waitfordb: initial tool — env-configured wait-for-DB init container

Small Go tool + distroless container used as a K8s initContainer to block an
app until its database (Postgres/MySQL) is reachable. Env-var configured
(WAITFORDB_* + libpq PG* fallback), configurable timeout/interval, redacted
logs, exit codes. Woodpecker CI publishes docker-internal/waitfordb on tag.

integration_test.go

@@ -0,0 +1,94 @@
+//go:build integration
+
+// Integration test: runs a real postgres container that accepts connections a
+// few seconds after start and asserts waitfordb retries then succeeds. Run with:
+//
+//	go test -tags integration -run TestIntegration -v .
+//
+// Requires podman (or docker via WAITFORDB_TEST_RUNTIME). Skipped otherwise.
+package main
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/waitfordb/internal/config"
+	"git.unkin.net/unkin/waitfordb/internal/driver"
+	"git.unkin.net/unkin/waitfordb/internal/wait"
+)
+
+func TestIntegrationPostgresComesUpLate(t *testing.T) {
+	runtime := envOr("WAITFORDB_TEST_RUNTIME", "podman")
+	if _, err := exec.LookPath(runtime); err != nil {
+		t.Skipf("%s not available: %v", runtime, err)
+	}
+	image := envOr("WAITFORDB_TEST_PG_IMAGE", "docker.io/library/postgres:16")
+
+	name := "waitfordb-it-" + time.Now().Format("150405")
+	// Publish on a fixed host port; map to a random-ish high port.
+	hostPort := "55432"
+	args := []string{
+		"run", "-d", "--rm", "--name", name,
+		"-e", "POSTGRES_PASSWORD=secret",
+		"-e", "POSTGRES_USER=app",
+		"-e", "POSTGRES_DB=appdb",
+		"-p", hostPort + ":5432",
+		image,
+	}
+	if out, err := exec.Command(runtime, args...).CombinedOutput(); err != nil {
+		t.Skipf("cannot start container (%s): %v\n%s", runtime, err, out)
+	}
+	t.Cleanup(func() {
+		_ = exec.Command(runtime, "rm", "-f", name).Run()
+	})
+
+	t.Setenv("WAITFORDB_HOST", "127.0.0.1")
+	t.Setenv("WAITFORDB_PORT", hostPort)
+	t.Setenv("WAITFORDB_USER", "app")
+	t.Setenv("WAITFORDB_PASSWORD", "secret")
+	t.Setenv("WAITFORDB_DATABASE", "appdb")
+	t.Setenv("WAITFORDB_SSLMODE", "disable")
+	t.Setenv("WAITFORDB_TIMEOUT", "60s")
+	t.Setenv("WAITFORDB_INTERVAL", "1s")
+
+	cfg, err := config.Load(os.Getenv)
+	if err != nil {
+		t.Fatalf("config: %v", err)
+	}
+	drv, err := driver.Get(cfg.Driver)
+	if err != nil {
+		t.Fatalf("driver: %v", err)
+	}
+	pinger, err := drv.Open(cfg)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer pinger.Close()
+
+	res := wait.Run(context.Background(),
+		wait.Params{Timeout: cfg.Timeout, Interval: cfg.Interval, ConnectTimeout: cfg.ConnectTimeout},
+		pinger.Ping,
+		func(n int, err error, _, _, _ time.Duration) {
+			t.Logf("attempt %d failed as expected: %v", n, err)
+		},
+		wait.RealClock{})
+
+	if !res.OK {
+		t.Fatalf("expected readiness, got %+v", res)
+	}
+	t.Logf("ready after %s (%d attempts)", res.Elapsed, res.Attempts)
+	if res.Attempts < 1 {
+		t.Errorf("expected at least one attempt")
+	}
+}
+
+func envOr(k, def string) string {
+	if v := strings.TrimSpace(os.Getenv(k)); v != "" {
+		return v
+	}
+	return def
+}