fix(sessions): honor zero recent events in database service

[White-Mouse]

Summary

• honor GetSessionConfig(num_recent_events=0) in DatabaseSessionService
• add cross-backend regression coverage for zero recent events in test_get_session_with_config
• keep existing positive-limit and no-config behavior unchanged

Motivation

GetSessionConfig documents num_recent_events=0 as returning no events. InMemory and Sqlite already preserve that behavior, and ADK call sites use num_recent_events=0 when they only need to check whether a session exists. DatabaseSessionService used a truthy check, so 0 skipped the SQL LIMIT and returned the full event history.

Testing

• uv run --extra test pytest tests/unittests/sessions/test_session_service.py::test_get_session_with_config -q
• uv run --extra test pytest tests/unittests/sessions -q
• uvx pyink --check src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py
• uv run python -m py_compile src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py
• git diff --check

[boyangsvl]

/adk-pr-analyze

[adk-bot]

🔍 ADK Pull Request Analysis: PR #5965

Title: fix(sessions): honor zero recent events in database service
Author: @White-Mouse
Status: open
Impact: 8 additions, 1 deletions across 2 files

Executive Summary

1. Core Objective: Fixes DatabaseSessionService to properly honor GetSessionConfig(num_recent_events=0) by returning an empty event history instead of skipping the limit and returning the full session history.
2. Justification & Value: Justified Fix - Crucial for database efficiency and consistency across all session service backends (matching InMemory and Sqlite services) when client code only seeks to check if a session exists without loading messages.
3. Alignment with Principles: Pass - Restores correct contract representation, adheres to zero vs None semantic standards, contains proper target typing, and incorporates high-integrity unit tests.
4. Recommendation: Approve

Detailed Findings & Analysis

1. Objectives & Impact ("What does it do?")

• Context & Background: In ADK, session fetching can be parameterized with a GetSessionConfig context. When checking if a session exists without wanting to pull the historical stream, clients request num_recent_events=0.
• Implementation Mechanism: Previously, database_session_service.py used a truthy check if config and config.num_recent_events:. Because 0 is falsy under Python's default truthy evaluation, the SQL pagination limit stmt = stmt.limit(...) was bypassed, loading and returning the entire event history. The PR switches this to if config and config.num_recent_events is not None:, correctly compiling a LIMIT 0 statement so that zero events are loaded.
• Affected Surface: This update has zero breaking changes to public APIs. It fixes internal SQL pagination boundaries in database_session_service.py.

2. Justification & Value ("Is it a valid and useful change?")

• Workspace Verification:
  • Investigated current workspace files: database_session_service.py.
  • Found that the baseline condition did indeed perform a truthy check on config.num_recent_events (which mistakenly interpreted 0 as "no limit").
• Value Assessment: Highly valuable. This prevents wasteful memory allocation and excessive queries in large-scale production applications using database session services when session existence verification is performed.
• Alternative Approaches: The chosen approach (is not None) is the cleanest and most standard way to differentiate between an unconfigured limit (None) and a zero-bounded limit (0).
• Scope & Depth: Systematic Fix & Root Cause. It directly corrects the core logical gap where truthiness was used in place of explicit type bounds.

3. Principle & Style Alignment Checklist ("Does it follow rules?")

• Public API & Visibility Boundaries:
  • Status: Pass
  • Analysis: No packages or exposed APIs are modified structurally. The public behavior aligns precisely with documented expectations.
• Code Quality, Typing & Conventions:
  • Status: Pass
  • Analysis: Leverages existing annotations (from __future__ import annotations is correctly present in database_session_service.py). It avoids any typing anti-patterns, keeping the changes highly focused and readable.
• Robustness & Edge Cases:
  • Status: Pass
  • Analysis: Perfectly discriminates between falsy values (0) and unprovided optional filters (None), preventing unexpected fallbacks.
• Test Integrity & Quality:
  • Status: Pass
  • Analysis: Incorporates strong verification coverage in test_session_service.py by asserting that num_recent_events=0 yields an empty list of events. Tests follow explicit Arrange-Act-Assert formatting.

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Phase Summary & Verification Results

• Google CLA Verification: PASS (Status SUCCESS confirmed via status checks rollup API).
• GitHub Workflow Status: PASS (All check runs, including agent-triage-pull-request and formatting/security scanning, succeeded).
• Recommendation: Fully recommended for immediate approval