container/init: handle unwrapped errors

This is much cleaner from both the return statement and the error handling.

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

container/errors.go

@@ -6,6 +6,49 @@ import (
 	"syscall"
 )
 
+// messageFromError returns a printable error message for a supported concrete type.
+func messageFromError(err error) (string, bool) {
+	if m, ok := messagePrefixP[MountError, *MountError]("cannot ", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefixP[os.PathError, *os.PathError]("cannot ", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefixP[AbsoluteError, *AbsoluteError]("", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefix[OpRepeatError]("", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefix[OpStateError]("", err); ok {
+		return m, ok
+	}
+
+	return zeroString, false
+}
+
+// messagePrefix checks and prefixes the error message of a non-pointer error.
+// While this is usable for pointer errors, such use should be avoided as nil check is omitted.
+func messagePrefix[T error](prefix string, err error) (string, bool) {
+	var targetError T
+	if errors.As(err, &targetError) {
+		return prefix + targetError.Error(), true
+	}
+	return zeroString, false
+}
+
+// messagePrefixP checks and prefixes the error message of a pointer error.
+func messagePrefixP[V any, T interface {
+	*V
+	error
+}](prefix string, err error) (string, bool) {
+	var targetError T
+	if errors.As(err, &targetError) && targetError != nil {
+		return prefix + targetError.Error(), true
+	}
+	return zeroString, false
+}
+
 type MountError struct {
 	Source, Target, Fstype string
 

container/errors_test.go

@@ -8,6 +8,52 @@ import (
 	"testing"
 )
 
+func TestMessageFromError(t *testing.T) {
+	testCases := []struct {
+		name   string
+		err    error
+		want   string
+		wantOk bool
+	}{
+		{"mount", &MountError{
+			Source: SourceTmpfsEphemeral,
+			Target: "/sysroot/tmp",
+			Fstype: FstypeTmpfs,
+			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
+			Data:   zeroString,
+			Errno:  syscall.EINVAL,
+		}, "cannot mount tmpfs on /sysroot/tmp: invalid argument", true},
+
+		{"path", &os.PathError{
+			Op:   "mount",
+			Path: "/sysroot",
+			Err:  errUnique,
+		}, "cannot mount /sysroot: unique error injected by the test suite", true},
+
+		{"absolute", &AbsoluteError{"etc/mtab"},
+			`path "etc/mtab" is not absolute`, true},
+
+		{"repeat", OpRepeatError("autoetc"),
+			"autoetc is not repeatable", true},
+
+		{"state", OpStateError("overlay"),
+			"impossible overlay state reached", true},
+
+		{"unsupported", errUnique, zeroString, false},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, ok := messageFromError(tc.err)
+			if got != tc.want {
+				t.Errorf("messageFromError: %q, want %q", got, tc.want)
+			}
+			if ok != tc.wantOk {
+				t.Errorf("messageFromError: ok = %v, want %v", ok, tc.wantOk)
+			}
+		})
+	}
+}
+
 func TestMountError(t *testing.T) {
 	testCases := []struct {
 		name  string

container/init.go

@@ -184,12 +184,16 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 		}
 
 		if err := op.early(state, k); err != nil {
+			if m, ok := messageFromError(err); ok {
+				k.fatal(m)
+			} else {
 				k.printBaseErr(err,
 					fmt.Sprintf("cannot prepare op at index %d:", i))
 				k.beforeExit()
 				k.exit(1)
 			}
 		}
+	}
 
 	if err := k.mount(SourceTmpfsRootfs, intermediateHostPath, FstypeTmpfs, MS_NODEV|MS_NOSUID, zeroString); err != nil {
 		k.fatalf("cannot mount intermediate root: %v", err)
@@ -224,12 +228,16 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 		// ops already checked during early setup
 		k.verbosef("%s %s", op.prefix(), op)
 		if err := op.apply(state, k); err != nil {
+			if m, ok := messageFromError(err); ok {
+				k.fatal(m)
+			} else {
 				k.printBaseErr(err,
 					fmt.Sprintf("cannot apply op at index %d:", i))
 				k.beforeExit()
 				k.exit(1)
 			}
 		}
+	}
 
 	// setup requiring host root complete at this point
 	if err := k.mount(hostDir, hostDir, zeroString, MS_SILENT|MS_REC|MS_PRIVATE, zeroString); err != nil {