fix: retrospective review fixes for merged PR #97

[ikeikeikeike]

Retrospective /code-review sweep of merged PR #97 (kind: compose — wrap an existing docker-compose.yml as an engine), which shipped without pre-merge review.

Finding: the only mysql-usable readiness probe is a bare TCP dial

kind: compose's ReadyCheck picks a probe via extras["compose.ready_probe"], supporting tcp (default) / redis / postgres / http. There is no mysql option — so a compose-wrapped mysql service (the exact motivating use case in the sibling, unmerged PR #96) has no way to get a real readiness check; it is stuck on the bare-dial tcp default.

That reintroduces, for kind: compose, the exact race plugins/engine/mysql/docker.go's dockerReadyCheck already had to fix for the bundled Docker backend (see its TestDockerReadyCheck_NoRaceWithTemporaryServer regression test): a Docker host port's forwarding accepts the TCP handshake the instant the container starts, regardless of whether the process inside is listening yet. The official mysql:8.x image runs a --skip-networking "temporary server" to bootstrap the datadir before restarting as the final, network-enabled server — a dial-only probe goes ready during that window.

Confirmed empirically, not just by reasoning: brought up a fresh mysql:8.4 container via docker compose up -d and polled the actual probeOnce function from this package every 100ms. Results:

16:52:41.424 tcp=true(<nil>) mysql=false(EOF)
...  (~1.6s of tcp=true / mysql=false)
16:52:43.058 tcp=true(<nil>) mysql=true(<nil>)

For ~1.6 seconds, the current tcp probe (bare dial+close) reported the service ready while mysqld was still bootstrapping and any real connection attempt got EOF — i.e., bough create would declare a compose-wrapped mysql engine ready before it can actually serve a query, breaking any post_create hook (e.g. a migration) that connects immediately after Up returns.

Fix

Add a mysql compose.ready_probe option that reads the server's unsolicited Initial Handshake Packet and checks its protocol-version byte (0x0a, unchanged since MySQL 4.1) instead of just dialing — the same technique the mysql plugin's own conformance regression test already relies on (plugins/engine/mysql/conformance_test.go's mysqlHandshakeOnce).

• plugins/engine/compose/probe.go: new mysqlProbe, wired into probeOnce's "mysql" case, error message updated.
• plugins/engine/compose/compose.go: package doc updated to describe the option and why it matters for mysql specifically.
• README.md: compose.ready_probe documentation updated to list mysql and call out the risk of leaving it unset for a wrapped mysql service.
• plugins/engine/compose/probe_test.go: TestProbeOnce_MySQL (valid handshake), TestProbeOnce_MySQL_WrongProtocolVersion, and TestProbeOnce_MySQL_AcceptThenClose (the regression guard for the accept-then-close docker-proxy race a bare TCP probe cannot see).

Default behavior for existing tcp/redis/postgres/http users is unchanged — this only adds the missing, correct option for mysql.

Test plan

• go build ./...
• go test ./... (full repo green, including new plugins/engine/compose tests)
• go test ./plugins/engine/compose/... -race -v green
• golangci-lint run ./... — 0 issues
• Empirically verified against a real mysql:8.4 container via docker compose up -d (see finding above) — not just unit tests against a fake TCP server