Explain Channel Behavior: Nil, Closed, and Blocking

Problem Statement

During a Google SRE interview, you're asked to explain the nuanced behavior of Go channels. A production outage was caused by a goroutine leak due to improper channel handling. Walk through all channel edge cases.

Channel Behavior Matrix

Operation	Nil Channel	Closed Channel	Open Channel
ch <- value	Blocks forever	PANIC	Sends or blocks
<-ch	Blocks forever	Returns zero value + false	Receives or blocks
close(ch)	PANIC	PANIC	Closes successfully

The Goroutine Leak Scenario

package main

import (
    "fmt"
    "runtime"
    "time"
)

func leakyFunction() {
    ch := make(chan int)
    
    go func() {
        // This goroutine will block forever
        // because no one ever sends to ch
        val := <-ch
        fmt.Println("Received:", val)
    }()
    
    // Function returns, but goroutine is stuck
}

func main() {
    for i := 0; i < 1000; i++ {
        leakyFunction()
    }
    
    time.Sleep(time.Second)
    fmt.Printf("Goroutines: %d\n", runtime.NumGoroutine())
    // Output: Goroutines: 1001 (leaked!)
}

The Fix: Context-Based Cancellation

func properFunction(ctx context.Context) {
    ch := make(chan int)
    
    go func() {
        select {
        case val := <-ch:
            fmt.Println("Received:", val)
        case <-ctx.Done():
            fmt.Println("Cancelled, exiting goroutine")
            return
        }
    }()
}

Interview Questions

1. Q: What happens if you read from a nil channel?
   A: It blocks forever. This is actually useful in select statements to disable a case.
2. Q: Why would you intentionally use a nil channel?
   A: To dynamically disable a select case without removing the code.
3. Q: How do you detect a closed channel?
   A: Use the two-value receive: val, ok := <-ch. If ok is false, channel is closed.

Real-World Production Issue

At Twitch, a service leaked 50,000 goroutines because websocket handlers were waiting on a channel that was never closed when clients disconnected. The fix: Always use context cancellation for cleanup.