cmd/snap-update-ns: add CurrentProfileFromChanges made

The new function takes the previously existing algorithm out of
executeMountProfileUpdate, as the original location makes it very tricky
to test due to the presence of all the system interactions through
change.Perform.

Add some simple tests that show how keep, mount and unmount changes are
preserved.

Signed-off-by: Zygmunt Krynicki <[email protected]>

cmd/snap-update-ns/update.go

@@ -92,17 +92,40 @@ func executeMountProfileUpdate(upCtx MountProfileUpdateContext) error {
 
 	// Compute the new current profile so that it contains only changes that were made
 	// and save it back for next runs.
-	var currentAfter osutil.MountProfile
-	for i := len(changesMade) - 1; i >= 0; i-- {
-		change := changesMade[i]
+	currentAfter := CurrentProfileFromChangesMade(changesMade)
+	return upCtx.SaveCurrentProfile(&currentAfter)
+}
+
+// CurrentProfileFromChangesMade computes a new mount profile a slice of changes.
+//
+// The return value collects mount profile entries from changes of type "mount"
+// and "keep" while discarding the changes of type "unmount". The order in
+// which entries are collected depends on their type.
+//
+// The sequence of changes, as produced by NeededChanges, is computed by
+// looking at two mount profiles, examining the most recent change and working
+// backwards. Since the most recent change is the last entry in the mount
+// profile, the first change describes the last mount entry of the old mount
+// profile.
+//
+// The "keep" changes are thus collected in reverse order - the order of their
+// true appearance, while "mount" changes are collected in the order of their
+// appearance as this represents the actual order of performed mount
+// operations.
+func CurrentProfileFromChangesMade(changes []*Change) osutil.MountProfile {
+	var profile osutil.MountProfile
+
+	for i := len(changes) - 1; i >= 0; i-- {
+		change := changes[i]
 		if change.Action == Keep {
-			currentAfter.Entries = append(currentAfter.Entries, change.Entry)
+			profile.Entries = append(profile.Entries, change.Entry)
 		}
 	}
-	for _, change := range changesMade {
+	for _, change := range changes {
 		if change.Action == Mount {
-			currentAfter.Entries = append(currentAfter.Entries, change.Entry)
+			profile.Entries = append(profile.Entries, change.Entry)
 		}
 	}
-	return upCtx.SaveCurrentProfile(&currentAfter)
+
+	return profile
 }

cmd/snap-update-ns/update_test.go

@@ -371,6 +371,64 @@ func (s *updateSuite) TestKeepSyntheticMountsLP2043993(c *C) {
 	c.Check(saved.Entries[1], DeepEquals, desiredMountEntry)
 }
 
+func (s *updateSuite) TestCurrentProfileFromChangesMade(c *C) {
+	// Changes are computed back-to-front, starting from the last entry in
+	// the mount profile.
+	changes := []*update.Change{
+		{Action: update.Mount, Entry: osutil.MountEntry{Dir: "/dir-1"}},
+		{Action: update.Mount, Entry: osutil.MountEntry{Dir: "/dir-2"}},
+		{Action: update.Mount, Entry: osutil.MountEntry{Dir: "/dir-3"}},
+	}
+	profile := update.CurrentProfileFromChangesMade(changes)
+	c.Check(profile, DeepEquals, osutil.MountProfile{Entries: []osutil.MountEntry{
+		{Dir: "/dir-1"},
+		{Dir: "/dir-2"},
+		{Dir: "/dir-3"},
+	}})
+
+	// When we keep a number of entries we are not still processing them
+	// back-to-front but the order of actual changes is front-to-back.
+	changes = []*update.Change{
+		{Action: update.Keep, Entry: osutil.MountEntry{Dir: "/dir-3"}},
+		{Action: update.Keep, Entry: osutil.MountEntry{Dir: "/dir-2"}},
+		{Action: update.Keep, Entry: osutil.MountEntry{Dir: "/dir-1"}},
+	}
+	profile = update.CurrentProfileFromChangesMade(changes)
+	c.Check(profile, DeepEquals, osutil.MountProfile{Entries: []osutil.MountEntry{
+		{Dir: "/dir-1"},
+		{Dir: "/dir-2"},
+		{Dir: "/dir-3"},
+	}})
+
+	// When we unmount things they just don't appear in the resulting profile.
+	changes = []*update.Change{
+		{Action: update.Unmount, Entry: osutil.MountEntry{Dir: "/dir-1"}},
+		{Action: update.Unmount, Entry: osutil.MountEntry{Dir: "/dir-2"}},
+		{Action: update.Unmount, Entry: osutil.MountEntry{Dir: "/dir-3"}},
+	}
+	profile = update.CurrentProfileFromChangesMade(changes)
+	c.Check(profile, DeepEquals, osutil.MountProfile{})
+
+	// When we have a mixture of changes unmount entries are removed, keep
+	// entries are retained first, back to front and then mount entries are
+	// retained in the order in which they were executed.
+	changes = []*update.Change{
+		{Action: update.Unmount, Entry: osutil.MountEntry{Dir: "/old-2"}},
+		{Action: update.Unmount, Entry: osutil.MountEntry{Dir: "/old-1"}},
+		{Action: update.Keep, Entry: osutil.MountEntry{Dir: "/same-2"}},
+		{Action: update.Keep, Entry: osutil.MountEntry{Dir: "/same-1"}},
+		{Action: update.Mount, Entry: osutil.MountEntry{Dir: "/new-1"}},
+		{Action: update.Mount, Entry: osutil.MountEntry{Dir: "/new-2"}},
+	}
+	profile = update.CurrentProfileFromChangesMade(changes)
+	c.Check(profile, DeepEquals, osutil.MountProfile{Entries: []osutil.MountEntry{
+		{Dir: "/same-1"},
+		{Dir: "/same-2"},
+		{Dir: "/new-1"},
+		{Dir: "/new-2"},
+	}})
+}
+
 // testProfileUpdateContext implements MountProfileUpdateContext and is suitable for testing.
 type testProfileUpdateContext struct {
 	loadCurrentProfile func() (*osutil.MountProfile, error)