Race Conditions

A race condition occurs when two or more goroutines access shared data concurrently, and at least one of the accesses is a write. This can lead to unpredictable behavior and hard-to-debug issues.

Understanding Race Conditions#

Race conditions happen when the outcome of a program depends on the timing or order of execution of multiple goroutines. If the accesses to shared data aren't properly synchronized, the program may behave incorrectly or produce different results on different runs.

A Simple Example of a Race Condition#

Here's a simple program that demonstrates a race condition:

example.go

package main
 
import (
    string">"fmt"
    string">"sync"
)
 
var msg string
var wg sync.WaitGroup
 
func updateMessage(s string) {
    defer wg.Done()
    msg = s
}
 
func main() {
    msg = string">"Hello, world!"
    
    wg.Add(2)
    go updateMessage(string">"Hello, one")
    go updateMessage(string">"Hello, two")
    wg.Wait()
    
    fmt.Println(msg)
}

In this program:

We have a global variable msg
We launch two goroutines that both modify msg
The final value of msg depends on which goroutine executes last
The output could be either "Hello, one" or "Hello, two"

Detecting Race Conditions#

Go includes a built-in race detector that can help identify race conditions in your code. To use it, run your program with the -race flag:

example.sh

go run -race .

If we run our example with the race detector:

example.sh

go run -race .
==================
WARNING: DATA RACE
Write at 0x0000011d09b0 by goroutine 8:
  main.updateMessage()
      /path/to/file.go:13 +0x78
  main.main·dwrap·3()
      /path/to/file.go:22 +0x47
 
Previous write at 0x0000011d09b0 by goroutine 7:
  main.updateMessage()
      /path/to/file.go:13 +0x78
  main.main·dwrap·2()
      /path/to/file.go:21 +0x47
 
Goroutine 8 (running) created at:
  main.main()
      /path/to/file.go:22 +0x164
 
Goroutine 7 (finished) created at:
  main.main()
      /path/to/file.go:21 +0xeb
==================
Hello, two
Found 1 data race(s)
exit status 66

The race detector tells us:

There's a race condition on a memory address
Two goroutines are writing to the same location
The specific lines of code involved in the race

The Risks of Race Conditions#

Race conditions can lead to several problems:

Inconsistent results: Your program produces different results on each run
Data corruption: Data structures may be left in an inconsistent state
Memory corruption: In low-level languages, this can cause crashes
Subtle bugs: The program works most of the time but fails occasionally

Testing for Race Conditions#

You can also use the race detector when running tests:

example.sh

go test -race ./...

Here's a test that would detect our race condition:

example.go

package main
 
import (
    string">"testing"
)
 
func Test_updateMessage(t *testing.T) {
    msg = string">"Hello, world!"
 
    wg.Add(2)
    go updateMessage(string">"Hello, one")
    go updateMessage(string">"Hello, two")
    wg.Wait()
 
    string">"comment">// The test doesn't actually fail, but the race detector
    string">"comment">// will report the race condition
    if msg != string">"Hello, one" && msg != string">"Hello, two" {
        t.Error(string">"Expected message to be updated")
    }
}

Real-World Example#

Let's look at a more realistic example involving a shared counter:

example.go

package main
 
import (
    string">"fmt"
    string">"sync"
)
 
func main() {
    string">"comment">// Variable for bank balance
    var bankBalance int
    
    string">"comment">// Print starting balance
    fmt.Printf(string">"Initial account balance: $%d.00\n", bankBalance)
    
    string">"comment">// Define weekly revenue
    incomes := []int{500, 10, 50, 100}
    
    var wg sync.WaitGroup
    wg.Add(len(incomes))
    
    string">"comment">// Loop through 52 weeks and add income
    for i, income := range incomes {
        go func(i int, income int) {
            defer wg.Done()
            
            for week := 1; week <= 52; week++ {
                string">"comment">// This is a race condition - multiple goroutines
                string">"comment">// are reading and writing bankBalance without synchronization
                temp := bankBalance
                temp += income
                bankBalance = temp
                
                fmt.Printf(string">"On week %d, you earned $%d.00 from source %d\n", 
                          week, income, i)
            }
        }(i, income)
    }
    
    wg.Wait()
    
    string">"comment">// Print final balance
    fmt.Printf(string">"Final bank balance: $%d.00\n", bankBalance)
}

In this example, we have multiple goroutines updating a shared bankBalance variable. Because these updates aren't synchronized, the final balance might be incorrect.

Summary#

Race conditions occur when multiple goroutines access shared data concurrently
They lead to unpredictable behavior and inconsistent results
Go's race detector can help identify race conditions
Race conditions are a common source of bugs in concurrent programs
Always be cautious when sharing data between goroutines

In the next lab, we'll learn how to use mutexes to protect shared data and prevent race conditions.

Race Conditions

Understanding Race Conditions#

A Simple Example of a Race Condition#

Detecting Race Conditions#

The Risks of Race Conditions#

Testing for Race Conditions#

Real-World Example#

Summary#

Your Progress