defaultLabel: "GRDB.DatabasePool",
purpose: "writer")
// Readers
var readerConfiguration = DatabasePool.readerConfiguration(configuration)
// Readers can't allow dangling transactions because there's no
// guarantee that one can get the same reader later in order to close
// an opened transaction.
readerConfiguration.allowsUnsafeTransactions = false
var readerCount = 0
readerPool = Pool(
maximumCount: configuration.maximumReaderCount,
qos: configuration.readQoS,
makeElement: {
readerCount += 1 // protected by Pool (TODO: document this protection behavior)
return try SerializedDatabase(
path: path,
configuration: readerConfiguration,
defaultLabel: "GRDB.DatabasePool",
purpose: "reader.\(readerCount)")
})
// Set up journal mode unless readonly
if !configuration.readonly {
switch configuration.journalMode {
case .default, .wal:
try writer.sync {
try $0.setUpWALMode()
}
}
}
setupSuspension()
// Be a nice iOS citizen, and don't consume too much memory
// See https://github.com/groue/GRDB.swift/#memory-management
#if os(iOS)
if configuration.automaticMemoryManagement {
setupMemoryManagement()
}
#endif
}
deinit {
// Remove block-based Notification observers.
suspensionObservers.forEach(NotificationCenter.default.removeObserver(_:))
// Undo job done in setupMemoryManagement()
//
// https://developer.apple.com/library/mac/releasenotes/Foundation/RN-Foundation/index.html#10_11Error
// Explicit unregistration is required before macOS 10.11.
NotificationCenter.default.removeObserver(self)
// Close reader connections before the writer connection.
// Context: https://github.com/groue/GRDB.swift/issues/739
readerPool = nil
}
/// Returns a Configuration suitable for readonly connections on a
/// WAL database.
private static func readerConfiguration(_ configuration: Configuration) -> Configuration {
var configuration = configuration
configuration.readonly = true
// Readers use deferred transactions by default.
// Other transaction kinds are forbidden by SQLite in read-only connections.
configuration.defaultTransactionKind = .deferred
// <https://www.sqlite.org/wal.html#sometimes_queries_return_sqlite_busy_in_wal_mode>
// > But there are some obscure cases where a query against a WAL-mode
// > database can return SQLITE_BUSY, so applications should be prepared
// > for that happenstance.
// >
// > - If another database connection has the database mode open in
// > exclusive locking mode [...]
// > - When the last connection to a particular database is closing,
// > that connection will acquire an exclusive lock for a short time
// > while it cleans up the WAL and shared-memory files [...]
// > - If the last connection to a database crashed, then the first new
// > connection to open the database will start a recovery process. An
// > exclusive lock is held during recovery. [...]
//
// The whole point of WAL readers is to avoid SQLITE_BUSY, so let's
// setup a busy handler for pool readers, in order to workaround those
// "obscure cases" that may happen when the database is shared between
// multiple processes.
if configuration.readonlyBusyMode == nil {
configuration.readonlyBusyMode = .timeout(10)
}
return configuration
}
}
// @unchecked because of databaseSnapshotCount, readerPool and suspensionObservers
extension DatabasePool: @unchecked Sendable { }
extension DatabasePool {
// MARK: - Memory management
/// Frees as much memory as possible, by disposing non-essential memory.
///
/// This method is synchronous, and blocks the current thread until all
/// database accesses are completed.
///
/// This method closes all read-only connections, unless the
/// ``Configuration/persistentReadOnlyConnections`` configuration flag
/// is set.
///
/// - warning: This method can prevent concurrent reads from executing,
/// until it returns. Prefer ``releaseMemoryEventually()`` if you intend
/// to keep on using the database while releasing memory.
public func releaseMemory() {
// Release writer memory
writer.sync { $0.releaseMemory() }
if configuration.persistentReadOnlyConnections {
// Keep existing readers
readerPool?.forEach { reader in
reader.sync { $0.releaseMemory() }
}
} else {
// Release readers memory by closing all connections.
//
// We must use a barrier in order to guarantee that memory has been
// freed (reader connections closed) when the method exits, as
// documented.
//
// Without the barrier, connections would only close _eventually_ (after
// their eventual concurrent jobs have completed).
readerPool?.barrier {
readerPool?.removeAll()
}
}
}
/// Eventually frees as much memory as possible, by disposing
/// non-essential memory.
///
/// This method eventually closes all read-only connections, unless the
/// ``Configuration/persistentReadOnlyConnections`` configuration flag
/// is set.
///
/// Unlike ``releaseMemory()``, this method does not prevent concurrent
/// database accesses when it is executing. But it does not notify when
/// non-essential memory has been freed.
public func releaseMemoryEventually() {
if configuration.persistentReadOnlyConnections {
// Keep existing readers
readerPool?.forEach { reader in
reader.async { $0.releaseMemory() }
}
} else {
// Release readers memory by eventually closing all reader connections
// (they will close after their current jobs have completed).
readerPool?.removeAll()
}
// Release writer memory eventually.
writer.async { $0.releaseMemory() }
}
#if os(iOS)
/// Listens to UIApplicationDidEnterBackgroundNotification and
/// UIApplicationDidReceiveMemoryWarningNotification in order to release
/// as much memory as possible.
private func setupMemoryManagement() {
let center = NotificationCenter.default
center.addObserver(
self,
selector: #selector(DatabasePool.applicationDidReceiveMemoryWarning(_:)),
name: UIApplication.didReceiveMemoryWarningNotification,
object: nil)
center.addObserver(
self,
selector: #selector(DatabasePool.applicationDidEnterBackground(_:)),
name: UIApplication.didEnterBackgroundNotification,
object: nil)
}
@objc
private func applicationDidEnterBackground(_ notification: NSNotification) {
guard let application = notification.object as? UIApplication else {
return
}
let task: UIBackgroundTaskIdentifier = application.beginBackgroundTask(expirationHandler: nil)
if task == .invalid {
// Release memory synchronously
releaseMemory()
} else {
// Release memory eventually.
//
// We don't know when reader connections will be closed (because
// they may be currently in use), so we don't quite know when
// reader memory will be freed (which would be the ideal timing for
// ending our background task).
//
// So let's just end the background task after the writer connection
// has freed its memory. That's better than nothing.
releaseMemoryEventually()
writer.async { _ in
application.endBackgroundTask(task)
}
}
}
@objc
private func applicationDidReceiveMemoryWarning(_ notification: NSNotification) {
releaseMemoryEventually()
}
#endif
}
extension DatabasePool: DatabaseReader {
public func close() throws {
try readerPool?.barrier {
// Close writer connection first. If we can't close it,
// don't close readers.
//
// This allows us to exit this method as fully closed (read and
// writes fail), or not closed at all (reads and writes succeed).
//
// Unfortunately, this introduces a regression for
// https://github.com/groue/GRDB.swift/issues/739.
// TODO: fix this regression.
try writer.sync { try $0.close() }
// OK writer is closed. Now close readers and
// eventually prevent any future read access
defer { readerPool = nil }
try readerPool?.forEach { reader in
try reader.sync { try $0.close() }
}
}
}
// MARK: - Interrupting Database Operations
public func interrupt() {
writer.interrupt()
readerPool?.forEach { $0.interrupt() }
}
// MARK: - Database Suspension
func suspend() {
if configuration.readonly {
// read-only WAL connections can't acquire locks and do not need to
// be suspended.
return
}
writer.suspend()
}
func resume() {
if configuration.readonly {
// read-only WAL connections can't acquire locks and do not need to
// be suspended.
return
}
writer.resume()
}
private func setupSuspension() {
if configuration.observesSuspensionNotifications {
let center = NotificationCenter.default
suspensionObservers.append(center.addObserver(
forName: Database.suspendNotification,
object: nil,
queue: nil,
using: { [weak self] _ in self?.suspend() }
))
suspensionObservers.append(center.addObserver(
forName: Database.resumeNotification,
object: nil,
queue: nil,
using: { [weak self] _ in self?.resume() }
))
}
}
// MARK: - Reading from Database
@_disfavoredOverload // SR-15150 Async overloading in protocol implementation fails
public func read<T>(_ value: (Database) throws -> T) throws -> T {
GRDBPrecondition(currentReader == nil, "Database methods are not reentrant.")
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
return try readerPool.get { reader in
try reader.sync { db in
try db.isolated {
try db.clearSchemaCacheIfNeeded()
return try value(db)
}
}
}
}
public func asyncRead(_ value: @escaping (Result<Database, Error>) -> Void) {
guard let readerPool else {
value(.failure(DatabaseError.connectionIsClosed()))
return
}
readerPool.asyncGet { result in
do {
let (reader, releaseReader) = try result.get()
// Second async jump because that's how `Pool.async` has to be used.
reader.async { db in
defer {
try? db.commit() // Ignore commit error
releaseReader(.reuse)
}
do {
// The block isolation comes from the DEFERRED transaction.
try db.beginTransaction(.deferred)
try db.clearSchemaCacheIfNeeded()
value(.success(db))
} catch {
value(.failure(error))
}
}
} catch {
value(.failure(error))
}
}
}
@_disfavoredOverload // SR-15150 Async overloading in protocol implementation fails
public func unsafeRead<T>(_ value: (Database) throws -> T) throws -> T {
GRDBPrecondition(currentReader == nil, "Database methods are not reentrant.")
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
return try readerPool.get { reader in
try reader.sync { db in
try db.clearSchemaCacheIfNeeded()
return try value(db)
}
}
}
public func asyncUnsafeRead(_ value: @escaping (Result<Database, Error>) -> Void) {
guard let readerPool else {
value(.failure(DatabaseError.connectionIsClosed()))
return
}
readerPool.asyncGet { result in
do {
let (reader, releaseReader) = try result.get()
// Second async jump because that's how `Pool.async` has to be used.
reader.async { db in
defer {
releaseReader(.reuse)
}
do {
try db.clearSchemaCacheIfNeeded()
value(.success(db))
} catch {
value(.failure(error))
}
}
} catch {
value(.failure(error))
}
}
}
public func unsafeReentrantRead<T>(_ value: (Database) throws -> T) throws -> T {
if let reader = currentReader {
return try reader.reentrantSync(value)
} else if writer.onValidQueue {
return try writer.execute(value)
} else {
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
return try readerPool.get { reader in
try reader.sync { db in
try db.clearSchemaCacheIfNeeded()
return try value(db)
}
}
}
}
public func concurrentRead<T>(_ value: @escaping (Database) throws -> T) -> DatabaseFuture<T> {
// The semaphore that blocks until futureResult is defined:
let futureSemaphore = DispatchSemaphore(value: 0)
var futureResult: Result<T, Error>? = nil
asyncConcurrentRead { dbResult in
// Fetch and release the future
futureResult = dbResult.flatMap { db in Result { try value(db) } }
futureSemaphore.signal()
}
return DatabaseFuture {
// Block the future until results are fetched
_ = futureSemaphore.wait(timeout: .distantFuture)
return try futureResult!.get()
}
}
public func spawnConcurrentRead(_ value: @escaping (Result<Database, Error>) -> Void) {
asyncConcurrentRead(value)
}
/// Performs an asynchronous read access.
///
/// This method must be called from the writer dispatch queue, outside of
/// any transaction. You'll get a fatal error otherwise.
///
/// The `value` function is guaranteed to see the database in the last
/// committed state at the moment this method is called. Eventual
/// concurrent database updates are not visible from the function.
///
/// This method returns as soon as the isolation guarantee described above
/// has been established.
///
/// In the example below, the number of players is fetched concurrently with
/// the player insertion. Yet it is guaranteed to be zero:
///
/// ```swift
/// try writer.asyncWriteWithoutTransaction { db in
/// // Delete all players
/// try Player.deleteAll()
///
/// // Count players concurrently
/// writer.asyncConcurrentRead { dbResult in
/// do {
/// let db = try dbResult.get()
/// // Guaranteed to be zero
/// let count = try Player.fetchCount(db)
/// } catch {
/// // Handle error
/// }
/// }
///
/// // Insert a player
/// try Player(...).insert(db)
/// }
/// ```
///
/// - parameter value: A function that accesses the database.
public func asyncConcurrentRead(_ value: @escaping (Result<Database, Error>) -> Void) {
// Check that we're on the writer queue...
writer.execute { db in
// ... and that no transaction is opened.
GRDBPrecondition(!db.isInsideTransaction, """
must not be called from inside a transaction. \
If this error is raised from a DatabasePool.write block, use \
DatabasePool.writeWithoutTransaction instead (and use \
transactions when needed).
""")
}
// The semaphore that blocks the writing dispatch queue until snapshot
// isolation has been established:
let isolationSemaphore = DispatchSemaphore(value: 0)
do {
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
let (reader, releaseReader) = try readerPool.get()
reader.async { db in
defer {
try? db.commit() // Ignore commit error
releaseReader(.reuse)
}
do {
// https://www.sqlite.org/isolation.html
//
// > In WAL mode, SQLite exhibits "snapshot isolation". When
// > a read transaction starts, that reader continues to see
// > an unchanging "snapshot" of the database file as it
// > existed at the moment in time when the read transaction
// > started. Any write transactions that commit while the
// > read transaction is active are still invisible to the
// > read transaction, because the reader is seeing a
// > snapshot of database file from a prior moment in time.
//
// That's exactly what we need. But what does "when read
// transaction starts" mean?
//
// http://www.sqlite.org/lang_transaction.html
//
// > Deferred [transaction] means that no locks are acquired
// > on the database until the database is first accessed.
// > [...] Locks are not acquired until the first read or
// > write operation. [...] Because the acquisition of locks
// > is deferred until they are needed, it is possible that
// > another thread or process could create a separate
// > transaction and write to the database after the BEGIN
// > on the current thread has executed.
//
// Now that's precise enough: SQLite defers snapshot
// isolation until the first SELECT:
//
// Reader Writer
// BEGIN DEFERRED TRANSACTION
// UPDATE ... (1)
// Here the change (1) is visible from the reader
// SELECT ...
// UPDATE ... (2)
// Here the change (2) is not visible from the reader
//
// We thus have to perform a select that establishes the
// snapshot isolation before we release the writer queue:
//
// Reader Writer
// BEGIN DEFERRED TRANSACTION
// SELECT anything
// UPDATE ... (1)
// Here the change (1) is not visible from the reader
//
// Since any select goes, use `PRAGMA schema_version`.
try db.beginTransaction(.deferred)
try db.clearSchemaCacheIfNeeded()
} catch {
isolationSemaphore.signal()
value(.failure(error))
return
}
// Now that we have an isolated snapshot of the last commit, we
// can release the writer queue.
isolationSemaphore.signal()
value(.success(db))
}
} catch {
isolationSemaphore.signal()
value(.failure(error))
}
// Block the writer queue until snapshot isolation success or error
_ = isolationSemaphore.wait(timeout: .distantFuture)
}
/// Invalidates open read-only SQLite connections.
///
/// After this method is called, read-only database access methods will use
/// new SQLite connections.
///
/// Eventual concurrent read-only accesses are not interrupted, and
/// proceed until completion.
///
/// - This method closes all read-only connections, even if the
/// ``Configuration/persistentReadOnlyConnections`` configuration flag
/// is set.
public func invalidateReadOnlyConnections() {
readerPool?.removeAll()
}
/// Returns a reader that can be used from the current dispatch queue,
/// if any.
private var currentReader: SerializedDatabase? {
guard let readerPool else {
return nil
}
var readers: [SerializedDatabase] = []
readerPool.forEach { reader in
// We can't check for reader.onValidQueue here because
// Pool.forEach() runs its closure argument in some arbitrary
// dispatch queue. We thus extract the reader so that we can query
// it below.
readers.append(reader)
}
// Now the readers array contains some readers. The pool readers may
// already be different, because some other thread may have started
// a new read, for example.
//
// This doesn't matter: the reader we are looking for is already on
// its own dispatch queue. If it exists, is still in use, thus still
// in the pool, and thus still relevant for our check:
return readers.first { $0.onValidQueue }
}
// MARK: - WAL Snapshot Transactions
// swiftlint:disable:next line_length
#if SQLITE_ENABLE_SNAPSHOT || (!GRDBCUSTOMSQLITE && !GRDBCIPHER && (compiler(>=5.7.1) || !(os(macOS) || targetEnvironment(macCatalyst))))
/// Returns a long-lived WAL snapshot transaction on a reader connection.
func walSnapshotTransaction() throws -> WALSnapshotTransaction {
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
let (reader, releaseReader) = try readerPool.get()
return try WALSnapshotTransaction(onReader: reader, release: { isInsideTransaction in
// Discard the connection if the transaction could not be
// properly ended. If we'd reuse it, the next read would
// fail because we'd fail starting a read transaction.
releaseReader(isInsideTransaction ? .discard : .reuse)
})
}
/// Returns a long-lived WAL snapshot transaction on a reader connection.
///
/// - important: The `completion` argument is executed in a serial
/// dispatch queue, so make sure you use the transaction asynchronously.
func asyncWALSnapshotTransaction(_ completion: @escaping (Result<WALSnapshotTransaction, Error>) -> Void) {
guard let readerPool else {
completion(.failure(DatabaseError.connectionIsClosed()))
return
}
readerPool.asyncGet { result in
completion(result.flatMap { reader, releaseReader in
Result {
try WALSnapshotTransaction(onReader: reader, release: { isInsideTransaction in
// Discard the connection if the transaction could not be
// properly ended. If we'd reuse it, the next read would
// fail because we'd fail starting a read transaction.
releaseReader(isInsideTransaction ? .discard : .reuse)
})
}
})
}
}
#endif
// MARK: - Database Observation
public func _add<Reducer: ValueReducer>(
observation: ValueObservation<Reducer>,
scheduling scheduler: some ValueObservationScheduler,
onChange: @escaping (Reducer.Value) -> Void)
-> AnyDatabaseCancellable
{
if configuration.readonly {
// The easy case: the database does not change
return _addReadOnly(
observation: observation,
scheduling: scheduler,
onChange: onChange)
} else if observation.requiresWriteAccess {
// Observe from the writer database connection.
return _addWriteOnly(
observation: observation,
scheduling: scheduler,
onChange: onChange)
} else {
// DatabasePool can perform concurrent observation
return _addConcurrent(
observation: observation,
scheduling: scheduler,
onChange: onChange)
}
}
/// A concurrent observation fetches the initial value without waiting for
/// the writer.
private func _addConcurrent<Reducer: ValueReducer>(
observation: ValueObservation<Reducer>,
scheduling scheduler: some ValueObservationScheduler,
onChange: @escaping (Reducer.Value) -> Void)
-> AnyDatabaseCancellable
{
assert(!configuration.readonly, "Use _addReadOnly(observation:) instead")
assert(!observation.requiresWriteAccess, "Use _addWriteOnly(observation:) instead")
let observer = ValueConcurrentObserver(
dbPool: self,
scheduler: scheduler,
trackingMode: observation.trackingMode,
reducer: observation.makeReducer(),
events: observation.events,
onChange: onChange)
return observer.start()
}
}
extension DatabasePool: DatabaseWriter {
// MARK: - Writing in Database
@_disfavoredOverload // SR-15150 Async overloading in protocol implementation fails
public func writeWithoutTransaction<T>(_ updates: (Database) throws -> T) rethrows -> T {
try writer.sync(updates)
}
@_disfavoredOverload // SR-15150 Async overloading in protocol implementation fails
public func barrierWriteWithoutTransaction<T>(_ updates: (Database) throws -> T) throws -> T {
guard let readerPool else {
throw DatabaseError.connectionIsClosed()
}
return try readerPool.barrier {
try writer.sync(updates)
}
}
public func asyncBarrierWriteWithoutTransaction(_ updates: @escaping (Result<Database, Error>) -> Void) {
guard let readerPool else {
updates(.failure(DatabaseError.connectionIsClosed()))
return
}
readerPool.asyncBarrier {
self.writer.sync { updates(.success($0)) }
}
}
/// Wraps database operations inside a database transaction.
///
/// The `updates` function runs in the writer dispatch queue, serialized
/// with all database updates.
///
/// If `updates` throws an error, the transaction is rollbacked and the
/// error is rethrown. If it returns
/// ``Database/TransactionCompletion/rollback``, the transaction is also
/// rollbacked, but no error is thrown.
///
/// For example:
///
/// ```swift
/// try dbPool.writeInTransaction { db in
/// try Player(name: "Arthur").insert(db)
/// try Player(name: "Barbara").insert(db)
/// return .commit
/// }
/// ```
///
/// - precondition: This method is not reentrant.
/// - parameters:
/// - kind: The transaction type (default nil). If nil, the transaction
/// type is the ``Configuration/defaultTransactionKind`` of the
/// the ``configuration``.
/// - updates: A function that updates the database.
/// - throws: The error thrown by `updates`, or by the wrapping transaction.
public func writeInTransaction(
_ kind: Database.TransactionKind? = nil,
_ updates: (Database) throws -> Database.TransactionCompletion)
throws
{
try writer.sync { db in
try db.inTransaction(kind) {
try updates(db)
}
}
}
public func unsafeReentrantWrite<T>(_ updates: (Database) throws -> T) rethrows -> T {
try writer.reentrantSync(updates)
}
public func asyncWriteWithoutTransaction(_ updates: @escaping (Database) -> Void) {
writer.async(updates)
}
}
extension DatabasePool {
// MARK: - Snapshots
/// Creates a database snapshot that serializes accesses to an unchanging
/// database content, as it exists at the moment the snapshot is created.
///
/// It is a programmer error to create a snapshot from the writer dispatch
/// queue when a transaction is opened:
///
/// ```swift
/// try dbPool.write { db in
/// try Player.deleteAll()
///
/// // fatal error: makeSnapshot() must not be called from inside a transaction
/// let snapshot = try dbPool.makeSnapshot()
/// }
/// ```
///
/// To avoid this fatal error, create the snapshot *before* or *after*
/// the transaction:
///
/// ```swift
/// let snapshot = try dbPool.makeSnapshot() // OK
///
/// try dbPool.writeWithoutTransaction { db in
/// let snapshot = try dbPool.makeSnapshot() // OK
///
/// try db.inTransaction {
/// try Player.deleteAll()
/// return .commit
/// }
///
/// // OK
/// let snapshot = try dbPool.makeSnapshot() // OK
/// }
///
/// let snapshot = try dbPool.makeSnapshot() // OK
/// ```
public func makeSnapshot() throws -> DatabaseSnapshot {
// Sanity check
if writer.onValidQueue {
writer.execute { db in
GRDBPrecondition(
!db.isInsideTransaction,
"makeSnapshot() must not be called from inside a transaction.")
}
}
return try DatabaseSnapshot(
path: path,
configuration: DatabasePool.readerConfiguration(writer.configuration),
defaultLabel: "GRDB.DatabasePool",
purpose: "snapshot.\($databaseSnapshotCount.increment())")
}
// swiftlint:disable:next line_length
#if SQLITE_ENABLE_SNAPSHOT || (!GRDBCUSTOMSQLITE && !GRDBCIPHER && (compiler(>=5.7.1) || !(os(macOS) || targetEnvironment(macCatalyst))))
/// Creates a database snapshot that allows concurrent accesses to an
/// unchanging database content, as it exists at the moment the snapshot
/// is created.
///
/// - note: [**🔥 EXPERIMENTAL**](https://github.com/groue/GRDB.swift/blob/master/README.md#what-are-experimental-features)
///
/// A ``DatabaseError`` of code `SQLITE_ERROR` is thrown if the SQLite
/// database is not in the [WAL mode](https://www.sqlite.org/wal.html),
/// or if this method is called from a write transaction, or if the
/// wal file is missing or truncated (size zero).
///
/// Related SQLite documentation: <https://www.sqlite.org/c3ref/snapshot_get.html>
public func makeSnapshotPool() throws -> DatabaseSnapshotPool {
try unsafeReentrantRead { db in
try DatabaseSnapshotPool(db)
}
}
#endif
}