propagate connector upload errors

Author: stevensJourney

Sources/PowerSync/Kotlin/wrapQueryCursor.swift

@@ -60,3 +60,26 @@ func wrapQueryCursorTyped<RowType, ReturnType>(
         resultType
     )
 }
+
+
+/// Throws a `PowerSyncException` using a helper provided by the Kotlin SDK.
+/// We can't directly throw Kotlin `PowerSyncException`s from Swift, but we can delegate the throwing
+/// to the Kotlin implementation.
+/// Our Kotlin SDK methods handle thrown Kotlin `PowerSyncException` correctly.
+/// The flow of events is as follows
+///     Swift code calls `throwKotlinPowerSyncError`
+///          This method calls the Kotlin helper `throwPowerSyncException` which is annotated as being able to throw `PowerSyncException`
+///             The Kotlin helper throws the provided `PowerSyncException`. Since the method is annotated the exception propagates back to Swift, but in a form which can propagate back
+///             to any calling Kotlin stack.
+/// This only works for SKIEE methods which have an associated completion handler which handles annotated errors.
+/// This seems to only apply for Kotlin suspending function bindings.
+func throwKotlinPowerSyncError (message: String, cause: String? = nil) throws {
+    try throwPowerSyncException(
+        exception: PowerSyncKotlin.PowerSyncException(
+            message: message,
+            cause: PowerSyncKotlin.KotlinThrowable(
+                message: cause ?? message
+            )
+        )
+    )
+}

Sources/PowerSync/kotlin/PowerSyncBackendConnectorAdapter.swift

@@ -12,23 +12,31 @@ internal class PowerSyncBackendConnectorAdapter: KotlinPowerSyncBackendConnector
         self.swiftBackendConnector = swiftBackendConnector
         self.db = db
     }
-
+    
     override func __fetchCredentials() async throws -> KotlinPowerSyncCredentials? {
         do {
             let result = try await swiftBackendConnector.fetchCredentials()
             return result?.kotlinCredentials
         } catch {
             db.logger.error("Error while fetching credentials", tag: logTag)
+            /// We can't use throwKotlinPowerSyncError here since the Kotlin connector 
+            /// runs this in a Job - this seems to break the SKIEE error propagation.
+            /// returning nil here should still cause a retry
             return nil
         }
     }
 
     override func __uploadData(database: KotlinPowerSyncDatabase) async throws {
         do {
             // Pass the Swift DB protocal to the connector
-            return  try await swiftBackendConnector.uploadData(database: db)
+            return try await swiftBackendConnector.uploadData(database: db)
         } catch {
             db.logger.error("Error while uploading data: \(error)", tag: logTag)
+            // Relay the error to the Kotlin SDK
+            try throwKotlinPowerSyncError(
+                message: "Connector errored while uploading data: \(error.localizedDescription)",
+                cause: error.localizedDescription,
+            )
         }
     }
 }