zeus
95 lines
4.0 KiB
Swift
95 lines
4.0 KiB
Swift
// swiftlint:disable:next line_length
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#if SQLITE_ENABLE_SNAPSHOT || (!GRDBCUSTOMSQLITE && !GRDBCIPHER && (compiler(>=5.7.1) || !(os(macOS) || targetEnvironment(macCatalyst))))
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/// An instance of WALSnapshot records the state of a WAL mode database for some
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/// specific point in history.
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///
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/// We use `WALSnapshot` to help `ValueObservation` check for changes
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/// that would happen between the initial fetch, and the start of the
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/// actual observation. This class has no other purpose, and is not intended to
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/// become public.
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///
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/// It does not work with SQLCipher, because SQLCipher does not support
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/// `SQLITE_ENABLE_SNAPSHOT` correctly: we have linker errors.
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/// See <https://github.com/ericsink/SQLitePCL.raw/issues/452>.
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///
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/// With custom SQLite builds, it only works if `SQLITE_ENABLE_SNAPSHOT`
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/// is defined.
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///
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/// With system SQLite, it can only work when the SDK exposes the C apis and
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/// their availability, which means XCode 14 (identified with Swift 5.7).
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///
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/// Yes, this is an awfully complex logic.
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///
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/// See <https://www.sqlite.org/c3ref/snapshot.html>.
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final class WALSnapshot: @unchecked Sendable {
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// @unchecked because sqlite3_snapshot has no threading requirements.
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// <https://www.sqlite.org/c3ref/snapshot.html>
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let sqliteSnapshot: UnsafeMutablePointer<sqlite3_snapshot>
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init(_ db: Database) throws {
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var sqliteSnapshot: UnsafeMutablePointer<sqlite3_snapshot>?
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let code = withUnsafeMutablePointer(to: &sqliteSnapshot) {
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return sqlite3_snapshot_get(db.sqliteConnection, "main", $0)
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}
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guard code == SQLITE_OK else {
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// <https://www.sqlite.org/c3ref/snapshot_get.html>
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//
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// > The following must be true for sqlite3_snapshot_get() to succeed. [...]
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// >
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// > 1. The database handle must not be in autocommit mode.
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// > 2. Schema S of database connection D must be a WAL
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// > mode database.
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// > 3. There must not be a write transaction open on schema S
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// > of database connection D.
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// > 4. One or more transactions must have been written to the
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// > current wal file since it was created on disk (by any
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// > connection). This means that a snapshot cannot be taken
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// > on a wal mode database with no wal file immediately
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// > after it is first opened. At least one transaction must
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// > be written to it first.
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// Test condition 1:
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if sqlite3_get_autocommit(db.sqliteConnection) != 0 {
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throw DatabaseError(resultCode: code, message: """
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Can't create snapshot because database is in autocommit mode.
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""")
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}
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// Test condition 2:
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if let journalMode = try? String.fetchOne(db, sql: "PRAGMA journal_mode"),
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journalMode != "wal"
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{
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throw DatabaseError(resultCode: code, message: """
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Can't create snapshot because database is not in WAL mode.
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""")
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}
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// Condition 3 can't happen because GRDB only calls this
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// initializer from read transactions.
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//
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// Hence it is condition 4 that is false:
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throw DatabaseError(resultCode: code, message: """
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Can't create snapshot from a missing or empty wal file.
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""")
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}
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guard let sqliteSnapshot else {
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throw DatabaseError(resultCode: .SQLITE_INTERNAL) // WTF SQLite?
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}
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self.sqliteSnapshot = sqliteSnapshot
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}
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deinit {
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sqlite3_snapshot_free(sqliteSnapshot)
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}
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/// Compares two WAL snapshots.
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///
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/// `a.compare(b) < 0` iff a is older than b.
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///
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/// See <https://www.sqlite.org/c3ref/snapshot_cmp.html>.
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func compare(_ other: WALSnapshot) -> CInt {
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sqlite3_snapshot_cmp(sqliteSnapshot, other.sqliteSnapshot)
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}
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}
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#endif
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