container/vfs: wrap decoder errors

This passes line information and handles strconv errors so it reads better.

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

container/mount.go

@@ -149,7 +149,7 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 	return p.mountinfo(func(d *vfs.MountInfoDecoder) error {
 		n, err := d.Unfold(targetKFinal)
 		if err != nil {
-			if errors.Is(err, ESTALE) {
+			if errors.As(err, new(vfs.UnfoldTargetError)) {
 				return msg.WrapErr(err,
 					fmt.Sprintf("mount point %q never appeared in mountinfo", targetKFinal))
 			}

container/mount_test.go

@@ -109,7 +109,7 @@ func TestRemount(t *testing.T) {
 			{"close", expectArgs{0xdeadbeef}, nil, errUnique},
 		}}, errUnique},
 
-		{"mountinfo stale", func(k syscallDispatcher) error {
+		{"mountinfo no match", func(k syscallDispatcher) error {
 			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
 		}, [][]kexpect{{
 			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/.hakurei", nil},
@@ -118,7 +118,7 @@ func TestRemount(t *testing.T) {
 			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/.hakurei", nil},
 			{"close", expectArgs{0xdeadbeef}, nil, nil},
 			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-		}}, msg.WrapErr(syscall.ESTALE, `mount point "/sysroot/.hakurei" never appeared in mountinfo`)},
+		}}, msg.WrapErr(&vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/sysroot/.hakurei")}, `mount point "/sysroot/.hakurei" never appeared in mountinfo`)},
 
 		{"mountinfo", func(k syscallDispatcher) error {
 			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
@@ -128,7 +128,7 @@ func TestRemount(t *testing.T) {
 			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
 			{"close", expectArgs{0xdeadbeef}, nil, nil},
 			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile("\x00"), nil},
-		}}, wrapErrSuffix(vfs.ErrMountInfoFields, `cannot parse mountinfo:`)},
+		}}, wrapErrSuffix(&vfs.DecoderError{Op: "parse", Line: 0, Err: vfs.ErrMountInfoFields}, `cannot parse mountinfo:`)},
 
 		{"mount", func(k syscallDispatcher) error {
 			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)

container/path_test.go

@@ -248,7 +248,7 @@ func TestProcPaths(t *testing.T) {
 					t.Fatalf("WriteFile: error = %v", err)
 				}
 
-				wantErr := wrapErrSuffix(vfs.ErrMountInfoFields, "cannot parse mountinfo:")
+				wantErr := wrapErrSuffix(&vfs.DecoderError{Op: "parse", Line: 0, Err: vfs.ErrMountInfoFields}, "cannot parse mountinfo:")
 				if err := newProcPaths(direct{}, tempDir).mountinfo(func(d *vfs.MountInfoDecoder) error { return d.Decode(new(*vfs.MountInfo)) }); !errors.Is(err, wantErr) {
 					t.Fatalf("mountinfo: error = %v, want %v", err, wantErr)
 				}

container/vfs/mountinfo.go

@@ -24,6 +24,32 @@ var (
 	ErrMountInfoSep    = errors.New("bad optional fields separator")
 )
 
+type DecoderError struct {
+	Op   string
+	Line int
+	Err  error
+}
+
+func (e *DecoderError) Unwrap() error { return e.Err }
+func (e *DecoderError) Error() string {
+	var s string
+
+	var numError *strconv.NumError
+	switch {
+	case errors.As(e.Err, &numError) && numError != nil:
+		s = "numeric field " + strconv.Quote(numError.Num) + " " + numError.Err.Error()
+
+	default:
+		s = e.Err.Error()
+	}
+
+	var atLine string
+	if e.Line >= 0 {
+		atLine = " at line " + strconv.Itoa(e.Line)
+	}
+	return e.Op + " mountinfo" + atLine + ": " + s
+}
+
 type (
 	// A MountInfoDecoder reads and decodes proc_pid_mountinfo(5) entries from an input stream.
 	MountInfoDecoder struct {
@@ -32,6 +58,7 @@ type (
 
 		current  *MountInfo
 		parseErr error
+		curLine  int
 		complete bool
 	}
 
@@ -132,9 +159,12 @@ func (d *MountInfoDecoder) Entries() iter.Seq[*MountInfoEntry] {
 
 func (d *MountInfoDecoder) Err() error {
 	if err := d.s.Err(); err != nil {
-		return err
+		return &DecoderError{"scan", d.curLine, err}
 	}
-	return d.parseErr
+	if d.parseErr != nil {
+		return &DecoderError{"parse", d.curLine, d.parseErr}
+	}
+	return nil
 }
 
 func (d *MountInfoDecoder) scan() bool {
@@ -160,6 +190,7 @@ func (d *MountInfoDecoder) scan() bool {
 		d.current.Next = m
 		d.current = d.current.Next
 	}
+	d.curLine++
 	return true
 }
 

container/vfs/mountinfo_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"iter"
+	"os"
 	"path"
 	"reflect"
 	"slices"
@@ -15,62 +16,102 @@ import (
 	"hakurei.app/container/vfs"
 )
 
+func TestDecoderError(t *testing.T) {
+	testCases := []struct {
+		name    string
+		err     *vfs.DecoderError
+		want    string
+		target  error
+		targetF error
+	}{
+		{"errno", &vfs.DecoderError{Op: "parse", Line: 0xdeadbeef, Err: syscall.ENOTRECOVERABLE},
+			"parse mountinfo at line 3735928559: state not recoverable", syscall.ENOTRECOVERABLE, syscall.EROFS},
+
+		{"strconv", &vfs.DecoderError{Op: "parse", Line: 0xdeadbeef, Err: &strconv.NumError{Func: "Atoi", Num: "meow", Err: strconv.ErrSyntax}},
+			`parse mountinfo at line 3735928559: numeric field "meow" invalid syntax`, strconv.ErrSyntax, os.ErrInvalid},
+
+		{"unfold", &vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/proc/nonexistent")},
+			"unfold mountinfo: mount point /proc/nonexistent never appeared in mountinfo", vfs.UnfoldTargetError("/proc/nonexistent"), os.ErrNotExist},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Run("error", func(t *testing.T) {
+				if got := tc.err.Error(); got != tc.want {
+					t.Errorf("Error: %s, want %s", got, tc.want)
+				}
+			})
+
+			t.Run("is", func(t *testing.T) {
+				if !errors.Is(tc.err, tc.target) {
+					t.Errorf("Is: unexpected false")
+				}
+				if errors.Is(tc.err, tc.targetF) {
+					t.Errorf("Is: unexpected true")
+				}
+			})
+		})
+	}
+}
+
 func TestMountInfo(t *testing.T) {
 	testCases := []mountInfoTest{
 		{"count", sampleMountinfoBase + `
 21 20 0:53/ /mnt/test rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoFields, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoFields},
+			"", nil, nil, nil},
 
 		{"sep", sampleMountinfoBase + `
 21 20 0:53 / /mnt/test rw,relatime shared:212 _ tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoSep, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoSep},
+			"", nil, nil, nil},
 
 		{"id", sampleMountinfoBase + `
 id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			strconv.ErrSyntax, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: &strconv.NumError{Func: "Atoi", Num: "id", Err: strconv.ErrSyntax}},
+			"", nil, nil, nil},
 
 		{"parent", sampleMountinfoBase + `
 21 parent 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			strconv.ErrSyntax, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: &strconv.NumError{Func: "Atoi", Num: "parent", Err: strconv.ErrSyntax}}, "", nil, nil, nil},
 
 		{"devno", sampleMountinfoBase + `
 21 20 053 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "unexpected EOF", nil, nil, nil},
+			nil, "parse mountinfo at line 6: unexpected EOF", nil, nil, nil},
 
 		{"maj", sampleMountinfoBase + `
 21 20 maj:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "expected integer", nil, nil, nil},
+			nil, "parse mountinfo at line 6: expected integer", nil, nil, nil},
 
 		{"min", sampleMountinfoBase + `
 21 20 0:min / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "expected integer", nil, nil, nil},
+			nil, "parse mountinfo at line 6: expected integer", nil, nil, nil},
 
 		{"mountroot", sampleMountinfoBase + `
 21 20 0:53  /mnt/test rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},
 
 		{"target", sampleMountinfoBase + `
 21 20 0:53 /  rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},
 
 		{"vfs options", sampleMountinfoBase + `
 21 20 0:53 / /mnt/test  - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},
 
 		{"FS type", sampleMountinfoBase + `
-21 20 0:53 / /mnt/test rw,relatime -   rw
+21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755
-21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
+21 20 0:53 / /mnt/test rw,relatime -   rw`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 7, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},
 
 		{"base", sampleMountinfoBase, nil, "", []*wantMountInfo{
 			m(15, 20, 0, 3, "/", "/proc", "rw,relatime", o(), "proc", "/proc", "rw", syscall.MS_RELATIME, nil),
@@ -266,9 +307,9 @@ func (tc *mountInfoTest) check(t *testing.T, d *vfs.MountInfoDecoder, funcName s
 		})
 	} else if tc.wantNode != nil || tc.wantCollectF != nil {
 		panic("invalid test case")
-	} else if _, err := d.Unfold("/"); !errors.Is(err, tc.wantErr) {
+	} else if _, err := d.Unfold("/"); !reflect.DeepEqual(err, tc.wantErr) {
 		if tc.wantError == "" {
-			t.Errorf("Unfold: error = %v, wantErr %v",
+			t.Errorf("Unfold: error = %#v, wantErr %#v",
 				err, tc.wantErr)
 		} else if err != nil && err.Error() != tc.wantError {
 			t.Errorf("Unfold: error = %q, wantError %q",
@@ -276,9 +317,9 @@ func (tc *mountInfoTest) check(t *testing.T, d *vfs.MountInfoDecoder, funcName s
 		}
 	}
 
-	if err := gotErr(); !errors.Is(err, tc.wantErr) {
+	if err := gotErr(); !reflect.DeepEqual(err, tc.wantErr) {
 		if tc.wantError == "" {
-			t.Errorf("%s: error = %v, wantErr %v",
+			t.Errorf("%s: error = %#v, wantErr %#v",
 				funcName, err, tc.wantErr)
 		} else if err != nil && err.Error() != tc.wantError {
 			t.Errorf("%s: error = %q, wantError %q",

container/vfs/unfold.go

@@ -4,9 +4,14 @@ import (
 	"iter"
 	"path"
 	"strings"
-	"syscall"
 )
 
+type UnfoldTargetError string
+
+func (e UnfoldTargetError) Error() string {
+	return "mount point " + string(e) + " never appeared in mountinfo"
+}
+
 // MountInfoNode positions a [MountInfoEntry] in its mount hierarchy.
 type MountInfoNode struct {
 	*MountInfoEntry
@@ -65,7 +70,8 @@ func (d *MountInfoDecoder) Unfold(target string) (*MountInfoNode, error) {
 	}
 
 	if targetIndex == -1 {
-		return nil, syscall.ESTALE
+		// target does not exist in parsed mountinfo
+		return nil, &DecoderError{Op: "unfold", Line: -1, Err: UnfoldTargetError(targetClean)}
 	}
 
 	for _, cur := range mountinfo {

container/vfs/unfold_test.go

@@ -1,11 +1,9 @@
 package vfs_test
 
 import (
-	"errors"
 	"reflect"
 	"slices"
 	"strings"
-	"syscall"
 	"testing"
 
 	"hakurei.app/container/vfs"
@@ -26,7 +24,7 @@ func TestUnfold(t *testing.T) {
 			"no match",
 			sampleMountinfoBase,
 			"/mnt",
-			syscall.ESTALE, nil, nil, nil,
+			&vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/mnt")}, nil, nil, nil,
 		},
 		{
 			"cover",
@@ -55,7 +53,7 @@ func TestUnfold(t *testing.T) {
 			d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
 			got, err := d.Unfold(tc.target)
 
-			if !errors.Is(err, tc.wantErr) {
+			if !reflect.DeepEqual(err, tc.wantErr) {
 				t.Errorf("Unfold: error = %v, wantErr %v",
 					err, tc.wantErr)
 			}