Frédéric G. MARAND - 23 avril 2023 The end of execution (In a Go program) Devoxx FR 2023 1 

© 2023 OSInet The problem to solve 2 

© 2023 OSInet 3 Why we can’t have nice things What we want: ● defer / recover is a real nice Go pair of features ● Let’s use them both ! How Go breaks them: ● func main() doesn’t return a process exit code to the OS ● os.Exit() emits a process exit code, but skips defer. ● os.Exit(0) in SUT panics ● os.Exit(nonZero) in SUT kills the test 

© 2023 OSInet 4 How execution ends Functions / goroutines: ● return ○ Runs defer-red code ○ Terminates goroutine if called using go fn() ○ Does not set exit code in main ● panic ○ Prepare stack trace ○ Return (see above) ○ If not recovered, dump trace ● os.Exit, syscall.Exit, log.Fatal* ○ Exit without returning Goroutines: ● All userland goroutines ○ End of return ● os.Exit, syscall.Exit, log.Fatal* ○ No defer-red code 
 Exit without return-ing ● runtime.Goexit ○ Runs defer-red code ○ Ends goroutine ○ Does not handle #1 finishing program: deadlock Two separate notions: the call flow, and the scheduling (goroutine) 

© 2023 OSInet 5 os.Exit() vs defer-red code 

© 2023 OSInet 6 runtime.Goexit vs main() package main import ( "log" "runtime" ) func main() { defer func() { log.Println("Deferred code") }() log.Println("In main") runtime.Goexit() // DEADLOCK PANIC ! log.Println("Nothing after os.Exit") } 

© 2023 OSInet 7 os.Exit() in tests package problem3 import ( "log" "os" ) func Exit0() { log.Println("In Exit0") os.Exit(0) } func Exit1() { log.Println("In Exit1") os.Exit(1) } func TestExit(t *testing.T) { for _, test := range [...]struct { name string fn func() }{ {"Exit0", Exit0}, {"Exit1", Exit1}, } { t.Run(fmt.Sprintf("%#v", test.name), 
 func(t *testing.T) { defer func() { rec := recover() switch x := rec.(type) { case nil: t.Fatal("nothing recovered") case string: if !strings.Contains(x, 
 "unexpected call to os.Exit(0)") { t.Fatalf("unexpected string: %q", x) } t.Log("successful recovery") default: t.Fatalf("unexpected recovery: %#v", x) } }() test.fn() }) } } 

© 2023 OSInet 8 os.Exit() in tests: 0 vs 1 % go test -v -run TestExit/Exit0 === RUN TestExit === RUN TestExit/"Exit0" 2023/01/09 14:40:02 In Exit0 problem3_test.go:27: successful recovery --- PASS: TestExit (0.00s) --- PASS: TestExit/"Exit0" (0.00s) PASS ok code.osinet.fr/fgm/go__deferplus/step01-problem/problem3 0.107s 
 % go test -v -run TestExit/Exit1 === RUN TestExit === RUN TestExit/"Exit1" 2023/01/09 14:40:05 In Exit1 exit status 1 FAIL code.osinet.fr/fgm/go__deferplus/step01-problem/problem3 0.100s 

© 2023 OSInet 9 os.Exit() vs os.Exit(1) vs syscall.Exit(n) // Exit causes the current program to exit with the given status code. // Conventionally, code zero indicates success, non-zero an error. // The program terminates immediately; deferred functions are not run. // // For portability, the status code should be in the range [0, 125]. func Exit(code int) { if code == 0 { if testlog.PanicOnExit0() { // We were told to panic on calls to os.Exit(0). // This is used to fail tests that make an early // unexpected call to os.Exit(0). panic("unexpected call to os.Exit(0) during test") } // Give race detector a chance to fail the program. // Racy programs do not have the right to finish successfully. runtime_beforeExit() } syscall.Exit(code) } 

© 2023 OSInet Practical solutions 10 

© 2023 OSInet 11 Combining os.Exit() and defer package main import ( "log" "os" ) var someCondition = true func main() { var exitCode = 0 defer func() { log.Println("Deferred code") os.Exit(exitCode) }() log.Println("Dans main") if someCondition { // Instead of os.Exit(1) exitCode = 1 return } } 

© 2023 OSInet 12 A better way to exit from func main() package main import "os" func main() { os.Exit(int(realMain())) } // realMain is the actual main. Exit codes must be in [0, 125]. func realMain() byte { // Do whatever you want. Use defer freely. var someExitCode byte // Return will set the exit code, just like in C and many others. return someExitCode } 

© 2023 OSInet ● When running tests, log.Fatal* allows finishing the test early with a message ● The downside is that this uses os.Exit(), hence skipping defer execution ● A better equivalent is t.FailNow which uses the testing package’s own test termination mechanism, based on runtime.Goexit(), which allows defer to work after a failure, unlike log.Fatal*. 13 Keep defers: avoiding exit in tests 

© 2023 OSInet 14 Setup and cleanup in tests Using defer: ● setup functions ○ return their actual result ○ return a defer-rable func ● tests ○ call setup functions ○ need to defer each such defer-rable in order ○ defer-red run : ■ in the test scope ■ before the end of the test Using testing.Cleanup(): ● setup functions ○ return their actual result ○ call t.Cleanup with their teardown callback ● tests ○ call setup functions ○ no cleanup work needed ○ cleanups run : ■ in the setup scope ■ after the end of the test 

© 2023 OSInet 15 Testing an os.Exit-ing(0) function func TestExit(t *testing.T) { for _, test := range [...]struct { name string fn func() }{{"Exit0", Exit0},{"Exit1", Exit1}} { t.Run(fmt.Sprintf("%#v", test.name), 
 func(t *testing.T) { defer func() { rec := recover() switch x := rec.(type) { case nil: t.Fatal("nothing recovered") case string: if !strings.Contains(x, 
 "unexpected call to os.Exit(0)") { t.Fatalf("unexpected: %q", x) } t.Log("successful recovery") default: t.Fatalf("unexpected: %#v", x) } }() test.fn() }) } } As seen previously, this only works if the exit code is zero: otherwise in os.Exit, the value of PanicOnExit0 will not be considered. It might even be false with alternative test runners using internal.testlog.SetPanicOnExit 0 

© 2023 OSInet 16 Testing an os.Exit-ing(nonZero) func Method: ● Test creates a new command using the same test binary running itself ● Runs itself recursively, passing a flag to identify the recursive call ● Outer code receives the exit code of the recursive call Examples: ● https://blog.antoine-augusti.fr/ 2015/12/testing-an-os-exit- scenario-in-golang/ ● https://github.com/kohirens/ tmpltoapp/blob/ 46e536e50eaf55ea60d66bf815c 9a4d6f51d27b9/ main_test.go#L31 

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