learning-go-for-sre-interview/98-http-server-experiments/loadlib/loadlib_test.go

package loadlib

import (
	"errors"
	"fmt"
	"net"
	"testing"
	"time"
)

func TestBurnDeterministic(t *testing.T) {
	// Same difficulty must give the same checksum: the /compute endpoint returns
	// it, and a flaky checksum would mean a non-deterministic loop.
	for _, d := range []int{1, 3, 10} {
		if a, b := Burn(d), Burn(d); a != b {
			t.Errorf("Burn(%d) not deterministic: %d != %d", d, a, b)
		}
	}
}

func TestBurnDifficultyChangesResult(t *testing.T) {
	// Different difficulties should not collide (they run different iteration
	// counts); a collision here would suggest the loop body ignores i.
	if Burn(1) == Burn(2) {
		t.Error("Burn(1) == Burn(2): difficulty has no effect on the checksum")
	}
}

func TestIterationsLinearAndFloored(t *testing.T) {
	if got := Iterations(2); got != 2*burnIters {
		t.Errorf("Iterations(2) = %d, want %d", got, 2*burnIters)
	}
	// difficulty < 1 is clamped to 1 so /compute?difficulty=0 still does work.
	for _, d := range []int{0, -5} {
		if got := Iterations(d); got != burnIters {
			t.Errorf("Iterations(%d) = %d, want %d (floor of 1)", d, got, burnIters)
		}
	}
	// Cost is monotonic in difficulty.
	if Iterations(3) <= Iterations(2) {
		t.Error("Iterations not increasing with difficulty")
	}
}

func TestPercentileEdges(t *testing.T) {
	if got := Percentile(nil, 0.5); got != 0 {
		t.Errorf("Percentile(empty) = %v, want 0", got)
	}
	s := []time.Duration{10, 20, 30, 40, 50}
	cases := []struct {
		p    float64
		want time.Duration
	}{
		{-1, 10},  // clamped to first
		{0, 10},   // first
		{0.5, 30}, // middle
		{1, 50},   // last
		{2, 50},   // clamped to last
	}
	for _, c := range cases {
		if got := Percentile(s, c.p); got != c.want {
			t.Errorf("Percentile(%v, %v) = %v, want %v", s, c.p, got, c.want)
		}
	}
	if got := Percentile([]time.Duration{99}, 0.9); got != 99 {
		t.Errorf("Percentile(single) = %v, want 99", got)
	}
}

func TestLatenciesSnapshot(t *testing.T) {
	var l Latencies
	// Add out of order; Snapshot must sort before reporting.
	for _, d := range []time.Duration{50, 10, 30, 40, 20} {
		l.Add(d * time.Millisecond)
	}
	if l.Len() != 5 {
		t.Fatalf("Len = %d, want 5", l.Len())
	}
	p50, p99, max := l.Snapshot()
	if p50 != 30*time.Millisecond {
		t.Errorf("p50 = %v, want 30ms", p50)
	}
	if max != 50*time.Millisecond {
		t.Errorf("max = %v, want 50ms", max)
	}
	// Nearest-rank: index int(4*0.99)=3 → 40ms for this 5-sample set.
	if p99 != 40*time.Millisecond {
		t.Errorf("p99 = %v, want 40ms", p99)
	}
}

func TestLatenciesMerge(t *testing.T) {
	var a, b Latencies
	a.Add(10 * time.Millisecond)
	b.Add(20 * time.Millisecond)
	a.Merge(&b)
	if a.Len() != 2 {
		t.Fatalf("after merge Len = %d, want 2", a.Len())
	}
}

func TestClassify(t *testing.T) {
	timeoutErr := &net.OpError{
		Op:  "dial",
		Err: timeoutError{},
	}
	headerTimeout := fmt.Errorf("net/http: timeout awaiting response headers")

	cases := []struct {
		name string
		err  error
		want ErrClass
	}{
		{"nil", nil, ErrNone},
		{"client fds", errors.New("dial tcp 127.0.0.1:8098: socket: too many open files"), ErrClientFdLimit},
		{"client ports", errors.New("dial tcp 127.0.0.1:8098: connect: can't assign requested address"), ErrClientPortLimit},
		{"refused", errors.New("dial tcp 127.0.0.1:8098: connect: connection refused"), ErrRefused},
		{"reset", errors.New("read tcp: connection reset by peer"), ErrConnReset},
		{"eof", errors.New("unexpected EOF"), ErrConnReset},
		{"plain net timeout", timeoutErr, ErrTimeout},
		{"server cant accept", headerTimeoutNetErr{}, ErrServerCantAccept},
		{"header timeout string", headerTimeout, ErrTimeout}, // not a net.Error, but has "timeout"
		{"deadline", errors.New("context deadline exceeded"), ErrTimeout},
		{"other", errors.New("something weird"), ErrOther},
	}
	for _, c := range cases {
		if got := Classify(c.err); got != c.want {
			t.Errorf("%s: Classify(%v) = %v, want %v", c.name, c.err, got, c.want)
		}
	}
}

// timeoutError is a minimal net.Error whose Timeout() reports true, used to build
// a synthetic *net.OpError the way the runtime would on a real dial timeout.
type timeoutError struct{}

func (timeoutError) Error() string   { return "i/o timeout" }
func (timeoutError) Timeout() bool   { return true }
func (timeoutError) Temporary() bool { return true }

// headerTimeoutNetErr is a net.Error whose message is the exact one net/http
// produces when ResponseHeaderTimeout fires — the "server can't accept" signal.
type headerTimeoutNetErr struct{}

func (headerTimeoutNetErr) Error() string   { return "net/http: timeout awaiting response headers" }
func (headerTimeoutNetErr) Timeout() bool   { return true }
func (headerTimeoutNetErr) Temporary() bool { return true }