package dbreads

import (
	"context"
	"errors"
	"testing"
	"time"
)

type unitsSeam struct {
	n   int
	err error
}

func (u *unitsSeam) count(context.Context) (int, error) { return u.n, u.err }

func gib(n uint64) uint64 { return n << 30 }

func TestAdmissionTwoReadsThenBusy(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{}
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(100), gib(200), nil })
	ctx := context.Background()
	for _, id := range []string{"r1", "r2"} {
		if err := a.admitRead(ctx, id); err != nil {
			t.Fatalf("%s: %v", id, err)
		}
	}
	if err := a.admitRead(ctx, "r3"); !errors.Is(err, errBusy) {
		t.Fatalf("a third read: %v", err)
	}
	// The probe still has its connection (spec §7.1, codex 9).
	if !a.opened() || !a.opened() || !a.admitProbe(ctx) || !a.opened() {
		t.Fatal("two reads and a probe do not fit three connections")
	}
	if a.opened() {
		t.Fatal("a fourth connection was allowed")
	}
}

// T12, unit half: free space that fits one read admits one.
func TestAdmissionDiskFitsOneOfTwo(t *testing.T) {
	t.Parallel()
	a := newAdmission((&unitsSeam{}).count, func() (uint64, uint64, error) { return gib(12), gib(20), nil })
	ctx := context.Background()
	if err := a.admitRead(ctx, "r1"); err != nil {
		t.Fatal(err)
	}
	if err := a.admitRead(ctx, "r2"); !errors.Is(err, errNoRoom) {
		t.Fatalf("a second read on a box with room for one: %v", err)
	}
	if err := a.grow("r1", int64(gib(8))); !errors.Is(err, errNoRoom) {
		t.Fatalf("growing past the floor and 10%%: %v", err)
	}
	if err := a.grow("r1", int64(gib(6))); err != nil {
		t.Fatalf("growing within room: %v", err)
	}
}

// An orphan (a unit this agent is not running) blocks every admission until
// it is gone, and no size is guessed for it (spec §5.7, codex 8).
func TestAdmissionRefusesBesideAnOrphan(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{n: 1}
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(100), gib(200), nil })
	ctx := context.Background()
	if err := a.admitRead(ctx, "r1"); !errors.Is(err, errOrphans) {
		t.Fatalf("beside an orphan: %v", err)
	}
	if a.admitProbe(ctx) {
		t.Fatal("a probe beside an orphan")
	}
	u.n = 0
	if err := a.admitRead(ctx, "r1"); err != nil {
		t.Fatalf("once the orphan is gone: %v", err)
	}
}

// A finished read's reservation is held until no db-read unit is left, since
// systemd removes the scratch only at unit stop.
func TestAdmissionHoldsAReservationUntilItsUnitIsGone(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{}
	// 12 GiB free of 20: r1 grown to 6 leaves room for r2's floor only once
	// r1's reservation is released.
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(12), gib(20), nil })
	ctx := context.Background()
	if err := a.admitRead(ctx, "r1"); err != nil {
		t.Fatal(err)
	}
	a.opened()
	if err := a.grow("r1", int64(gib(6))); err != nil {
		t.Fatal(err)
	}
	u.n = 1
	a.closed()
	a.finish("r1")
	// The unit is still deactivating: one unit, no connection. Refused.
	if err := a.admitRead(ctx, "r2"); !errors.Is(err, errOrphans) {
		t.Fatalf("while the unit deactivates: %v", err)
	}
	u.n = 0
	if err := a.admitRead(ctx, "r2"); err != nil {
		t.Fatalf("after the unit is gone the reservation is free: %v", err)
	}
}

// Review Focus 5: a unit listing that fails admits nothing and skips the
// probe rather than guessing.
func TestAdmissionFailsClosedWhenUnitsCannotBeListed(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{err: errors.New("systemctl: not found")}
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(100), gib(200), nil })
	if err := a.admitRead(context.Background(), "r1"); err == nil {
		t.Fatal("admitted with no unit listing")
	}
	if a.admitProbe(context.Background()) {
		t.Fatal("probed with no unit listing")
	}
}

// A read admitted and then lost in the election started no worker: its
// floor goes at once, even while another read's unit is running.
func TestAdmissionAbandonFreesAtOnce(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{}
	// 17 GiB of 20: room for two floors (17-10 = 7), not three.
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(17), gib(20), nil })
	ctx := context.Background()
	if err := a.admitRead(ctx, "running"); err != nil {
		t.Fatal(err)
	}
	a.opened()
	u.n = 1 // its worker unit is up, so finished reservations stay held
	if err := a.admitRead(ctx, "lost"); err != nil {
		t.Fatal(err)
	}
	a.abandon("lost") // it lost the election; no worker started
	if err := a.admitRead(ctx, "next"); err != nil {
		t.Fatalf("the lost read's floor is still held: %v", err)
	}
}

// Plan review finding 10: a retried read keeps its first attempt's
// reservation while that worker's unit is still stopping, and settle waits
// for the unit. Mutation: retry leaves held[id] in place instead of moving
// it to a retired key; the grow below then fits and this test is red.
func TestAdmissionRetryHoldsTheOldReservation(t *testing.T) {
	t.Parallel()
	u := &unitsSeam{}
	a := newAdmission(u.count, func() (uint64, uint64, error) { return gib(12), gib(20), nil })
	ctx := context.Background()
	if err := a.admitRead(ctx, "r1"); err != nil {
		t.Fatal(err)
	}
	if err := a.grow("r1", int64(gib(6))); err != nil {
		t.Fatal(err)
	}
	u.n = 1 // the first worker's connection is closed; its unit is stopping
	a.retry("r1", 1)
	if err := a.grow("r1", int64(gib(5))); !errors.Is(err, errNoRoom) {
		t.Fatalf("the stopping worker's scratch was not held: %v", err)
	}
	short, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
	defer cancel()
	if err := a.settle(short, 10*time.Millisecond); !errors.Is(err, context.DeadlineExceeded) {
		t.Fatalf("settle returned beside a live unit: %v", err)
	}
	u.n = 0
	if err := a.settle(ctx, 10*time.Millisecond); err != nil {
		t.Fatal(err)
	}
	if err := a.grow("r1", int64(gib(5))); err != nil {
		t.Fatalf("after the unit stopped: %v", err)
	}
}

func TestAdmissionFailsClosedWhenTheDiskCannotBeRead(t *testing.T) {
	t.Parallel()
	a := newAdmission((&unitsSeam{}).count, func() (uint64, uint64, error) { return 0, 0, errors.New("statfs") })
	if err := a.admitRead(context.Background(), "r1"); !errors.Is(err, errNoRoom) {
		t.Fatalf("admitted with no disk reading: %v", err)
	}
}