PostgresDSN with prefix falls back to non-prefixed env vars by accident

Created by: slimsag

An input to PostgresDSN without a database name, like:

export CODEINSIGHTS_PGDATASOURCE=postgres://postgres:[email protected]:5435

(note how there is no /databasename at the end) will, instead of having an empty "" database name have the value of PGDATABASE - NOT CODEINSIGHTS_PGDATABASE.

This seems to be an affect of having an empty string for the database, and the pgx library falling back to PGDATABASE in that case.

This affects all prefixed DSNs, e.g. CODEINTEL_PGDATASOURCE is also affected and other variants like specifying CODEINTEL_PGUSER but not CODEINTEL_PGDATABASE will cause it to fallback to unprefixed PGDATABASE.

Discovered via https://github.com/sourcegraph/sourcegraph/pull/17738

Workaround: make sure the DSN has all options present, e.g.:

export CODEINSIGHTS_PGDATASOURCE=postgres://postgres:[email protected]:5435/database