package httpapi

import (
	"context"
	"errors"
	"log/slog"
	"net"
	"net/http"
	"time"
)

// Timeouts of the listening server. A named type rather than literals inside main: the test floor
// asserts this class of defect on a REAL server, and a *http.Server built inside func main is not
// reachable from a test (PD-2 survived P0 for exactly that reason).
type Timeouts struct {
	ReadHeader time.Duration
	// Read bounds the WHOLE request, body included. Without it a client can pin a connection
	// forever, and it does not need a body-reading handler to do it: net/http drains an unread
	// body inside chunkWriter.writeHeader, before the response goes out, and that read inherits
	// the connection deadline.
	//
	// It does NOT bound a long RESPONSE, and a streaming handler needs nothing from it: net/http
	// clears the deadline itself once the request has arrived — before the handler when nothing
	// remains to read, at body EOF otherwise (server.go:2059-2062) — and nothing re-arms it while
	// the handler runs. Nor may a handler clear it by hand: on a half-fed request that drain is
	// the only bound left, and clearing the deadline before the header write hangs the handler
	// inside WriteHeader for as long as the client keeps the socket. Measured both ways (PD-51).
	Read time.Duration
	Idle time.Duration
	// Shutdown is how long a drain may take before in-flight handlers lose their context.
	Shutdown time.Duration
}

// DefaultTimeouts is what the daemon runs with.
//
// No WriteTimeout: the SSE stream is a long-lived response and a write deadline set here would cut
// it. Read is deliberately short — a request that legitimately takes longer than this to ARRIVE is
// an upload, and an upload extends its own deadline as it makes progress.
func DefaultTimeouts() Timeouts {
	return Timeouts{
		ReadHeader: 10 * time.Second,
		Read:       30 * time.Second,
		Idle:       2 * time.Minute,
		Shutdown:   15 * time.Second,
	}
}

// Server owns the listener lifecycle: serve, then drain, then cancel.
type Server struct {
	http     *http.Server
	stopBase context.CancelFunc
	grace    time.Duration
	log      *slog.Logger
}

// NewServer configures the listening server the daemon runs. The handler is whatever New returned.
//
// It takes no Timeouts on purpose. When it did, main passed DefaultTimeouts() and every test passed
// its own, so the one call that decided what SHIPPED was the one nothing observed — and replacing it
// with a bare Timeouts{} left the whole battery green while the binary re-acquired PD-2 (measured).
// With nothing to pass there is nothing to get wrong, and the test floor asserts on this very
// constructor. Tests that need short deadlines use serverWithTimeouts.
func NewServer(addr string, h http.Handler, log *slog.Logger) *Server {
	return serverWithTimeouts(addr, h, log, DefaultTimeouts())
}

func serverWithTimeouts(addr string, h http.Handler, log *slog.Logger, t Timeouts) *Server {
	// The base context is NOT the signal context (PD-9): a signal must start the drain, not end
	// every in-flight request at once. It is cancelled after Shutdown returns, which is the point
	// where the grace period is spent and a still-running handler is one we no longer wait for.
	base, cancel := context.WithCancel(context.Background())
	return &Server{
		http: &http.Server{
			Addr:              addr,
			Handler:           h,
			ReadHeaderTimeout: t.ReadHeader,
			ReadTimeout:       t.Read,
			IdleTimeout:       t.Idle,
			MaxHeaderBytes:    1 << 16,
			BaseContext:       func(net.Listener) context.Context { return base },
			// net/http's own errors (bad TLS records, malformed requests) reach slog instead of
			// the default logger's stderr, where nothing structured would find them.
			ErrorLog: slog.NewLogLogger(log.Handler(), slog.LevelWarn),
		},
		stopBase: cancel,
		grace:    t.Shutdown,
		log:      log,
	}
}

// Listen opens the configured address. Separate from Run so that a caller — the daemon, a test —
// knows the port is bound (and, with :0, which one) before anything is served on it.
func (s *Server) Listen(ctx context.Context) (net.Listener, error) {
	var lc net.ListenConfig
	return lc.Listen(ctx, "tcp", s.http.Addr)
}

// Run serves until ctx is cancelled, then drains for the grace period. Returns nil on a clean stop.
func (s *Server) Run(ctx context.Context, ln net.Listener) error {
	errc := make(chan error, 1)
	go func() { errc <- s.http.Serve(ln) }()

	select {
	case err := <-errc:
		s.stopBase()
		if errors.Is(err, http.ErrServerClosed) {
			return nil
		}
		return err
	case <-ctx.Done():
	}

	s.log.Info("shutting down", "grace_seconds", int(s.grace.Seconds()))
	started := time.Now()
	shutdownCtx, cancel := context.WithTimeout(context.Background(), s.grace)
	defer cancel()
	err := s.http.Shutdown(shutdownCtx)
	s.stopBase()
	if errors.Is(err, context.DeadlineExceeded) {
		// A drain that ran out of time is a slow request, not a failed service. Returning the error
		// makes the process exit non-zero, and under Restart=on-failure an ordinary stop then reads
		// to systemd as a crash.
		s.log.Warn("stopped: drain deadline exceeded, in-flight requests were cancelled",
			"after_ms", time.Since(started).Milliseconds())
		return nil
	}
	if err != nil {
		return err
	}
	s.log.Info("stopped", "drained_ms", time.Since(started).Milliseconds())
	return nil
}