filesystem: add back the mountsByPath map

Add back the mountsByPath map, which indexes all Mounts by mountpoint.
This is needed again.

To avoid confusion, also rename two local variables named mountsByPath.

mountsByPath won't contain nil entries, so also make AllFilesystems()
use it instead of mountsByDevice.  This shouldn't change its behavior.

Update #339

filesystem/mountpoint.go

@@ -38,13 +38,15 @@ import (
 )
 
 var (
-	// This map holds data about the state of the system's filesystems.
+	// These maps hold data about the state of the system's filesystems.
 	//
-	// It only contains one Mount per filesystem, even if there are
+	// They only contain one Mount per filesystem, even if there are
 	// additional bind mounts, since we want to store fscrypt metadata in
-	// only one place per filesystem.  If it is ambiguous which Mount should
-	// be used, an explicit nil entry is stored.
+	// only one place per filesystem.  When it is ambiguous which Mount
+	// should be used for a filesystem, mountsByDevice will contain an
+	// explicit nil entry, and mountsByPath won't contain an entry.
 	mountsByDevice map[DeviceNumber]*Mount
+	mountsByPath   map[string]*Mount
 	// Used to make the mount functions thread safe
 	mountMutex sync.Mutex
 	// True if the maps have been successfully initialized.
@@ -197,18 +199,18 @@ func findMainMount(filesystemMounts []*Mount) *Mount {
 	// since non-last mounts were already excluded earlier.
 	//
 	// Also build the set of all mounted subtrees.
-	mountsByPath := make(map[string]*mountpointTreeNode)
+	filesystemMountsByPath := make(map[string]*mountpointTreeNode)
 	allSubtrees := make(map[string]bool)
 	for _, mnt := range filesystemMounts {
-		mountsByPath[mnt.Path] = &mountpointTreeNode{mount: mnt}
+		filesystemMountsByPath[mnt.Path] = &mountpointTreeNode{mount: mnt}
 		allSubtrees[mnt.Subtree] = true
 	}
 
 	// Divide the mounts into non-overlapping trees of mountpoints.
-	for path, mntNode := range mountsByPath {
+	for path, mntNode := range filesystemMountsByPath {
 		for path != "/" && mntNode.parent == nil {
 			path = filepath.Dir(path)
-			if parent := mountsByPath[path]; parent != nil {
+			if parent := filesystemMountsByPath[path]; parent != nil {
 				mntNode.parent = parent
 				parent.children = append(parent.children, mntNode)
 			}
@@ -233,7 +235,7 @@ func findMainMount(filesystemMounts []*Mount) *Mount {
 	// *all* mounted subtrees.  Equivalently, select a mountpoint tree in
 	// which every uncontained subtree is mounted.
 	var mainMount *Mount
-	for _, mntNode := range mountsByPath {
+	for _, mntNode := range filesystemMountsByPath {
 		mnt := mntNode.mount
 		if mntNode.parent != nil {
 			continue
@@ -260,8 +262,10 @@ func findMainMount(filesystemMounts []*Mount) *Mount {
 
 // This is separate from loadMountInfo() only for unit testing.
 func readMountInfo(r io.Reader) error {
-	mountsByPath := make(map[string]*Mount)
 	mountsByDevice = make(map[DeviceNumber]*Mount)
+	mountsByPath = make(map[string]*Mount)
+	allMountsByDevice := make(map[DeviceNumber][]*Mount)
+	allMountsByPath := make(map[string]*Mount)
 
 	scanner := bufio.NewScanner(r)
 	for scanner.Scan() {
@@ -281,19 +285,22 @@ func readMountInfo(r io.Reader) error {
 		// Note this overrides the info if we have seen the mountpoint
 		// earlier in the file. This is correct behavior because the
 		// mountpoints are listed in mount order.
-		mountsByPath[mnt.Path] = mnt
+		allMountsByPath[mnt.Path] = mnt
 	}
 	// For each filesystem, choose a "main" Mount and discard any additional
 	// bind mounts.  fscrypt only cares about the main Mount, since it's
-	// where the fscrypt metadata is stored.  Store all main Mounts in
-	// mountsByDevice so that they can be found by device number later.
-	allMountsByDevice := make(map[DeviceNumber][]*Mount)
-	for _, mnt := range mountsByPath {
+	// where the fscrypt metadata is stored.  Store all the main Mounts in
+	// mountsByDevice and mountsByPath so that they can be found later.
+	for _, mnt := range allMountsByPath {
 		allMountsByDevice[mnt.DeviceNumber] =
 			append(allMountsByDevice[mnt.DeviceNumber], mnt)
 	}
 	for deviceNumber, filesystemMounts := range allMountsByDevice {
-		mountsByDevice[deviceNumber] = findMainMount(filesystemMounts)
+		mnt := findMainMount(filesystemMounts)
+		mountsByDevice[deviceNumber] = mnt // may store an explicit nil entry
+		if mnt != nil {
+			mountsByPath[mnt.Path] = mnt
+		}
 	}
 	return nil
 }
@@ -329,11 +336,9 @@ func AllFilesystems() ([]*Mount, error) {
 		return nil, err
 	}
 
-	mounts := make([]*Mount, 0, len(mountsByDevice))
-	for _, mount := range mountsByDevice {
-		if mount != nil {
-			mounts = append(mounts, mount)
-		}
+	mounts := make([]*Mount, 0, len(mountsByPath))
+	for _, mount := range mountsByPath {
+		mounts = append(mounts, mount)
 	}
 
 	sort.Sort(PathSorter(mounts))

filesystem/mountpoint_test.go

@@ -90,6 +90,12 @@ func TestLoadMountInfoBasic(t *testing.T) {
 	if mnt.ReadOnly {
 		t.Error("Wrong readonly flag")
 	}
+	if len(mountsByPath) != 1 {
+		t.Error("mountsByPath doesn't contain exactly one entry")
+	}
+	if mountsByPath[mnt.Path] != mnt {
+		t.Error("mountsByPath doesn't contain the correct entry")
+	}
 }
 
 // Test that Mount.Device is set to the mountpoint's source device if