package datadir_test

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
	"sneak.berlin/go/webhooker/internal/datadir"
)

// holderEnv names the directory the re-executed test binary should
// lock and hold. When it is unset the child test does nothing, so an
// ordinary run is unaffected.
const holderEnv = "WEBHOOKER_DATADIR_LOCK_HOLDER"

// holderReadyPrefix labels the child's one-line report that it holds
// the lock, so the parent can find it among the testing package's own
// output on the same descriptor.
const holderReadyPrefix = "DATADIR-LOCK-HELD "

// holderReadyTimeout bounds the wait for the child to take the lock.
// It only has to cover process start on a loaded shared host.
const holderReadyTimeout = 60 * time.Second

// holderHold is how long the child keeps the lock if nothing kills it.
// A sleep rather than a bare block, so the runtime's deadlock detector
// has a pending timer and the child cannot outlive a killed test run
// by more than this.
const holderHold = 10 * time.Minute

// TestLockHolder is the child half of the two-process tests below. It
// takes the lock on the directory named by holderEnv, reports the lock
// file on standard output, and then holds it until it is killed.
func TestLockHolder(t *testing.T) {
	t.Parallel()

	dir := os.Getenv(holderEnv)
	if dir == "" {
		return
	}

	lock, err := datadir.Acquire(dir)
	require.NoError(t, err)

	// Written to the descriptor directly: the parent reads fd 1, not
	// the testing package's buffered report.
	_, err = fmt.Fprintf(
		os.Stdout, "%s%s\n", holderReadyPrefix, lock.Path(),
	)
	require.NoError(t, err)

	time.Sleep(holderHold)
}

// startHolder re-executes this test binary as a separate process that
// takes and holds the lock on dir, and returns once that process
// actually holds it. The child is killed when the test ends.
func startHolder(t *testing.T, dir string) *exec.Cmd {
	t.Helper()

	//nolint:gosec // Re-executing this test binary, with a fixed arg.
	cmd := exec.CommandContext(
		t.Context(), os.Args[0], "-test.run", "^TestLockHolder$",
	)

	cmd.Env = append(os.Environ(), holderEnv+"="+dir)
	cmd.Stderr = os.Stderr

	stdout, err := cmd.StdoutPipe()
	require.NoError(t, err)
	require.NoError(t, cmd.Start())

	t.Cleanup(func() {
		_ = cmd.Process.Kill()
		_ = cmd.Wait()
	})

	ready := make(chan string, 1)

	go func() {
		scanner := bufio.NewScanner(stdout)
		for scanner.Scan() {
			after, found := strings.CutPrefix(
				scanner.Text(), holderReadyPrefix,
			)
			if found {
				ready <- after

				break
			}
		}

		close(ready)

		// Keep draining so the child never blocks on a full pipe.
		_, _ = io.Copy(io.Discard, stdout)
	}()

	select {
	case path, ok := <-ready:
		require.True(
			t, ok, "holder exited without taking the lock",
		)
		require.Equal(t, filepath.Join(dir, datadir.LockFileName), path)
	case <-time.After(holderReadyTimeout):
		t.Fatal("timed out waiting for the holder to take the lock")
	}

	return cmd
}

// TestSecondInstanceRefused is the regression test for the duplicate
// delivery this package exists to prevent: a real second process
// pointed at a data directory a live process already holds must be
// refused, with an error that names the directory.
func TestSecondInstanceRefused(t *testing.T) {
	t.Parallel()

	dir := t.TempDir()

	startHolder(t, dir)

	lock, err := datadir.Acquire(dir)

	require.Error(t, err, "the second instance took the lock too")
	require.Nil(t, lock)
	require.ErrorIs(
		t, err, datadir.ErrLocked,
		"the refusal must be distinguishable from any other failure",
	)
	assert.Contains(
		t, err.Error(), dir,
		"the refusal must name the directory it is about",
	)
}

// TestRestartAfterHardKill is the other half of the regression: a
// process killed with SIGKILL runs no cleanup and leaves its lock file
// behind, and the next start must not be blocked by it. This is what a
// pidfile would get wrong; the kernel drops a flock when the
// descriptor closes, however the process died.
func TestRestartAfterHardKill(t *testing.T) {
	t.Parallel()

	dir := t.TempDir()

	holder := startHolder(t, dir)

	require.NoError(t, holder.Process.Kill())

	// Wait for the kill to have actually happened. Re-acquiring while
	// the corpse still holds a descriptor would be a race, and would
	// make this test pass or fail on scheduling.
	_ = holder.Wait()

	require.FileExists(
		t, filepath.Join(dir, datadir.LockFileName),
		"the stale lock file is what must not block the restart",
	)

	lock, err := datadir.Acquire(dir)
	require.NoError(
		t, err, "a hard-killed instance must not block the next start",
	)

	require.NoError(t, lock.Release())
}

// TestSecondFdInSameProcessRefused pins the flock(2) property the
// tests in cmd/webhooker rely on: descriptors are locked
// independently, so a second acquisition is denied even when it comes
// from the process that already holds the lock.
func TestSecondFdInSameProcessRefused(t *testing.T) {
	t.Parallel()

	dir := t.TempDir()

	first, err := datadir.Acquire(dir)
	require.NoError(t, err)

	defer func() { _ = first.Release() }()

	_, err = datadir.Acquire(dir)
	require.ErrorIs(t, err, datadir.ErrLocked)
}

// TestReleaseAllowsReacquire covers the clean-shutdown path: the lock
// is released on exit, so a restart is not blocked by the previous
// run.
func TestReleaseAllowsReacquire(t *testing.T) {
	t.Parallel()

	dir := t.TempDir()

	first, err := datadir.Acquire(dir)
	require.NoError(t, err)
	require.NoError(t, first.Release())

	second, err := datadir.Acquire(dir)
	require.NoError(t, err)
	require.NoError(t, second.Release())
}

// TestAcquireCreatesDataDir covers a first start against a DATA_DIR
// that does not exist yet, which is the normal case for a fresh
// deployment: the lock is taken before anything else creates it.
func TestAcquireCreatesDataDir(t *testing.T) {
	t.Parallel()

	dir := filepath.Join(t.TempDir(), "nested", "data")

	lock, err := datadir.Acquire(dir)
	require.NoError(t, err)

	defer func() { _ = lock.Release() }()

	assert.Equal(t, dir, lock.Dir())
	assert.FileExists(t, filepath.Join(dir, datadir.LockFileName))
}

// TestAcquireEmptyDir rejects an empty directory rather than locking
// the process's working directory.
func TestAcquireEmptyDir(t *testing.T) {
	t.Parallel()

	_, err := datadir.Acquire("")
	require.ErrorIs(t, err, datadir.ErrNoDir)
}

// TestAcquireUnusableDir reports an unusable DATA_DIR clearly, naming
// it, instead of failing later and deeper.
func TestAcquireUnusableDir(t *testing.T) {
	t.Parallel()

	file := filepath.Join(t.TempDir(), "not-a-directory")
	require.NoError(t, os.WriteFile(file, nil, 0o600))

	_, err := datadir.Acquire(file)

	require.Error(t, err)
	assert.Contains(t, err.Error(), file)
}