Program stability concepts in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of deferred function with panic

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    defer fmt.Println("deferred")
    panic("panic occurred")
    fmt.Println("after panic")
}

A deferred

B

deferred
panic: panic occurred

C

after panic
deferred

D

panic: panic occurred
deferred

💡 Hint

Remember that deferred functions run before the program exits due to panic.

In Go, deferred functions run even if a panic occurs. So "deferred" prints before the panic message.

❓ Predict Output

intermediate

Recover usage in goroutine

What will this Go program print?

Go

package main
import (
    "fmt"
    "time"
)
func safeGo() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered from panic")
        }
    }()
    panic("panic in goroutine")
}
func main() {
    go safeGo()
    time.Sleep(100 * time.Millisecond)
    fmt.Println("main finished")
}

A

main finished
Recovered from panic

B

Recovered from panic
main finished

C

panic: panic in goroutine
main finished

D panic: panic in goroutine

💡 Hint

Recover only works if called inside deferred function in the same goroutine.

The panic inside safeGo is recovered, so "Recovered from panic" prints. Then main prints "main finished".

❓ Predict Output

advanced

Race condition detection output

What will happen when running this Go program with the race detector enabled?

Go

package main
import (
    "fmt"
    "sync"
)
func main() {
    var x int
    var wg sync.WaitGroup
    wg.Add(2)
    go func() {
        x = 1
        wg.Done()
    }()
    go func() {
        x = 2
        wg.Done()
    }()
    wg.Wait()
    fmt.Println(x)
}

A Prints 1 or 2, race detector reports data race

B Always prints 1, no race detected

C Compilation error due to missing synchronization

D Always prints 2, no race detected

💡 Hint

Two goroutines write to the same variable without synchronization.

The variable x is written concurrently without locks, causing a data race detected by the race detector.

❓ Predict Output

advanced

Effect of closing a channel twice

What happens when this Go program runs?

Go

package main
func main() {
    ch := make(chan int)
    close(ch)
    close(ch)
}

A Compilation error

B Deadlock error

C Panic: close of closed channel

D Program exits normally

💡 Hint

Closing a channel twice is not allowed in Go.

Closing a channel that is already closed causes a runtime panic.

❓ Predict Output

expert

Output of select with nil channel

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var ch chan int // nil channel
    select {
    case ch <- 1:
        fmt.Println("sent 1")
    case <-ch:
        fmt.Println("received")
    default:
        fmt.Println("default case")
    }
}

A default case

B sent 1

C received

D Deadlock

💡 Hint

Operations on nil channels block forever unless default case is present.

Both send and receive on nil channel block, so select chooses default immediately.

Practice

1. What is the main purpose of using recover() in Go programs?

easy

A. To declare a new variable

B. To start a new goroutine

C. To catch a panic and prevent the program from crashing

D. To handle normal errors returned by functions

Solution

1. Step 1: Understand panic and recover

   Panic causes a program to stop running immediately unless recovered.

2. Step 2: Role of recover()

   Recover is used inside a deferred function to catch the panic and allow the program to continue.

3. Final Answer:

   To catch a panic and prevent the program from crashing -> Option C

4. Quick Check:

   recover() catches panic = C [OK]

Hint: recover() only works inside deferred functions [OK]

Common Mistakes:

• Thinking recover() handles normal errors
• Using recover() outside defer
• Confusing recover() with panic()

2. Which of the following is the correct syntax to defer a function call in Go?

easy

A. defer fmt.Println("Hello")

B. defer = fmt.Println("Hello")

C. defer: fmt.Println("Hello")

D. defer -> fmt.Println("Hello")

Solution

1. Step 1: Recall defer syntax

   In Go, defer is a keyword placed before a function call to delay its execution.

2. Step 2: Check each option

   Only defer fmt.Println("Hello") uses 'defer' correctly before the function call without extra symbols.

3. Final Answer:

   defer fmt.Println("Hello") -> Option A

4. Quick Check:

   defer + function call = D [OK]

Hint: defer must be followed directly by a function call [OK]

Common Mistakes:

• Adding '=' after defer
• Using ':' or '->' after defer
• Not calling a function after defer

3. What will be the output of this Go program?

package main
import "fmt"
func main() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered from panic:", r)
        }
    }()
    fmt.Println("Start")
    panic("Something went wrong")
    fmt.Println("End")
}

medium

A. Start
panic: Something went wrong

B. Start
End
Recovered from panic: Something went wrong

C. Start
End

D. Start
Recovered from panic: Something went wrong

Solution

1. Step 1: Trace program execution

   "Start" is printed first. Then panic is called, which triggers the deferred function.

2. Step 2: Deferred function recovers panic

   The deferred anonymous function calls recover(), catches the panic, and prints the recovery message. Execution does not continue after panic call, so "End" is not printed.

3. Final Answer:

   Start Recovered from panic: Something went wrong -> Option D

4. Quick Check:

   panic caught by recover = A [OK]

Hint: Deferred recover runs on panic, then execution continues after panic [OK]

Common Mistakes:

• Thinking code after panic does not run
• Missing recover() inside defer
• Confusing panic message with normal print

4. Identify the error in this Go code that tries to recover from a panic:

package main
import "fmt"
func main() {
    if r := recover(); r != nil {
        fmt.Println("Recovered:", r)
    }
    panic("Oops")
}

medium

A. recover() must be called inside a deferred function

B. panic() must be called before recover()

C. recover() cannot be used with panic()

D. fmt.Println() cannot be used inside if statement

Solution

1. Step 1: Understand recover usage

   recover() only works if called inside a deferred function; here it is called directly in main.

2. Step 2: Analyze code flow

   Since recover() is not deferred, it cannot catch the panic, so the program will crash.

3. Final Answer:

   recover() must be called inside a deferred function -> Option A

4. Quick Check:

   recover inside defer = B [OK]

Hint: Always call recover() inside defer to catch panics [OK]

Common Mistakes:

• Calling recover() directly without defer
• Thinking recover() catches errors, not panics
• Placing panic before recover without defer

5. You want to write a stable Go program that logs a message when a panic occurs but continues running. Which code snippet correctly implements this behavior?

hard

A. func main() {
if r := recover(); r != nil {
fmt.Println("Panic caught:", r)
}
panic("Crash")
fmt.Println("Program continues")
}

B. func main() {
defer func() {
if r := recover(); r != nil {
fmt.Println("Panic caught:", r)
}
}()
panic("Crash")
fmt.Println("Program continues")
}

C. func main() {
panic("Crash")
defer func() {
if r := recover(); r != nil {
fmt.Println("Panic caught:", r)
}
}()
fmt.Println("Program continues")
}

D. func main() {
defer fmt.Println("Program continues")
panic("Crash")
}

Solution

1. Step 1: Check defer and recover placement

   func main() { defer func() { if r := recover(); r != nil { fmt.Println("Panic caught:", r) } }() panic("Crash") fmt.Println("Program continues") } correctly defers a function that calls recover() to catch panic and print a message.

2. Step 2: Verify program flow

   After panic, deferred recover runs, preventing crash and allowing program to continue, executing code after panic.

3. Final Answer:

   Uses defer func() with recover() before panic("Crash") -> Option B

4. Quick Check:

   defer + recover to catch panic = A [OK]

Hint: Defer recover before panic to handle it safely [OK]

Common Mistakes:

• Calling recover() outside defer
• Placing defer after panic
• Not handling panic at all