[ramiscaleai]

Phoenix.Socket.PoolDrainer computes the number of rounds as div(total, size) + 1. Stream.chunk_every/2 only produces ceil(total / size) batches, so when the number of connections is an exact multiple of :batch_size the drainer believes there is one more round than there really is. Two effects:

• The [:phoenix, :socket_drain] telemetry event, and the log line built from it, report a wrong number of rounds, e.g. "round 2 of 3" for a drain made of two batches.
• The sleep between batches is guarded by index < rounds, so the drainer also sleeps :batch_interval after the last batch, when there is nothing left to do. The example in the Phoenix.Endpoint.socket/3 docs (150k connections with the default :batch_size of 10_000 and :batch_interval of 2_000) is such a case: the drainer needs 15 x 2s = 30s, the whole default :shutdown, instead of 14 x 2s = 28s.

The fix computes rounds with a ceiling division and reports length(pids) as the count of each batch (the special case for the last batch is no longer needed).

Tests

Nothing exercised PoolDrainer.terminate/2, so this adds test/phoenix/socket/pool_drainer_test.exs. It drains an isolated PoolSupervisor pool and checks:

• every connection receives phx_drain;
• a partial last batch (5 connections, batches of 2: 3 rounds, counts 2/2/1);
• an exact multiple (4 connections, batches of 2: 2 rounds);
• no extra wait after the last batch;
• no events when there are no connections.

mix test test/phoenix/socket/pool_drainer_test.exs

Before the fix, the exact multiple test fails (rounds: 3, expected 2) and the wait test fails (5001ms elapsed for a 5000ms interval): 5 tests, 2 failures. After the fix: 5 tests, 0 failures. The rest of test/phoenix gives the same results with and without this change.

I ran this locally on Elixir 1.14 / OTP 25, in a scratch copy of the repository with a small compatibility shim (mix.exs requires 1.15). lib/ and the test file are identical to this PR; CI covers the supported versions.

AI disclosure: Claude Code (an AI coding agent) found this bug, wrote the fix and the tests, and drafted this description. The tests above were run locally.

🤖 Generated with Claude Code