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PgBouncer Debugging and Migration

1 post · Aug 2026

What this trail covers

Decagon successfully migrated from GCP's managed connection pooler to self-managed PgBouncer. This migration uncovered a latent bug in PgBouncer related to TLS buffering and packet handling, which caused connection stalls and pool exhaustion. The issue was mitigated by shortening the application name parameter and an upstream fix was contributed to PgBouncer to address the event loop deadlock. The investigation traced the bug through SQLAlchemy, PgBouncer, OpenSSL, and the Linux kernel's epoll interface.

All Decagon trails
  1. How we debugged a latent PgBouncer bug across four layers of the stack | DecagonThis post details the debugging process of a latent PgBouncer bug that caused connection stalls and pool exhaustion during a migration from GCP's managed connection pooler. The bug was traced across four layers: SQLAlchemy's QueuePool, PgBouncer's packet parser and event loop, OpenSSL's TLS record buffer, and the Linux kernel's epoll() interface. The root cause was identified as OpenSSL buffering decrypted data that PgBouncer's event loop could not see after a partial read, leading to a deadlock. Mitigation involved shortening the application name parameter and contributing an upstream fix to PgBouncer to check SSL_pending() before waiting on epoll().