Go 기술 면접: Goroutine, Channel, 동시성 패턴 완벽 가이드

package main

import (
	"fmt"
	"runtime"
	"sync"
)

func main() {
	// Print the number of OS threads available
	fmt.Println("GOMAXPROCS:", runtime.GOMAXPROCS(0))

	var wg sync.WaitGroup

	for i := 0; i < 10000; i++ {
		wg.Add(1)
		go func(id int) {
			defer wg.Done()
			// Each goroutine starts with ~2-8KB stack
			// OS threads typically start with 1-8MB
			_ = id
		}(i)
	}

	wg.Wait()
	fmt.Println("All 10,000 goroutines completed")
}

package main

import "fmt"

func safeGo(fn func()) {
	go func() {
		defer func() {
			if r := recover(); r != nil {
				fmt.Println("recovered from panic:", r)
			}
		}()
		fn() // execute the actual work
	}()
}

func main() {
	safeGo(func() {
		panic("something went wrong")
	})

	// Give goroutine time to complete
	select {}
}

package main

import "fmt"

func main() {
	// Unbuffered: send blocks until receive is ready
	ch := make(chan string)
	go func() {
		ch <- "hello" // blocks here until main reads
	}()
	msg := <-ch
	fmt.Println(msg)

	// Buffered: send does not block until buffer is full
	buf := make(chan int, 3)
	buf <- 1 // does not block (buffer has space)
	buf <- 2 // does not block
	buf <- 3 // does not block
	// buf <- 4 would block — buffer is full

	fmt.Println(<-buf, <-buf, <-buf) // 1 2 3
}

package main

import "fmt"

func producer(ch chan<- int, count int) {
	for i := 0; i < count; i++ {
		ch <- i
	}
	close(ch) // signal: no more values
}

func main() {
	ch := make(chan int, 5)
	go producer(ch, 5)

	// range exits automatically when channel closes
	for val := range ch {
		fmt.Println("received:", val)
	}

	// Reading from closed channel returns zero value + false
	val, ok := <-ch
	fmt.Printf("after close: val=%d, ok=%v\n", val, ok)
}

package main

import (
	"context"
	"fmt"
	"time"
)

func fetchFromAPI(ctx context.Context, url string) (string, error) {
	resultCh := make(chan string, 1)
	errCh := make(chan error, 1)

	go func() {
		// Simulate API call
		time.Sleep(200 * time.Millisecond)
		resultCh <- fmt.Sprintf("data from %s", url)
	}()

	select {
	case result := <-resultCh:
		return result, nil
	case err := <-errCh:
		return "", err
	case <-ctx.Done():
		// Context cancelled or timed out
		return "", ctx.Err()
	}
}

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
	defer cancel()

	result, err := fetchFromAPI(ctx, "https://api.example.com/data")
	if err != nil {
		fmt.Println("error:", err)
		return
	}
	fmt.Println(result)
}

package main

import (
	"fmt"
	"sync"
)

// generator produces values on a channel
func generator(nums ...int) <-chan int {
	out := make(chan int)
	go func() {
		for _, n := range nums {
			out <- n
		}
		close(out)
	}()
	return out
}

// square reads from input, squares each value
func square(in <-chan int) <-chan int {
	out := make(chan int)
	go func() {
		for n := range in {
			out <- n * n
		}
		close(out)
	}()
	return out
}

// fanIn merges multiple channels into one
func fanIn(channels ...<-chan int) <-chan int {
	var wg sync.WaitGroup
	merged := make(chan int)

	for _, ch := range channels {
		wg.Add(1)
		go func(c <-chan int) {
			defer wg.Done()
			for val := range c {
				merged <- val
			}
		}(ch)
	}

	go func() {
		wg.Wait()
		close(merged) // close after all inputs are drained
	}()

	return merged
}

func main() {
	in := generator(2, 3, 4, 5, 6)

	// Fan out: two goroutines reading from same channel
	c1 := square(in)
	c2 := square(in)

	// Fan in: merge results
	for result := range fanIn(c1, c2) {
		fmt.Println(result)
	}
}

package main

import (
	"context"
	"fmt"
	"golang.org/x/sync/errgroup"
)

func processItem(ctx context.Context, id int) error {
	// Check for cancellation before heavy work
	select {
	case <-ctx.Done():
		return ctx.Err()
	default:
	}

	if id == 7 {
		return fmt.Errorf("failed to process item %d", id)
	}
	fmt.Printf("processed item %d\n", id)
	return nil
}

func main() {
	g, ctx := errgroup.WithContext(context.Background())
	g.SetLimit(3) // maximum 3 concurrent goroutines

	for i := 0; i < 10; i++ {
		id := i
		g.Go(func() error {
			return processItem(ctx, id)
		})
	}

	// Wait blocks until all goroutines finish
	// Returns the first non-nil error
	if err := g.Wait(); err != nil {
		fmt.Println("pipeline error:", err)
	}
}

package main

import (
	"fmt"
	"sync"
	"sync/atomic"
)

// BAD: race condition — do not use in production
func unsafeCounter() int {
	counter := 0
	var wg sync.WaitGroup
	for i := 0; i < 1000; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			counter++ // DATA RACE: concurrent read/write
		}()
	}
	wg.Wait()
	return counter // result is non-deterministic
}

// GOOD: atomic operations for simple counters
func atomicCounter() int64 {
	var counter atomic.Int64
	var wg sync.WaitGroup
	for i := 0; i < 1000; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			counter.Add(1) // thread-safe atomic increment
		}()
	}
	wg.Wait()
	return counter.Load() // always 1000
}

// GOOD: mutex for complex shared state
type SafeMap struct {
	mu sync.RWMutex
	data map[string]int
}

func (m *SafeMap) Set(key string, val int) {
	m.mu.Lock()         // exclusive lock for writes
	defer m.mu.Unlock()
	m.data[key] = val
}

func (m *SafeMap) Get(key string) (int, bool) {
	m.mu.RLock()         // shared lock for reads
	defer m.mu.RUnlock()
	v, ok := m.data[key]
	return v, ok
}

func main() {
	fmt.Println("unsafe:", unsafeCounter())  // unpredictable
	fmt.Println("atomic:", atomicCounter())  // always 1000
}

package main

import (
	"context"
	"fmt"
	"time"
)

// worker simulates a long-running task
func worker(ctx context.Context, id int, results chan<- string) {
	select {
	case <-time.After(time.Duration(id*100) * time.Millisecond):
		results <- fmt.Sprintf("worker %d: done", id)
	case <-ctx.Done():
		results <- fmt.Sprintf("worker %d: cancelled (%v)", id, ctx.Err())
	}
}

func main() {
	// Parent context with 250ms deadline
	ctx, cancel := context.WithTimeout(context.Background(), 250*time.Millisecond)
	defer cancel()

	results := make(chan string, 5)

	// Launch 5 workers with increasing durations
	for i := 1; i <= 5; i++ {
		go worker(ctx, i, results)
	}

	// Collect all results
	for i := 0; i < 5; i++ {
		fmt.Println(<-results)
	}
}

package main

func main() {
	ch := make(chan int)
	ch <- 42 // DEADLOCK: unbuffered send with no receiver
	// The main goroutine blocks here forever
	// Go runtime detects this: "fatal error: all goroutines are asleep"
}

package main

import (
	"context"
	"fmt"
	"sync"
	"time"
)

// RateLimiter controls concurrent and temporal access
type RateLimiter struct {
	semaphore chan struct{}   // limits concurrency
	ticker    *time.Ticker    // limits rate
}

func NewRateLimiter(maxConcurrent int, interval time.Duration) *RateLimiter {
	return &RateLimiter{
		semaphore: make(chan struct{}, maxConcurrent),
		ticker:    time.NewTicker(interval),
	}
}

func (rl *RateLimiter) Execute(ctx context.Context, fn func() error) error {
	// Wait for rate limit tick
	select {
	case <-rl.ticker.C:
	case <-ctx.Done():
		return ctx.Err()
	}

	// Acquire concurrency slot
	select {
	case rl.semaphore <- struct{}{}:
	case <-ctx.Done():
		return ctx.Err()
	}

	defer func() { <-rl.semaphore }() // release slot
	return fn()
}

func main() {
	rl := NewRateLimiter(3, 100*time.Millisecond)
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()

	var wg sync.WaitGroup
	for i := 0; i < 10; i++ {
		wg.Add(1)
		go func(id int) {
			defer wg.Done()
			err := rl.Execute(ctx, func() error {
				fmt.Printf("[%v] processing %d\n", time.Now().Format("04:05.000"), id)
				time.Sleep(150 * time.Millisecond) // simulate work
				return nil
			})
			if err != nil {
				fmt.Printf("item %d: %v\n", id, err)
			}
		}(i)
	}
	wg.Wait()
}