fix: move timezone string creation before startTimestamp to avoid nesting assertion

[jding-xyz]

Problem

ArrowWriter.writeStreaming() crashes with a FlatBuffers assertion when writing any schema containing a Timestamp field with a timezone (e.g. timestamp[us, UTC]):

FlatBuffers/FlatBufferBuilder.swift:317: Assertion failed: Object serialization must not be nested

In toFBType() (ArrowWriterHelper.swift), the .timestamp case calls fbb.create(string: timezone) inside the startTimestamp/endTimestamp table context. FlatBuffers' create(string:) calls notNested() which asserts !isNested, but startTimestamp (which calls startTable) has already set isNested = true.

Fix

Move the fbb.create(string:) call before startTimestamp(). This is the standard FlatBuffers pattern: all child objects (strings, vectors, tables) must be created before starting their parent table.

// Before (buggy):
let startOffset = org_apache_arrow_flatbuf_Timestamp.startTimestamp(&fbb)
// ...
if let timezone = timestampType.timezone {
    let timezoneOffset = fbb.create(string: timezone) // ASSERTS
}

// After (fixed):
let timezoneOffset = timestampType.timezone.map { fbb.create(string: $0) }
let startOffset = org_apache_arrow_flatbuf_Timestamp.startTimestamp(&fbb)
// ...
if let offset = timezoneOffset {
    org_apache_arrow_flatbuf_Timestamp.add(timezone: offset, &fbb)
}

Testing

Added testTimestampWithTimezoneInMemoryToFromStream which writes a timestamp[us, UTC] column through writeStreaming and reads it back, verifying the schema (unit, timezone) and data survive the roundtrip. This test would crash with the nesting assertion before the fix.

AI disclosure

The code change and test were written with the assistance of Cursor (AI). I identified the bug by tracing the FlatBuffers assertion through the startTable/notNested call chain, reviewed all generated code, and verified correctness by running the test suite locally.

[kou]

Could you also add a new test for the case?

[kou]

Could you read https://arrow.apache.org/docs/dev/developers/overview.html#ai-generated-code carefully?

[jding-xyz]

Thanks for the review. I've pushed a test (testTimestampWithTimezoneInMemoryToFromStream) that does a write/read roundtrip with a timestamp[us, UTC] column. This exercises the exact code path that crashed before the fix.

I've read the AI policy. The fix and test were written with AI assistance (Cursor). I identified the issue when try to use this package. AI helped root cause by tracing FlatBuffers' startTable -> isNested = true -> create(string:) -> notNested() assertion chain, and I've reviewed and verified all generated code. I understand the fix and can debug it.

[kou]

+1