goAsync race conditionMedium

Missing mutex unlock on early error return

Time: 0:00

BugFind the bug and fix the code

What it should do

Deposit amount to balance safely with mutex. Example: safeDepositBalance(100, -10) -> -1

What happens instead

Subsequent operations hang indefinitely due to mutex remaining locked

package main

import (
	"fmt"
	"sync"
)

var mu sync.Mutex
var balance int

func safeDepositBalance(initial int, amount int) int {
	mu.Lock()
	balance = initial
	if amount < 0 {
		// Early return forgets to unlock mutex
		return -1
	}
	balance += amount
	mu.Unlock()
	return balance
}

func main() {
	fmt.Println(safeDepositBalance(100, -10))
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the async and race conditions pattern

A missing await, unbuffered channel deadlocks, ConcurrentModificationException, and data races under load.

Learn the async and race conditions pattern Step through your own code

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package main import ( "fmt" "sync" ) var mu sync.Mutex var balance int func safeDepositBalance(initial int, amount int) int { mu.Lock() balance = initial if amount < 0 { // Early return forgets to unlock mutex return -1 } balance += amount mu.Unlock() return balance } func main() { fmt.Println(safeDepositBalance(100, -10)) }

Missing mutex unlock on early error return

What it should do

What happens instead

More async and race conditions to practise

Missing mutex unlock on early error return

What it should do

What happens instead

More async and race conditions to practise