fmsg: implement suspend in writer

This removes the requirement to call fmsg.Exit on every exit path, and enables direct use of the "log" package. However, fmsg.BeforeExit is still encouraged when possible to catch exit on suspended output.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/init/main.go

@@ -2,6 +2,7 @@ package init0
 
 import (
 	"errors"
+	"log"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -24,17 +25,15 @@ const (
 func Main() {
 	// sharing stdout with shim
 	// USE WITH CAUTION
-	fmsg.SetPrefix("init")
+	fmsg.Prepare("init")
 
 	// setting this prevents ptrace
 	if err := internal.PR_SET_DUMPABLE__SUID_DUMP_DISABLE(); err != nil {
-		fmsg.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
-		panic("unreachable")
+		log.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
 	}
 
 	if os.Getpid() != 1 {
-		fmsg.Fatal("this process must run as pid 1")
-		panic("unreachable")
+		log.Fatal("this process must run as pid 1")
 	}
 
 	// receive setup payload
@@ -44,30 +43,29 @@ func Main() {
 	)
 	if f, err := proc.Receive(Env, &payload); err != nil {
 		if errors.Is(err, proc.ErrInvalid) {
-			fmsg.Fatal("invalid config descriptor")
+			log.Fatal("invalid config descriptor")
 		}
 		if errors.Is(err, proc.ErrNotSet) {
-			fmsg.Fatal("FORTIFY_INIT not set")
+			log.Fatal("FORTIFY_INIT not set")
 		}
 
-		fmsg.Fatalf("cannot decode init setup payload: %v", err)
-		panic("unreachable")
+		log.Fatalf("cannot decode init setup payload: %v", err)
 	} else {
-		fmsg.SetVerbose(payload.Verbose)
+		fmsg.Store(payload.Verbose)
 		closeSetup = f
 
 		// child does not need to see this
 		if err = os.Unsetenv(Env); err != nil {
-			fmsg.Printf("cannot unset %s: %v", Env, err)
+			log.Printf("cannot unset %s: %v", Env, err)
 			// not fatal
 		} else {
-			fmsg.VPrintln("received configuration")
+			fmsg.Verbose("received configuration")
 		}
 	}
 
 	// die with parent
 	if err := internal.PR_SET_PDEATHSIG__SIGKILL(); err != nil {
-		fmsg.Fatalf("prctl(PR_SET_PDEATHSIG, SIGKILL): %v", err)
+		log.Fatalf("prctl(PR_SET_PDEATHSIG, SIGKILL): %v", err)
 	}
 
 	cmd := exec.Command(payload.Argv0)
@@ -76,13 +74,13 @@ func Main() {
 	cmd.Env = os.Environ()
 
 	if err := cmd.Start(); err != nil {
-		fmsg.Fatalf("cannot start %q: %v", payload.Argv0, err)
+		log.Fatalf("cannot start %q: %v", payload.Argv0, err)
 	}
 	fmsg.Suspend()
 
 	// close setup pipe as setup is now complete
 	if err := closeSetup(); err != nil {
-		fmsg.Println("cannot close setup pipe:", err)
+		log.Println("cannot close setup pipe:", err)
 		// not fatal
 	}
 
@@ -119,7 +117,7 @@ func Main() {
 			}
 		}
 		if !errors.Is(err, syscall.ECHILD) {
-			fmsg.Println("unexpected wait4 response:", err)
+			log.Println("unexpected wait4 response:", err)
 		}
 
 		close(done)
@@ -132,9 +130,12 @@ func Main() {
 	for {
 		select {
 		case s := <-sig:
-			fmsg.VPrintln("received", s.String())
-			fmsg.Resume() // output could still be withheld at this point, so resume is called
-			fmsg.Exit(0)
+			if fmsg.Resume() {
+				fmsg.Verbosef("terminating on %s after process start", s.String())
+			} else {
+				fmsg.Verbosef("terminating on %s", s.String())
+			}
+			internal.Exit(0)
 		case w := <-info:
 			if w.wpid == cmd.Process.Pid {
 				// initial process exited, output is most likely available again
@@ -155,10 +156,10 @@ func Main() {
 				}()
 			}
 		case <-done:
-			fmsg.Exit(r)
+			internal.Exit(r)
 		case <-timeout:
-			fmsg.Println("timeout exceeded waiting for lingering processes")
-			fmsg.Exit(r)
+			log.Println("timeout exceeded waiting for lingering processes")
+			internal.Exit(r)
 		}
 	}
 }