Improve async support for Cursors #4102
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| parameters: Int, | ||
| binders: (SqlPreparedStatement.() -> Unit)?, | ||
| ) = execute(identifier, { AndroidQuery(sql, database, parameters) }, binders) { executeQuery(mapper) } | ||
| ) = execute(identifier, { AndroidQuery(sql, database, parameters) }, binders) { executeQuery(mapper) }.value |
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Could we return: QueryResult.Value here? (Only use GitHub Web, so maybe this is already the case).
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Yeah, I made a mistake here. The executeQuery() call in the trailing lambda there expected a (SqlCursor) -> R and returned R so the whole thing was evaluating into a QueryResult<QueryResult<T>> which isn't right.
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I hope we don't need to add transformers like map and flatMap to fix QueryResult<QueryResult<T>>...
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Looks good, gonna wait to approve until tests go green because they might uncover some bigger change that needs to be made |
| val first = cursor.next() | ||
| val result = mutableListOf<T>() | ||
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| // If the cursor isn't async, we want to preserve the blocking semantics and execute it synchronously | ||
| if (first is QueryResult.AsyncValue) { | ||
| QueryResult.AsyncValue { | ||
| if (first.await()) result.add(mapper(cursor)) else return@AsyncValue result | ||
| while (cursor.next().value) result.add(mapper(cursor)) | ||
| result | ||
| } | ||
| } else { | ||
| if (first.value) result.add(mapper(cursor)) else return@execute QueryResult.Value(result) | ||
| while (cursor.next().value) result.add(mapper(cursor)) | ||
| QueryResult.Value(result) |
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This change deserves extra attention because of how... special it is.
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merged cause green, I'm surprised this was needed, I assumed calling await on a QueryResult.Value would just block anyway and do the same thing as .value
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Explaining it here for posterity:
AsyncValues are evaluated lazily so that the getter block can be run in the consumer's coroutine scope. Because of this, when awaitAsList() (or awaitAsOne...()) is called, the actual work in the AsyncValue block is deferred until the final .await() is called, which happens after the call to execute() has completed. So by then, the driver may have closed the connection, and there will be no results in the cursor.
Basically the order of calls before:
awaitAs*()execute()mapper()cursor.next()(synchronously called in the mapper)onClose()(driver closes the connection)await()(returns the synchronous value)
and after:
awaitAs*()execute()mapper()(returns anAsyncValuewhich will be lazily evaluated)onClose()(driver closes the connection)await()(evaluates theAsyncValue)cursor.next()(connection has already been closed)
With drivers that implement async cursors, it's up to the driver to ensure that the connection (or whatever the underlying driver needs) stays alive until the coroutine is completed.
| val next = cursor.next() | ||
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| // If the cursor isn't async, we want to preserve the blocking semantics and execute it synchronously | ||
| if (next is QueryResult.AsyncValue) { | ||
| QueryResult.AsyncValue { | ||
| if (!next.await()) return@AsyncValue null | ||
| val value = mapper(cursor) | ||
| check(!cursor.next().value) { "ResultSet returned more than 1 row for $this" } | ||
| value | ||
| } | ||
| } else { | ||
| if (!next.value) return@execute QueryResult.Value(null) | ||
| val value = mapper(cursor) | ||
| check(!cursor.next().await()) { "ResultSet returned more than 1 row for $this" } | ||
| value | ||
| check(!cursor.next().value) { "ResultSet returned more than 1 row for $this" } | ||
| QueryResult.Value(value) |
...ync-extensions/src/commonMain/kotlin/app/cash/sqldelight/async/coroutines/QueryExtensions.kt
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...ync-extensions/src/commonMain/kotlin/app/cash/sqldelight/async/coroutines/QueryExtensions.kt
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SqlCursor.next()now returnsQueryResult<Boolean>which can be anAsyncValuefor async drivers.I'll follow up with changes to the R2DBC driver that uses this new API.