Cursor Exceptions and Type Casts (#23)

stevensJourney · web-flow · commit abca0c99adc0 · 2025-02-18T15:05:44.000+02:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## 1.0.0-Beta.7
+
+* Fixed an issue where throwing exceptions in the query `mapper` could cause a runtime crash.
+* Internally improved type casting.
+
 ## 1.0.0-Beta.6
 
 * BREAKING CHANGE: `watch` queries are now throwable and therefore will need to be accompanied by a `try` e.g.
diff --git a/Sources/PowerSync/Kotlin/KotlinPowerSyncDatabaseImpl.swift b/Sources/PowerSync/Kotlin/KotlinPowerSyncDatabaseImpl.swift
@@ -4,16 +4,14 @@ import PowerSyncKotlin
 final class KotlinPowerSyncDatabaseImpl: PowerSyncDatabaseProtocol {
     private let kotlinDatabase: PowerSyncKotlin.PowerSyncDatabase
 
-    var currentStatus: SyncStatus {
-        get { kotlinDatabase.currentStatus }
-    }
+    var currentStatus: SyncStatus { kotlinDatabase.currentStatus }
 
     init(
         schema: Schema,
         dbFilename: String
     ) {
         let factory = PowerSyncKotlin.DatabaseDriverFactory()
-        self.kotlinDatabase = PowerSyncDatabase(
+        kotlinDatabase = PowerSyncDatabase(
             factory: factory,
             schema: KotlinAdapter.Schema.toKotlin(schema),
             dbFilename: dbFilename
@@ -65,85 +63,97 @@ final class KotlinPowerSyncDatabaseImpl: PowerSyncDatabaseProtocol {
     }
 
     func execute(sql: String, parameters: [Any]?) async throws -> Int64 {
-        Int64(truncating: try await kotlinDatabase.execute(sql: sql, parameters: parameters))
+        try Int64(truncating: await kotlinDatabase.execute(sql: sql, parameters: parameters))
     }
 
     func get<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) -> RowType
     ) async throws -> RowType {
-        try await kotlinDatabase.get(
+        try safeCast(await kotlinDatabase.get(
             sql: sql,
             parameters: parameters,
             mapper: mapper
-        ) as! RowType
+        ), to: RowType.self)
     }
 
     func get<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) throws -> RowType
     ) async throws -> RowType {
-        try await kotlinDatabase.get(
-            sql: sql,
-            parameters: parameters,
-            mapper: { cursor in
-                try! mapper(cursor)
-            }
-        ) as! RowType
+        return try await wrapQueryCursorTyped(
+            mapper: mapper,
+            executor: { wrappedMapper in
+                try await self.kotlinDatabase.get(
+                    sql: sql,
+                    parameters: parameters,
+                    mapper: wrappedMapper
+                )
+            },
+            resultType: RowType.self
+        )
     }
 
     func getAll<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) -> RowType
     ) async throws -> [RowType] {
-        try await kotlinDatabase.getAll(
+        try safeCast(await kotlinDatabase.getAll(
             sql: sql,
             parameters: parameters,
             mapper: mapper
-        ) as! [RowType]
+        ), to: [RowType].self)
     }
 
     func getAll<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) throws -> RowType
     ) async throws -> [RowType] {
-        try await kotlinDatabase.getAll(
-            sql: sql,
-            parameters: parameters,
-            mapper: { cursor in
-                try! mapper(cursor)
-            }
-        ) as! [RowType]
+        try await wrapQueryCursorTyped(
+            mapper: mapper,
+            executor: { wrappedMapper in
+                try await self.kotlinDatabase.getAll(
+                    sql: sql,
+                    parameters: parameters,
+                    mapper: wrappedMapper
+                )
+            },
+            resultType: [RowType].self
+        )
     }
 
     func getOptional<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) -> RowType
     ) async throws -> RowType? {
-        try await kotlinDatabase.getOptional(
+        try safeCast(await kotlinDatabase.getOptional(
             sql: sql,
             parameters: parameters,
             mapper: mapper
-        ) as! RowType?
+        ), to: RowType?.self)
     }
 
     func getOptional<RowType>(
         sql: String,
         parameters: [Any]?,
         mapper: @escaping (SqlCursor) throws -> RowType
     ) async throws -> RowType? {
-        try await kotlinDatabase.getOptional(
-            sql: sql,
-            parameters: parameters,
-            mapper: { cursor in
-                try! mapper(cursor)
-            }
-        ) as! RowType?
+        try await wrapQueryCursorTyped(
+            mapper: mapper,
+            executor: { wrappedMapper in
+                try await self.kotlinDatabase.getOptional(
+                    sql: sql,
+                    parameters: parameters,
+                    mapper: wrappedMapper
+                )
+            },
+            resultType: RowType?.self
+        )
     }
 
     func watch<RowType>(
@@ -159,7 +169,7 @@ final class KotlinPowerSyncDatabaseImpl: PowerSyncDatabaseProtocol {
                         parameters: parameters,
                         mapper: mapper
                     ) {
-                        continuation.yield(values as! [RowType])
+                        try continuation.yield(safeCast(values, to: [RowType].self))
                     }
                     continuation.finish()
                 } catch {
@@ -177,14 +187,23 @@ final class KotlinPowerSyncDatabaseImpl: PowerSyncDatabaseProtocol {
         AsyncThrowingStream { continuation in
             Task {
                 do {
-                    for await values in try self.kotlinDatabase.watch(
+                    var mapperError: Error?
+                    for try await values in try self.kotlinDatabase.watch(
                         sql: sql,
                         parameters: parameters,
-                        mapper: { cursor in
-                            try! mapper(cursor)
-                        }
+                        mapper: { cursor in do {
+                            return try mapper(cursor)
+                        } catch {
+                            mapperError = error
+                            // The value here does not matter. We will throw the exception later
+                            // This is not ideal, this is only a workaround until we expose fine grained access to Kotlin SDK internals.
+                            return nil as RowType?
+                        } }
                     ) {
-                        continuation.yield(values as! [RowType])
+                        if mapperError != nil {
+                            throw mapperError!
+                        }
+                        try continuation.yield(safeCast(values, to: [RowType].self))
                     }
                     continuation.finish()
                 } catch {
@@ -195,33 +214,11 @@ final class KotlinPowerSyncDatabaseImpl: PowerSyncDatabaseProtocol {
     }
 
     public func writeTransaction<R>(callback: @escaping (any PowerSyncTransaction) throws -> R) async throws -> R {
-        return try await kotlinDatabase.writeTransaction(callback: TransactionCallback(callback: callback)) as! R
+        return try safeCast(await kotlinDatabase.writeTransaction(callback: TransactionCallback(callback: callback)), to: R.self)
     }
 
     public func readTransaction<R>(callback: @escaping (any PowerSyncTransaction) throws -> R) async throws -> R {
-        return try await kotlinDatabase.readTransaction(callback: TransactionCallback(callback: callback)) as! R
-    }
-}
-
-class TransactionCallback<R>: PowerSyncKotlin.ThrowableTransactionCallback {
-    let callback: (PowerSyncTransaction) throws -> R
-
-    init(callback: @escaping (PowerSyncTransaction) throws -> R) {
-        self.callback = callback
-    }
-
-    func execute(transaction: PowerSyncKotlin.PowerSyncTransaction) throws -> Any{
-        do {
-            return try callback(transaction)
-        } catch let error {
-            return PowerSyncKotlin.PowerSyncException(
-                message: error.localizedDescription,
-                cause: PowerSyncKotlin.KotlinThrowable(message: error.localizedDescription)
-            )
-        }
+        return try safeCast(await kotlinDatabase.readTransaction(callback: TransactionCallback(callback: callback)), to: R.self)
     }
 }
 
-enum PowerSyncError: Error {
-    case invalidTransaction
-}
diff --git a/Sources/PowerSync/Kotlin/SafeCastError.swift b/Sources/PowerSync/Kotlin/SafeCastError.swift
@@ -0,0 +1,19 @@
+enum SafeCastError: Error, CustomStringConvertible {
+    case typeMismatch(expected: Any.Type, actual: Any?)
+
+    var description: String {
+        switch self {
+        case let .typeMismatch(expected, actual):
+            let actualType = actual.map { String(describing: type(of: $0)) } ?? "nil"
+            return "Type mismatch: Expected \(expected), but got \(actualType)."
+        }
+    }
+}
+
+internal func safeCast<T>(_ value: Any?, to type: T.Type) throws -> T {
+    if let castedValue = value as? T {
+        return castedValue
+    } else {
+        throw SafeCastError.typeMismatch(expected: type, actual: value)
+    }
+}
diff --git a/Sources/PowerSync/Kotlin/TransactionCallback.swift b/Sources/PowerSync/Kotlin/TransactionCallback.swift
@@ -0,0 +1,35 @@
+import PowerSyncKotlin
+
+class TransactionCallback<R>: PowerSyncKotlin.ThrowableTransactionCallback {
+    let callback: (PowerSyncTransaction) throws -> R
+
+    init(callback: @escaping (PowerSyncTransaction) throws -> R) {
+        self.callback = callback
+    }
+
+    // The Kotlin SDK does not gracefully handle exceptions thrown from Swift callbacks.
+    // If a Swift callback throws an exception, it results in a `BAD ACCESS` crash.
+    //
+    // To prevent this, we catch the exception and return it as a `PowerSyncException`,
+    // allowing Kotlin to propagate the error correctly.
+    //
+    // This approach is a workaround. Ideally, we should introduce an internal mechanism
+    // in the Kotlin SDK to handle errors from Swift more robustly.
+    //
+    // Currently, we wrap the public `PowerSyncDatabase` class in Kotlin, which limits our
+    // ability to handle exceptions cleanly. Instead, we should expose an internal implementation
+    // from a "core" package in Kotlin that provides better control over exception handling
+    // and other functionality—without modifying the public `PowerSyncDatabase` API to include
+    // Swift-specific logic.
+    func execute(transaction: PowerSyncKotlin.PowerSyncTransaction) throws -> Any {
+        do {
+            return try callback(transaction)
+        } catch {
+            return PowerSyncKotlin.PowerSyncException(
+                message: error.localizedDescription,
+                cause: PowerSyncKotlin.KotlinThrowable(message: error.localizedDescription)
+            )
+        }
+    }
+}
+
diff --git a/Sources/PowerSync/Kotlin/wrapQueryCursor.swift b/Sources/PowerSync/Kotlin/wrapQueryCursor.swift
@@ -0,0 +1,51 @@
+
+// The Kotlin SDK does not gracefully handle exceptions thrown from Swift callbacks.
+// If a Swift callback throws an exception, it results in a `BAD ACCESS` crash.
+//
+// This approach is a workaround. Ideally, we should introduce an internal mechanism
+// in the Kotlin SDK to handle errors from Swift more robustly.
+//
+// This hoists any exceptions thrown in a cursor mapper in order for the error to propagate correctly.
+//
+// Currently, we wrap the public `PowerSyncDatabase` class in Kotlin, which limits our
+// ability to handle exceptions cleanly. Instead, we should expose an internal implementation
+// from a "core" package in Kotlin that provides better control over exception handling
+// and other functionality—without modifying the public `PowerSyncDatabase` API to include
+// Swift-specific logic.
+internal func wrapQueryCursor<RowType, ReturnType>(
+    mapper: @escaping (SqlCursor) throws -> RowType,
+    //    The Kotlin APIs return the results as Any, we can explicitly cast internally
+    executor: @escaping (_ wrappedMapper: @escaping (SqlCursor) -> RowType?) async throws -> ReturnType
+) async throws -> ReturnType {
+    var mapperException: Error?
+
+    // Wrapped version of the mapper that catches exceptions and sets `mapperException`
+    // In the case of an exception this will return an empty result.
+    let wrappedMapper: (SqlCursor) -> RowType? = { cursor in
+        do {
+            return try mapper(cursor)
+        } catch {
+            // Store the error in order to propagate it
+            mapperException = error
+            // Return nothing here. Kotlin should handle this as an empty object/row
+            return nil
+        }
+    }
+
+    let executionResult = try await executor(wrappedMapper)
+    if mapperException != nil {
+        //        Allow propagating the error
+        throw mapperException!
+    }
+
+    return executionResult
+}
+
+internal func wrapQueryCursorTyped<RowType, ReturnType>(
+    mapper: @escaping (SqlCursor) throws -> RowType,
+    //    The Kotlin APIs return the results as Any, we can explicitly cast internally
+    executor: @escaping (_ wrappedMapper: @escaping (SqlCursor) -> RowType?) async throws -> Any?,
+    resultType: ReturnType.Type
+) async throws -> ReturnType {
+    return try safeCast(await wrapQueryCursor(mapper: mapper, executor: executor), to: resultType)
+}
diff --git a/Tests/PowerSyncTests/Kotlin/KotlinPowerSyncDatabaseImplTests.swift b/Tests/PowerSyncTests/Kotlin/KotlinPowerSyncDatabaseImplTests.swift
