fixup! o/snapstate: only create link-component tasks when needed

Author: andrewphelpsj

overlord/snapstate/component.go

@@ -453,33 +453,40 @@ func (cc *componentInstallChoreographer) canDiscardOldRevision() bool {
 }
 
 // targetSnapAlreadyHasComponentRevision reports whether a non-current target
-// snap revision already records the exact component state we want to end up
-// with. In that case, we must skip creating link-component entirely. Otherwise,
-// the undo path would end up unlinking components that should remain linked.
+// snap revision already contains the desired component revision. In that case,
+// we must skip creating link-component entirely. Otherwise, the undo path
+// would end up unlinking a component that should remain linked for that
+// snap revision.
 func (cc *componentInstallChoreographer) targetSnapAlreadyHasComponentRevision() bool {
 	if !cc.snapst.IsInstalled() {
 		return false
 	}
 
+	targetSnapRevision := cc.snapsup.Revision()
+	targetCompSideInfo := cc.compsup.CompSideInfo
+
 	// only consider non-current revisions when checking for the component.
-	if cc.snapst.Current == cc.snapsup.Revision() {
-		// when refreshing to the current snap revision, component refreshes
-		// rely on unlink-current-component/link-component. thus, undo can
-		// restore the old component revision cleanly.
+	if cc.snapst.Current == targetSnapRevision {
+		// when operating on the current snap revision, we're either going to:
+		// * install a new component, for which we'll always need a
+		//   link-component task.
+		// * change revisions of an already present component. in that case, we
+		//   create unlink-current-component and then a later link-component.
 		return false
 	}
 
-	idx := cc.snapst.LastIndex(cc.snapsup.Revision())
+	idx := cc.snapst.LastIndex(targetSnapRevision)
 	if idx < 0 {
 		return false
 	}
+	// the target snap revision is already present at idx
 
-	cs := cc.snapst.Sequence.ComponentStateForRev(idx, cc.compsup.CompSideInfo.Component)
+	cs := cc.snapst.Sequence.ComponentStateForRev(idx, targetCompSideInfo.Component)
 	if cs == nil {
 		return false
 	}
 
-	return cs.CompType == cc.compsup.CompType && *cs.SideInfo == *cc.compsup.CompSideInfo
+	return cs.SideInfo.Equal(targetCompSideInfo)
 }
 
 func (cc *componentInstallChoreographer) BeforeLocalSystemMod(st *state.State, s *taskChainSpan) ([]*state.Task, error) {