In this problem, you are given a process that runs indefinitely and blocks the program. The only way to stop the program is to send a SIGINT. But you should stop the process gracefully first. If you receive SIGINT again, then kill the process immediately.

Solution

Line:

1. This goroutine executes a blocking receive for the operating system signal.
2. Create a channel to receive os.Signal value. This channel should be buffered.
3. Register the channel to receive a notification. We use os.Interrupt instead of syscall.SIGINT here so Go can handle the correct interrupt on the platform it is running on (operating system Agnostic).
4. Block until we receive a signal.
5. Remove all signal handlers. The sig channel will no longer receive any signal. So the next time the user press Ctrl-C, it will follow the default behavior of signals in a Go program. In this case, the SIGINT signal causes the program to exit.
6. Gracefully ask proc to kill itself.

Key takeaways

A little background

In a Unix-based operating system, processes can receive a variety of signals. Signals are the method of communication between processes. There are a lot of signals. For example we have SIGTERM, SIGQUIT, SIGKILL, etc.

When a process receives a signal, it interrupts its execution, and a signal handler is invoked. Every signal has a default action: ignore, kill the process, kill the process and make a core dump file, stop the process, and resume operation.

1. SIGINT is the signal sent when you press Ctrl+C. The default action for the signal is to kill the process.
2. SIGTERM and SIGQUIT are used to terminate a process. SIGTERM is the default signal when we use the kill command. Sometimes, when we send SIGTERM, the process does a clean-up routine before exiting.
3. SIGKILL is also used to terminate a process. SIGKILL can't be ignored and can not be changed by a program. So your Go application will not handle this signal. The process that receives SIGKILL might not execute any clean-up routine.

Go os and os/signal packages

You can use the os package to work with the operating system. This package provides a platform-independent interface.

You can use the os/signal package to handle the operating system signal. This package implements access to incoming signals. Signals are primarly used on Unix-like systems. But you can still use this on Windows. According to the documentation:

If Notify is called for os.Interrupt, ^C or ^BREAK will cause os.Interrupt to be sent on the channel, and the program will not exit.

How to capture signal pattern in Go

Create a goroutine that runs a blocking for os.Signal, where you:

1. Create a buffered channel.
2. Use signal.Notify to register what signal you want to capture and specify the channel that receive the signal from step 1.
3. Block for the signal to be received.
4. Do what you need to do with the signal.