sandbox/vfs: expose mountinfo line scanning

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

sandbox/vfs/mountinfo.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"iter"
 	"strconv"
 	"strings"
 	"syscall"
@@ -18,13 +19,23 @@ var (
 )
 
 type (
-	// MountInfo represents a /proc/pid/mountinfo document.
+	// A MountInfoDecoder reads and decodes proc_pid_mountinfo(5) entries from an input stream.
+	MountInfoDecoder struct {
+		s *bufio.Scanner
+		m *MountInfo
+
+		current  *MountInfo
+		parseErr error
+		complete bool
+	}
+
+	// MountInfo represents the contents of a proc_pid_mountinfo(5) document.
 	MountInfo struct {
 		Next *MountInfo
 		MountInfoEntry
 	}
 
-	// MountInfoEntry represents a line in /proc/pid/mountinfo.
+	// MountInfoEntry represents a proc_pid_mountinfo(5) entry.
 	MountInfoEntry struct {
 		// mount ID: a unique ID for the mount (may be reused after umount(2)).
 		ID int `json:"id"`
@@ -77,95 +88,145 @@ func (e *MountInfoEntry) Flags() (flags uintptr, unmatched []string) {
 	return
 }
 
-// ParseMountInfo parses a mountinfo file according to proc_pid_mountinfo(5).
+// NewMountInfoDecoder returns a new decoder that reads from r.
-func ParseMountInfo(r io.Reader) (*MountInfo, int, error) {
+//
-	var m, cur *MountInfo
+// The decoder introduces its own buffering and may read data from r beyond the mountinfo entries requested.
-	s := bufio.NewScanner(r)
+func NewMountInfoDecoder(r io.Reader) *MountInfoDecoder {
-
+	return &MountInfoDecoder{s: bufio.NewScanner(r)}
-	var n int
+}
-	for s.Scan() {
+
-		n++
+func (d *MountInfoDecoder) Decode(v **MountInfo) (err error) {
-
+	for d.scan() {
-		if cur == nil {
+	}
-			m = new(MountInfo)
+	err = d.Err()
-			cur = m
+	if err == nil {
-		} else {
+		*v = d.m
-			cur.Next = new(MountInfo)
+	}
-			cur = cur.Next
+	return
-		}
+}
-
+
-		// prevent proceeding with misaligned fields due to optional fields
+// Entries returns an iterator over mountinfo entries.
-		f := strings.Split(s.Text(), " ")
+func (d *MountInfoDecoder) Entries() iter.Seq[*MountInfoEntry] {
-		if len(f) < 10 {
+	return func(yield func(*MountInfoEntry) bool) {
-			return nil, -1, ErrMountInfoFields
+		for cur := d.m; cur != nil; cur = cur.Next {
-		}
+			if !yield(&cur.MountInfoEntry) {
-
+				return
-		// 36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
+			}
-		// (1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)
+		}
-
+		for d.scan() {
-		// (1) id
+			if !yield(&d.current.MountInfoEntry) {
-		if id, err := strconv.Atoi(f[0]); err != nil { // 0
+				return
-			return nil, -1, err
+			}
-		} else {
+		}
-			cur.ID = id
+	}
-		}
+}
-
+
-		// (2) parent
+func (d *MountInfoDecoder) Err() error {
-		if parent, err := strconv.Atoi(f[1]); err != nil { // 1
+	if err := d.s.Err(); err != nil {
-			return nil, -1, err
+		return err
-		} else {
+	}
-			cur.Parent = parent
+	return d.parseErr
-		}
+}
-
+
-		// (3) maj:min
+func (d *MountInfoDecoder) scan() bool {
-		if n, err := fmt.Sscanf(f[2], "%d:%d", &cur.Devno[0], &cur.Devno[1]); err != nil {
+	if d.complete {
-			return nil, -1, err
+		return false
-		} else if n != 2 {
+	}
-			// unreachable
+	if !d.s.Scan() {
-			return nil, -1, ErrMountInfoDevno
+		d.complete = true
-		}
+		return false
-
+	}
-		// (4) mountroot
+
-		cur.Root = Unmangle(f[3])
+	m := new(MountInfo)
-		if cur.Root == "" {
+	if err := parseMountInfoLine(d.s.Text(), &m.MountInfoEntry); err != nil {
-			return nil, -1, ErrMountInfoEmpty
+		d.parseErr = err
-		}
+		d.complete = true
-
+		return false
-		// (5) target
+	}
-		cur.Target = Unmangle(f[4])
+
-		if cur.Target == "" {
+	if d.current == nil {
-			return nil, -1, ErrMountInfoEmpty
+		d.m = m
-		}
+		d.current = d.m
-
+	} else {
-		// (6) vfs options (fs-independent)
+		d.current.Next = m
-		cur.VfsOptstr = Unmangle(f[5])
+		d.current = d.current.Next
-		if cur.VfsOptstr == "" {
+	}
-			return nil, -1, ErrMountInfoEmpty
+	return true
-		}
+}
-
+
-		// (7) optional fields, terminated by " - "
+func parseMountInfoLine(s string, ent *MountInfoEntry) error {
-		i := len(f) - 4
+	// prevent proceeding with misaligned fields due to optional fields
-		cur.OptFields = f[6:i]
+	f := strings.Split(s, " ")
-
+	if len(f) < 10 {
-		// (8) optional fields end marker
+		return ErrMountInfoFields
-		if f[i] != "-" {
+	}
-			return nil, -1, ErrMountInfoSep
+
-		}
+	// 36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
-		i++
+	// (1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)
-
+
-		// (9) FS type
+	// (1) id
-		cur.FsType = Unmangle(f[i])
+	if id, err := strconv.Atoi(f[0]); err != nil { // 0
-		if cur.FsType == "" {
+		return err
-			return nil, -1, ErrMountInfoEmpty
+	} else {
-		}
+		ent.ID = id
-		i++
+	}
-
+
-		// (10) source -- maybe empty string
+	// (2) parent
-		cur.Source = Unmangle(f[i])
+	if parent, err := strconv.Atoi(f[1]); err != nil { // 1
-		i++
+		return err
-
+	} else {
-		// (11) fs options (fs specific)
+		ent.Parent = parent
-		cur.FsOptstr = Unmangle(f[i])
+	}
-	}
+
-	return m, n, s.Err()
+	// (3) maj:min
+	if n, err := fmt.Sscanf(f[2], "%d:%d", &ent.Devno[0], &ent.Devno[1]); err != nil {
+		return err
+	} else if n != 2 {
+		// unreachable
+		return ErrMountInfoDevno
+	}
+
+	// (4) mountroot
+	ent.Root = Unmangle(f[3])
+	if ent.Root == "" {
+		return ErrMountInfoEmpty
+	}
+
+	// (5) target
+	ent.Target = Unmangle(f[4])
+	if ent.Target == "" {
+		return ErrMountInfoEmpty
+	}
+
+	// (6) vfs options (fs-independent)
+	ent.VfsOptstr = Unmangle(f[5])
+	if ent.VfsOptstr == "" {
+		return ErrMountInfoEmpty
+	}
+
+	// (7) optional fields, terminated by " - "
+	i := len(f) - 4
+	ent.OptFields = f[6:i]
+
+	// (8) optional fields end marker
+	if f[i] != "-" {
+		return ErrMountInfoSep
+	}
+	i++
+
+	// (9) FS type
+	ent.FsType = Unmangle(f[i])
+	if ent.FsType == "" {
+		return ErrMountInfoEmpty
+	}
+	i++
+
+	// (10) source -- maybe empty string
+	ent.Source = Unmangle(f[i])
+	i++
+
+	// (11) fs options (fs specific)
+	ent.FsOptstr = Unmangle(f[i])
+
+	return nil
 }

sandbox/vfs/mountinfo_test.go

@@ -2,6 +2,7 @@ package vfs_test
 
 import (
 	"errors"
+	"iter"
 	"reflect"
 	"slices"
 	"strconv"
@@ -122,57 +123,106 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			got, n, err := vfs.ParseMountInfo(strings.NewReader(tc.sample))
+			t.Run("decode", func(t *testing.T) {
-			if !errors.Is(err, tc.wantErr) {
+				var got *vfs.MountInfo
-				if tc.wantError == "" {
+				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
-					t.Errorf("ParseMountInfo: error = %v, wantErr %v",
+				err := d.Decode(&got)
-						err, tc.wantErr)
+				checkMountInfo(t, true,
-				} else if err != nil && err.Error() != tc.wantError {
+					tc.want, tc.wantErr, tc.wantError,
-					t.Errorf("ParseMountInfo: error = %q, wantError %q",
+					func(yield func(*vfs.MountInfoEntry) bool) {
-						err, tc.wantError)
+						for cur := got; cur != nil; cur = cur.Next {
-				}
+							if !yield(&cur.MountInfoEntry) {
-			}
+								return
+							}
+						}
+					}, func() error { return err })
+				t.Run("reuse", func(t *testing.T) {
+					checkMountInfo(t, false,
+						tc.want, tc.wantErr, tc.wantError,
+						d.Entries(), d.Err)
+				})
+			})
 
-			wantCount := len(tc.want)
+			t.Run("iter", func(t *testing.T) {
-			if tc.wantErr != nil || tc.wantError != "" {
+				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
-				wantCount = -1
+				checkMountInfo(t, false,
-			}
+					tc.want, tc.wantErr, tc.wantError,
-			if n != wantCount {
+					d.Entries(), d.Err)
-				t.Errorf("ParseMountInfo: got %d entries, want %d", n, wantCount)
-			}
 
-			i := 0
+				t.Run("reuse", func(t *testing.T) {
-			for cur := got; cur != nil; cur = cur.Next {
+					checkMountInfo(t, false,
-				if i == len(tc.want) {
+						tc.want, tc.wantErr, tc.wantError,
-					t.Errorf("ParseMountInfo: got more than %d entries", len(tc.want))
+						d.Entries(), d.Err)
-					break
+				})
-				}
+			})
 
-				if !reflect.DeepEqual(&cur.MountInfoEntry, &tc.want[i].MountInfoEntry) {
+			t.Run("yield", func(t *testing.T) {
-					t.Errorf("ParseMountInfo: entry %d\ngot:  %#v\nwant: %#v",
+				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
-						i, cur.MountInfoEntry, tc.want[i])
+				v := false
-				}
+				d.Entries()(func(entry *vfs.MountInfoEntry) bool { v = !v; return v })
+				d.Entries()(func(entry *vfs.MountInfoEntry) bool { return false })
 
-				flags, unmatched := cur.Flags()
+				checkMountInfo(t, false,
-				if flags != tc.want[i].flags {
+					tc.want, tc.wantErr, tc.wantError,
-					t.Errorf("Flags(%q): %#x, want %#x",
+					d.Entries(), d.Err)
-						cur.VfsOptstr, flags, tc.want[i].flags)
-				}
-				if !slices.Equal(unmatched, tc.want[i].unmatched) {
-					t.Errorf("Flags(%q): unmatched = %#q, want %#q",
-						cur.VfsOptstr, unmatched, tc.want[i].unmatched)
-				}
 
-				i++
+				t.Run("reuse", func(t *testing.T) {
-			}
+					checkMountInfo(t, false,
-
+						tc.want, tc.wantErr, tc.wantError,
-			if i != len(tc.want) {
+						d.Entries(), d.Err)
-				t.Errorf("ParseMountInfo: got %d entries, want %d", i, len(tc.want))
+				})
-			}
+			})
 		})
 	}
 }
 
+func checkMountInfo(t *testing.T, checkDecode bool,
+	want []*wantMountInfo, wantErr error, wantError string,
+	got iter.Seq[*vfs.MountInfoEntry], gotErr func() error) {
+	i := 0
+	for cur := range got {
+		if i == len(want) {
+			if !checkDecode && (wantErr != nil || wantError != "") {
+				continue
+			}
+
+			t.Errorf("ParseMountInfo: got more than %d entries", len(want))
+			break
+		}
+
+		if !reflect.DeepEqual(cur, &want[i].MountInfoEntry) {
+			t.Errorf("ParseMountInfo: entry %d\ngot:  %#v\nwant: %#v",
+				i, cur, want[i])
+		}
+
+		flags, unmatched := cur.Flags()
+		if flags != want[i].flags {
+			t.Errorf("Flags(%q): %#x, want %#x",
+				cur.VfsOptstr, flags, want[i].flags)
+		}
+		if !slices.Equal(unmatched, want[i].unmatched) {
+			t.Errorf("Flags(%q): unmatched = %#q, want %#q",
+				cur.VfsOptstr, unmatched, want[i].unmatched)
+		}
+
+		i++
+	}
+
+	if i != len(want) {
+		t.Errorf("ParseMountInfo: got %d entries, want %d", i, len(want))
+	}
+
+	if err := gotErr(); !errors.Is(err, wantErr) {
+		if wantError == "" {
+			t.Errorf("ParseMountInfo: error = %v, wantErr %v",
+				err, wantErr)
+		} else if err != nil && err.Error() != wantError {
+			t.Errorf("ParseMountInfo: error = %q, wantError %q",
+				err, wantError)
+		}
+	}
+}
+
 type wantMountInfo struct {
 	vfs.MountInfoEntry
 	flags     uintptr