interfaces/mount: WIP add mount.{Resolve,AliasesOf}

This patch adds a pair of functions that aim to assist in implementation
of Change.Needed.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

interfaces/mount/mountinfo.go

@@ -24,6 +24,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
+	"sort"
 	"strconv"
 	"strings"
 )
@@ -170,3 +172,113 @@ func parseMountOpts(opts string) map[string]string {
 	}
 	return result
 }
+
+func formatMountOpts(opts map[string]string) string {
+	flat := make([]string, 0, len(opts))
+	for key, value := range opts {
+		if value != "" {
+			flat = append(flat, fmt.Sprintf("%s=%s", key, value))
+		} else {
+			flat = append(flat, key)
+		}
+	}
+	sort.Strings(flat)
+	return strings.Join(flat, ",")
+}
+
+// resolve finds the mountinfo entry and a subdirectory therein to which a given absolute directory belongs to.
+func resolve(dir string, mountinfo []*InfoEntry) (subdir string, entry *InfoEntry, err error) {
+	dir = path.Clean(dir)
+	// Precondition: dir is an absolute path.
+	if !path.IsAbs(dir) {
+		return "", nil, fmt.Errorf("cannot resolve relative directory %q", dir)
+	}
+	//
+	// Precondition: there is something mounted on the root directory.
+	var hasRoot bool
+	for _, mie := range mountinfo {
+		if mie.MountDir == "/" {
+			hasRoot = true
+			break
+		}
+	}
+	if !hasRoot {
+		return "", nil, fmt.Errorf("cannot resolve directory %q: root directory is not mounted", dir)
+	}
+
+	// This loop is guaranteed to terminate because we eventually look for "/"
+	// which is guaranteed to be in mountinfo by the precondition above.
+	var foundEntry *InfoEntry
+	var rfrag, frag []string
+	search := dir
+	for foundEntry == nil {
+		// We have to iterate in reverse here because if more than one element
+		// is mounted at the same place then the kernel will list them in the
+		// order the operations were performed.
+		for i := len(mountinfo) - 1; i >= 0; i-- {
+			mie := mountinfo[i]
+			if search == mie.MountDir {
+				foundEntry = mie
+				break
+			}
+		}
+		if foundEntry == nil {
+			rfrag = append(rfrag, path.Base(search))
+			search = path.Dir(search)
+		}
+	}
+
+	// rfrag is reversed, let's correct that and re-construct the path.
+	frag = make([]string, 0, len(rfrag)+1)
+	frag = append(frag, foundEntry.Root)
+	for i := len(rfrag) - 1; i >= 0; i-- {
+		frag = append(frag, rfrag[i])
+	}
+	foundDir := path.Join(frag...)
+	return foundDir, foundEntry, nil
+}
+
+// AliasesOf finds aliases of a given directory in a given mountinfo table.
+//
+// When the kernel bind mounts something it produces another, indistinguishable
+// place where a given entry may be found. The algorithm works by looking at
+// MountSource (what is mounted) and MountOptions (how is it mounted).
+func AliasesOf(dir string, mountinfo []*InfoEntry) (map[string]bool, error) {
+	// Find what the directory represents so that we know what to look for.
+	dir = path.Clean(dir)
+	foundDir, foundEntry, err := resolve(dir, mountinfo)
+	if err != nil {
+		return nil, err
+	}
+
+	// Build a map of mountinfo entries indexed by DevMajor:DevMinor taking
+	// only entries where the whole filesystem is mounted (Root == "/"). Those
+	// aid in lookup of the canonical mount locations of the given one.
+	byMajMin := make(map[[2]int][]*InfoEntry, len(mountinfo))
+	// NOTE: we don't consider mount options relevant here.
+	for _, mie := range mountinfo {
+		if mie.Root == "/" {
+			key := [2]int{mie.DevMajor, mie.DevMinor}
+			byMajMin[key] = append(byMajMin[key], mie)
+		}
+	}
+
+	// Search for equivalent mount points by matching their DevMajor:DevMinor
+	// against what the searched directory resolved to as well as their Root
+	// (fragment mounted) against the fragment (relative to the root of the
+	// mounted filesystem) of the searched directory.
+	aliases := make(map[string]bool)
+	for i := len(mountinfo) - 1; i >= 0; i-- {
+		mie := mountinfo[i]
+		if mie.DevMajor == foundEntry.DevMajor && mie.DevMinor == foundEntry.DevMinor && mie.Root == foundDir {
+			// TODO: don't add if this was shadowed by something we saw was
+			// mounted here earlier. (This is why we iterate in reverse).
+			aliases[mie.MountDir] = true
+			for _, canonMie := range byMajMin[[2]int{mie.DevMajor, mie.DevMinor}] {
+				canonDir := path.Join(canonMie.MountDir, mie.Root)
+				aliases[canonDir] = true
+			}
+		}
+	}
+	return aliases, nil
+}

interfaces/mount/mountinfo_test.go

@@ -168,3 +168,131 @@ func (s *profileSuite) TestLoadMountInfo2(c *C) {
 	_, err := mount.LoadMountInfo(fname)
 	c.Assert(err, ErrorMatches, "*. no such file or directory")
 }
+
+// Check that AliesesOf bails out on relative paths.
+func (s *mountinfoSuite) TestAliasesOfPrecondition1(c *C) {
+	mountinfo := mockMountInfo(c,
+		"201 25 8:2 / / rw,reltime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered")
+	_, err := mount.AliasesOf("foo", mountinfo)
+	c.Assert(err, ErrorMatches, `cannot resolve relative directory "foo"`)
+}
+
+// Check that AliesesOf bails out if / is not mounted.
+func (s *mountinfoSuite) TestAliasesOfPrecondition2(c *C) {
+	mountinfo := mockMountInfo(c,
+		"201 25 8:2 /home/zyga/foo /home/zyga/bar rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered")
+	_, err := mount.AliasesOf("/foo", mountinfo)
+	c.Assert(err, ErrorMatches, `cannot resolve directory "/foo": root directory is not mounted`)
+}
+
+// Check that AliesesOf finds a simple bind mount.
+func (s *mountinfoSuite) TestAliasesOf1(c *C) {
+	// $ mkdir foo bar; mount --bind foo bar
+	mountinfo := mockMountInfo(c,
+		"201 25 8:2 / / rw,reltime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"201 25 8:2 /home/zyga/foo /home/zyga/bar rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered")
+	for _, symmetry := range []string{
+		"/home/zyga/foo", "/home/zyga/bar",
+	} {
+		aliases, err := mount.AliasesOf(symmetry, mountinfo)
+		c.Assert(err, IsNil)
+		c.Assert(aliases, DeepEquals, map[string]bool{
+			"/home/zyga/foo": true,
+			"/home/zyga/bar": true,
+		})
+	}
+}
+
+// Check that AliasesOf finds a trio of bind mounts.
+func (s *mountinfoSuite) TestAliasesOf2(c *C) {
+	// $ mkdir foo bar; mount --bind foo bar
+	// $ mkdir froz; mount --bind bar froz
+	mountinfo := mockMountInfo(c,
+		"201 25 8:2 / / rw,reltime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"201 25 8:2 /home/zyga/foo /home/zyga/bar rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"428 25 8:2 /home/zyga/foo /home/zyga/froz rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered")
+	for _, symmetry := range []string{
+		"/home/zyga/foo", "/home/zyga/bar", "/home/zyga/froz",
+	} {
+		aliases, err := mount.AliasesOf(symmetry, mountinfo)
+		c.Assert(err, IsNil)
+		c.Assert(aliases, DeepEquals, map[string]bool{
+			"/home/zyga/foo":  true,
+			"/home/zyga/bar":  true,
+			"/home/zyga/froz": true,
+		}, Commentf("symmetry: %q", symmetry))
+	}
+}
+
+// Check that AliasesOf is not confused by shadowed mounts.
+func (s *mountinfoSuite) TestAliasesOf3(c *C) {
+	// $ mkdir src dst dummy
+	// $ mount --bind dummy dst
+	// $ mount --bind src dst
+	mountinfo := mockMountInfo(c,
+		"25 0 8:2 / / rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"395 25 8:2 /home/zyga/dummy /home/zyga/dst rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"404 395 8:2 /home/zyga/src /home/zyga/dst rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"405 25 8:2 /home/zyga/src /home/zyga/dummy rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered")
+	for _, symmetry := range []string{
+		"/home/zyga/src", "/home/zyga/dst",
+	} {
+		aliases, err := mount.AliasesOf(symmetry, mountinfo)
+		c.Assert(err, IsNil)
+		// We don't see "dummy!" here.
+		c.Assert(aliases, DeepEquals, map[string]bool{
+			"/home/zyga/src": true,
+			"/home/zyga/dst": true,
+		}, Commentf("symmetry: %q", symmetry))
+	}
+}
+
+// Check that AliasesOf finds a trio of bind mounts in tmpfs in home.
+func (s *mountinfoSuite) TestAliasesOf4(c *C) {
+	// $ mkdir data; sudo mount -t tmpfs none data; cd data
+	// $ mkdir foo bar; sudo mount --bind foo bar
+	// $ mkdir froz; sudo mount --bind bar froz
+	mountinfo := mockMountInfo(c,
+		"25 0 8:2 / / rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"392 25 0:47 / /home/zyga/data rw,relatime shared:158 - tmpfs none rw",
+		"400 392 0:47 /foo /home/zyga/data/bar rw,relatime shared:158 - tmpfs none rw",
+		"408 392 0:47 /foo /home/zyga/data/froz rw,relatime shared:158 - tmpfs none rw")
+	for _, symmetry := range []string{
+		"/home/zyga/data/foo", "/home/zyga/data/bar", "/home/zyga/data/froz",
+	} {
+		aliases, err := mount.AliasesOf(symmetry, mountinfo)
+		c.Assert(err, IsNil)
+		c.Assert(aliases, DeepEquals, map[string]bool{
+			"/home/zyga/data/foo":  true,
+			"/home/zyga/data/bar":  true,
+			"/home/zyga/data/froz": true,
+		}, Commentf("symmetry: %q", symmetry))
+	}
+}
+
+// Check that AliasesOf finds all the canonical mounts.
+// Here the /home/zyga/data directory exists in two places, fully (not via a
+// sub-directory mount where Root != "/") so both of those should be found.
+func (s *mountinfoSuite) TestAliasesOf5(c *C) {
+	// $ mkdir data; sudo mount -t tmpfs none data; cd data
+	// $ mkdir foo bar; sudo mount --bind foo bar
+	// $ mkdir froz; sudo mount --bind bar froz
+	// $ mkdir data-bind; sudo mount --bind data data-bind -o ro
+	// ^ note, not recursive so only data itself is propagated
+	mountinfo := mockMountInfo(c,
+		"25 0 8:2 / / rw,relatime shared:1 - ext4 /dev/sda2 rw,errors=remount-ro,data=ordered",
+		"392 25 0:47 / /home/zyga/data rw,relatime shared:158 - tmpfs none rw",
+		"400 392 0:47 /foo /home/zyga/data/bar rw,relatime - tmpfs none rw",
+		"408 392 0:47 /foo /home/zyga/data/froz rw,relatime shared:158 - tmpfs none rw",
+		"416 25 0:47 / /home/zyga/data-bind ro,relatime shared:158 - tmpfs none rw")
+	for _, symmetry := range []string{
+		"/home/zyga/data", "/home/zyga/data-bind",
+	} {
+		aliases, err := mount.AliasesOf(symmetry, mountinfo)
+		c.Assert(err, IsNil)
+		c.Assert(aliases, DeepEquals, map[string]bool{
+			"/home/zyga/data":      true,
+			"/home/zyga/data-bind": true,
+		}, Commentf("symmetry: %q", symmetry))
+	}
+}