Race conditions interview questions

Question 1

Several worker processes call ensure_dir() at startup. Occasionally one crashes with FileExistsError. Why?

import os

def ensure_dir(path: str) -> str:
    """Create the directory if it doesn't exist yet."""
    if not os.path.exists(path):
        os.makedirs(path)
    return path

Accepted Answer

Line 7: This is a time-of-check to time-of-use race. Two processes can both see that the path doesn't exist, then both call makedirs, and the slower one raises FileExistsError. Make the operation itself idempotent instead of checking first.

Question 2

onQueryChange() runs on every keystroke. Users sometimes see results for "re" after typing "react". Why?

let results: string[] = [];

export async function onQueryChange(query: string, render: (r: string[]) => void) {
  const res = await fetch(`/api/search?q=${encodeURIComponent(query)}`);
  const data: { results: string[] } = await res.json();
  results = data.results;
  render(results);
}

Accepted Answer

Line 6: Requests can finish out of order. If the slow response for "re" arrives after the fast one for "react", it overwrites the newer results. Only the latest request should be allowed to update the UI.

Question 3

CountWords splits work across goroutines. Under load the process dies with "fatal error: concurrent map writes". Why?

func CountWords(docs []string) map[string]int {
	counts := make(map[string]int)
	var wg sync.WaitGroup
	for _, doc := range docs {
		wg.Add(1)
		go func(d string) {
			defer wg.Done()
			for _, w := range strings.Fields(d) {
				counts[w]++
			}
		}(doc)
	}
	wg.Wait()
	return counts
}

Accepted Answer

Line 9: Go maps are not safe for concurrent writes. Several goroutines update `counts` at once, which the runtime detects and treats as a fatal error that can't be recovered.

Race conditions: interview questions

How to spot it in code review

Why AI-generated code gets this wrong

Race conditions: 3 practice questions

Check-then-create directory

Search-as-you-type results

Counting words in goroutines

Attempts distribution