Will `defer` functions execute after a `log.Fatalln()` call in Go?

Deferred Functions and log.Fatalln() Calls

Problem:

Consider the following Go code:

db, err := sql.Open("postgres", "…")
if err != nil {
    log.Fatalln(err)
}
defer db.Close()

tpl, err := template.ParseGlob("")
if err != nil {
    log.Fatalln(err)
}

If template.ParseGlob("") returns an error, will the db.Close() function be called?

Answer:

No, the deferred functions will not be executed in this scenario.

Explanation:

log.Fatalln() is a logging function that prints an error message and then calls os.Exit(1) to terminate the program immediately. Here's a breakdown:

• log.Fatalln() is equivalent to log.Print() followed by os.Exit(1).
• os.Exit() causes the program to exit with the specified status code (in this case, 1 for failure).
• Deferred functions (such as db.Close()) are not run when os.Exit() is called.

In the code snippet, if the call to template.ParseGlob("") fails and log.Fatalln() is triggered, the program will exit immediately. As a result, the deferred function db.Close() will not be executed, potentially leaving the database connection open.

Demonstration:

To illustrate this behavior, the following code can be executed:

package main

import (
    "log"
    "os"
)

func main() {
    defer os.Exit(1) // Exit the program with status code 1
    log.Fatalln("Program terminated early")
}

Running this code will result in the error message being logged and the program exiting immediately, without any deferred functions being executed.

Avoidance:

To avoid issues with improperly closed resources or unhandled tasks, consider using log.Println() for error logging and handling errors gracefully without terminating the program.

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