Add .github/copilot-instructions.md for AI-assisted development

.github/copilot-instructions.md

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+# Copilot Instructions for sonic-dbsyncd
+
+## Project Overview
+
+sonic-dbsyncd provides Python-based database synchronization daemons for SONiC. The primary component is LLDP-SyncD, which syncs LLDP (Link Layer Discovery Protocol) data from the lldpd daemon into the SONiC Redis database. The repo also provides a generic `sonic_syncd` interface for building additional database sync daemons that bridge external data sources into SONiC's centralized Redis state.
+
+## Architecture
+
+```
+sonic-dbsyncd/
+├── src/                          # Source code root
+│   ├── __init__.py
+│   ├── lldp_syncd/               # LLDP database sync daemon
+│   │   ├── __init__.py
+│   │   ├── __main__.py           # Entry point for lldp_syncd
+│   │   ├── daemon.py             # Main daemon loop
+│   │   ├── main.py               # Startup logic
+│   │   └── conventions.py        # LLDP data formatting conventions
+│   └── sonic_syncd/              # Generic sync daemon interface
+│       ├── __init__.py
+│       └── interface.py          # Base interface for sync daemons
+├── tests/                        # Unit/integration tests
+├── setup.py                      # Python package setup
+├── README.md
+└── .github/                      # CI workflows
+```
+
+### Key Concepts
+- **SyncD pattern**: A daemon that subscribes to an external data source and writes structured data into SONiC's Redis database
+- **LLDP-SyncD**: Reads LLDP neighbor information from `lldpd` (via lldpctl) and populates `APPL_DB` LLDP tables
+- **Generic interface**: `sonic_syncd.interface` provides a base class that new sync daemons can implement
+- **Redis DB integration**: Uses `swsscommon` Python bindings to write to SONiC databases
+
+## Language & Style
+
+- **Primary language**: Python 3
+- **Style**: PEP 8 compliant
+- **Indentation**: 4 spaces
+- **Naming conventions**:
+  - Classes: `PascalCase` (e.g., `LldpSyncDaemon`)
+  - Functions/methods: `snake_case` (e.g., `sync_data`)
+  - Constants: `UPPER_SNAKE_CASE`
+  - Modules: `snake_case`
+- **Docstrings**: Use Python docstrings for public classes and methods
+- **Imports**: Standard library first, then third-party, then local
+
+## Build Instructions
+
+```bash
+# Install Python dependencies
+pip3 install -e .
+
+# Or build Debian package
+dpkg-buildpackage -us -uc -b
+
+# The package installs:
+#   - lldp_syncd module
+#   - sonic_syncd module
+```
+
+## Testing
+
+```bash
+# Run tests
+cd tests
+python3 -m pytest -v
+
+# Or run individual test files
+python3 -m pytest tests/test_lldp_syncd.py -v
+```
+
+### Test Structure
+- Tests are in the `tests/` directory
+- Use `pytest` framework
+- May require mock `swsscommon` module for unit testing without Redis
+
+## PR Guidelines
+
+- **Signed-off-by**: REQUIRED on all commits (`git commit -s`)
+- **CLA**: Sign the Linux Foundation EasyCLA
+- **Single commit per PR**: Squash commits before merge
+- **PEP 8**: Code must pass linting (flake8 or similar)
+- **Tests**: Add or update tests for any behavioral changes
+- **Reference issues**: Link related GitHub issues in PR description
+
+## Dependencies
+
+- **sonic-swss-common**: Python bindings (`swsscommon`) for Redis DB access
+- **lldpd**: External LLDP daemon whose data is consumed by lldp_syncd
+- **sonic-py-swsssdk**: SONiC Python SDK for database connectivity
+- **Redis**: Backend database for SONiC operational state
+
+## Gotchas
+
+- **lldpctl dependency**: LLDP-SyncD relies on `lldpctl` (lldpd CLI tool) being installed and accessible
+- **swsscommon import**: The `swsscommon` Python module is built from `sonic-swss-common` and must be installed in the Python path
+- **Daemon lifecycle**: The sync daemon runs as a systemd service; handle signals (SIGTERM) gracefully
+- **Data format**: LLDP data formatting in `conventions.py` must match what consumers (SNMP, CLI) expect
+- **Testing without hardware**: Unit tests must mock external dependencies (lldpctl, swsscommon)
+- **Python version**: Targets Python 3; do not use Python 2 constructs
+- **Entry points**: `lldp_syncd` is run as a module (`python3 -m lldp_syncd`)
+- **DB schema changes**: Any change to Redis table schema must be coordinated with consumers (e.g., `sonic-snmpagent`, `sonic-utilities`)