What Happens to Unfinished Goroutines When the Main Go Routine Exits?

Unresolved Goroutines at Main Exit or Mirrored Query Return

Consider the code snippet from the Go programming language book, where goroutines are spawned to retrieve responses from multiple URLs. The main goroutine exits after receiving the quickest response from the responses channel. The question arises: what happens to the unfinished goroutines when the main or parent goroutine exits or returns?

Main Goroutine Exit

When the main goroutine exits under normal circumstances (returns), the entire runtime system shuts down. Consequently, any goroutines stuck waiting to send on an unbuffered or full channel abruptly cease to exist without being canceled, run, or wait. This is similar to flash paper being ignited.

One might refer to this as a goroutine leak, analogous to unclosed or unfreed resources in other contexts. However, since the entire process terminates, there is no true leak; the resources are not retained indefinitely. The system essentially does a quick cleanup.

Mirrored Query Return with Main Goroutine Running

If the main goroutine has not exited and the mirroredQuery() function has returned, the remaining goroutines continue their execution or waiting until they complete their tasks and return. This includes waiting for a response, sending it to the channel, and returning. Upon return, they are terminated and evaporate.

The channel itself remains in existence, holding the strings, which could be considered a minor resource leak. However, this leak is resolved when the last reference to the channel disappears, prompting the channel (and its contents) to be garbage collected.

Unbuffered Channel

Had the channel in the code snippet been unbuffered, the slower goroutines would block while attempting to send to the channel. This would prevent their termination, keeping the channel and resources allocated until the program ends. This situation would be undesirable.

Buffered Channel and Cleanest Code

Using a buffered channel in the code snippet is the simplest approach that ensures the desired functionality without potential errors or resource leaks. It allows the slower goroutines to finish their tasks and terminate gracefully.

Conclusion

The fate of unfinished goroutines at main exit is abrupt termination. When the mirrored query returns while the main goroutine is still running, the remaining goroutines finish their tasks and evaporate. Using a buffered channel resolves any potential resource leaks associated with unfinished goroutines in this context.

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