fix: capacitor iOS Buffer read error

[stevensJourney]

This is an alternative solution to the one provided in #905.

Overview

Users have reported errors of the form

Run: failed(message: "Error in reading buffer")

When using the PowerSync Capacitor SDK to connect to a PowerSync service.

Background

The PowerSync SDK sync stream client sends binary payloads from the PowerSync service to the PowerSync Rust core by making SQL queries of the form

execute('SELECT powersync_control(?, ?)', ['line-binary', payload]))

The RSocket client we use to connect to the PowerSync service requires a polyfill for Buffer which is provided and bundled in the @powersync/common package. This results in the payload above being an instance of the polyfilled Buffer.

The Capacitor Community SQLite lib accepts buffer payloads as a dictionary mapping of numerical array indexes to numerical byte values.

/// Parameter parsing logic in the `run`/`execute` method.
                        } else if let obj = value as? [String: Any] {
                            if var keys = Array(obj.keys) as? [String] {
                                if #available(iOS 15.0, *) {
                                    keys.sort(using: .localizedStandard)
                                    var valuesArr: [UInt8] = []
                                    for key in keys {
                                        if let mVal = obj[key] {
                                            if let iVal = mVal as? Int {
                                                valuesArr.append(UInt8(iVal))
                                            } else {
                                                let msg: String = "Error in reading buffer"
                                                throw CapacitorSQLiteError.failed(message: msg)
                                            }
                                        } else {
                                            let msg: String = "Error in reading buffer"
                                            throw CapacitorSQLiteError.failed(message: msg)
                                        }
                                    }
                                    val.append(valuesArr)
                                } else {
                                    let msg: String = "Error buffer sorted not implemented"
                                    throw CapacitorSQLiteError.failed(message: msg)

                                }
                            }

The above parsing logic will fail with a Error in reading buffer error if either:

• a value associated with a key cannot be converted to an Int type
• a value associated with a key is nil

The Buffer polyfil in @powersync/common defines a few additional fields in it's implementation, which are passed through the native bridge to the Swift SQLite layer. These additional parameters don't meet the above parsing requirements, which cause the error above.

For example: in the polyfil, we see an parent member being declared.

		Object.defineProperty(Buffer.prototype, 'parent', {
		  enumerable: true,
		  get: function () {
		    if (!Buffer.isBuffer(this)) return undefined
		    return this.buffer
		  }
		});

When debugging the Swift parsing logic, we can see this value propagates from JS to Swift

Notice the value for key is parent which has a value which could not be cast to a numerical Int value

The Fix

Only the Buffer polyfil contains these incompatible member values. We can create a Uint8Array view over the Buffer and pass that to the SQLite lib instead. This should have minimal overhead.

Testing

For some reason, I have not been able to reproduce this issue previously... On the current main branch with an updated capacitor package from 7.4.x to 7.6.x I have been able to reproduce the issue.

I've confirmed manually that the example-capacitor demo connects and syncs correctly to a PowerSync service after applying these changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: 152ea89

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@powersync/capacitor	Patch

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[rkistner]

Out of interest, is it RSocket that uses the Buffer polyfill? It would be great if it would use plain Uint8Array natively, but that's probably not easy to change. Looks like there is an open issue for that, but hasn't seen much activity lately: rsocket/rsocket-js#273

Regardless of that, the Capacitor Uint8Array handling is likely still quite slow, but using a native Uint8Array is a step in the right direction at least.

[stevensJourney]

Out of interest, is it RSocket that uses the Buffer polyfill? It would be great if it would use plain Uint8Array natively, but that's probably not easy to change. Looks like there is an open issue for that, but hasn't seen much activity lately: rsocket/rsocket-js#273

Yeah that is the case. We might be able to override Buffer to Uint8Array in the bundling process - but I'm not sure what kinds of trouble that could cause.

Regardless of that, the Capacitor Uint8Array handling is likely still quite slow, but using a native Uint8Array is a step in the right direction at least.

Indeed 😄