fix(data): prevent empty string from overwriting step content on upsert

[giulio-leone]

Problem

When using cl.Step, content within steps occasionally disappears or appears empty when reloading a thread from history. This is caused by a race condition between the initial step.send() and the subsequent step.update().

cl.Step initializes input and output as empty strings (""). In ChainlitDataLayer.create_step, the SQL uses:

output = COALESCE(EXCLUDED.output, "Step".output)

Since empty string is not NULL, COALESCE accepts it as a valid value. If the background task for the initial send() (with empty output) finishes after the final update() (with actual content), the empty string overwrites the content.

Fix

Wrap EXCLUDED.output and EXCLUDED.input with NULLIF(..., ''):

output = COALESCE(NULLIF(EXCLUDED.output, ''), "Step".output)
input = COALESCE(NULLIF(EXCLUDED.input, ''), "Step".input)

This treats empty strings as NULL, so COALESCE falls back to the existing database content.

Test

Added test_create_step_uses_nullif_for_output_and_input that verifies the SQL contains the NULLIF protection.

Fixes #2789

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Summary by cubic

Prevent empty-string input/output from overwriting step content during upsert. Fixes a race where initial Step.send() could erase content saved by Step.update(), so step content persists when reloading threads.

• Bug Fixes
  • Use NULLIF(EXCLUDED.input/output, '') in ON CONFLICT updates so COALESCE keeps existing non-empty values.
  • Add regression test to verify NULLIF is applied to both fields.

^{Written for commit 8095410. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files