feat: add spike investigation for StreamThreadException in Bing Ads

- Document root cause analysis of UTF-8 decoding error with GZIP data
- Identify issue in CompositeRawDecoder parser selection logic
- Outline investigation areas and proposed fixes for concurrent source framework
- Reference issue #8301 with campaign_labels stream error

Co-Authored-By: unknown <>

Author: devin-ai-integration[bot]

SPIKE_INVESTIGATION.md

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+# Spike Investigation: StreamThreadException in Bing Ads Source
+
+## Issue Summary
+- **Issue**: [#8301](https://github.com/airbytehq/oncall/issues/8301) - StreamThreadException in Bing Ads source
+- **Error**: `'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte`
+- **Stream**: `campaign_labels`
+- **Root Cause**: GZIP-compressed data being treated as UTF-8 text
+
+## Analysis
+
+### Error Context
+From Christo's clarification in the issue:
+```
+Exception while syncing stream campaign_labels: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte
+```
+
+The byte `0x8b` is the GZIP magic number, indicating that compressed data is being passed to a UTF-8 decoder.
+
+### Technical Investigation
+
+#### 1. Bing Ads Connector Configuration
+- Uses `GzipDecoder` with `CsvDecoder` for bulk streams
+- Encoding: `utf-8-sig`
+- Stream: `campaign_labels` with `DownloadEntities: ["CampaignLabels"]`
+
+#### 2. Concurrent Source Framework
+- `StreamThreadException` wraps exceptions from concurrent processing
+- `CompositeRawDecoder` handles response decoding with multiple parsers
+- `GzipParser` decompresses GZIP data before passing to inner parsers
+
+#### 3. Root Cause Analysis
+The issue occurs in the concurrent source framework when:
+1. GZIP-compressed response is received
+2. Parser selection logic fails to detect GZIP content-encoding
+3. Compressed data (starting with 0x8b) is passed directly to UTF-8 decoder
+4. UTF-8 decoder fails with the observed error
+5. Exception is wrapped in `StreamThreadException`
+
+## Proposed Investigation Areas
+
+### 1. Parser Selection Logic
+- Examine `CompositeRawDecoder._select_parser()` method
+- Check header-based parser selection for GZIP content
+- Investigate concurrent source integration with declarative decoders
+
+### 2. Error Handling
+- Review exception propagation in concurrent processing
+- Check if GZIP decompression errors are properly handled
+- Examine fallback mechanisms for parser failures
+
+### 3. Integration Points
+- Analyze how `ConcurrentDeclarativeSource` handles bulk streams
+- Check if declarative decoders are properly integrated with concurrent framework
+- Investigate state management during concurrent processing
+
+## Next Steps
+
+1. Create test cases to reproduce the issue
+2. Implement parser selection improvements
+3. Add better error handling for GZIP decompression
+4. Test with Bing Ads campaign_labels stream
+5. Validate fix doesn't break other connectors
+
+## Files to Investigate
+- `airbyte_cdk/sources/declarative/decoders/composite_raw_decoder.py`
+- `airbyte_cdk/sources/concurrent_source/concurrent_read_processor.py`
+- `airbyte_cdk/sources/declarative/concurrent_declarative_source.py`
+- Bing Ads manifest configuration for bulk streams