pkg/eval: Test all branches of ExternalCmdExit.Error.

This addresses #944.

pkg/eval/exception_unix_test.go

@@ -3,6 +3,8 @@
 package eval
 
 import (
+	"fmt"
+	"syscall"
 	"testing"
 
 	"github.com/elves/elvish/pkg/tt"
@@ -10,6 +12,20 @@ import (
 
 func TestExternalCmdExit_Error(t *testing.T) {
 	tt.Test(t, tt.Fn("Error", error.Error), tt.Table{
-		tt.Args(ExternalCmdExit{0, "ls", 100}).Rets("ls exited with 0"),
+		tt.Args(ExternalCmdExit{0x0, "ls", 1}).Rets("ls exited with 0"),
+		tt.Args(ExternalCmdExit{0x100, "ls", 1}).Rets("ls exited with 1"),
+		// Note: all Unix'es have SIGINT = 2 and the syscall package has same
+		// string for it ("interrupt").
+		tt.Args(ExternalCmdExit{0x2, "ls", 1}).Rets("ls killed by signal interrupt"),
+		// 0x80 + signal for core dumped
+		tt.Args(ExternalCmdExit{0x82, "ls", 1}).Rets("ls killed by signal interrupt (core dumped)"),
+		// 0x7f + signal<<8 for stopped
+		tt.Args(ExternalCmdExit{0x27f, "ls", 1}).Rets("ls stopped by signal interrupt (pid=1)"),
+		// 0x057f + cause<<16 for trapped. SIGTRAP is 5 on all Unix'es but have
+		// different string representations on different OSes.
+		tt.Args(ExternalCmdExit{0x1057f, "ls", 1}).Rets(fmt.Sprintf(
+			"ls stopped by signal %s (pid=1) (trapped 1)", syscall.SIGTRAP)),
+		// 0xff is the only exit code that is not exited, signaled or stopped.
+		tt.Args(ExternalCmdExit{0xff, "ls", 1}).Rets("ls has unknown WaitStatus 255"),
 	})
 }