ca: extract the lookup of the active primary CA

This method had only one caller, which always looked for the active
root. This commit moves the lookup into the method to reduce the logic
in the one caller.

This is being done in preparation for a larger change. Keeping this
separate so it is easier to see.

The `storedRootID != primaryRoots.ActiveRootID` is being removed because
these can never be different.

The `storedRootID` comes from `provider.ActiveRoot`, the
`primaryRoots.ActiveRootID` comes from the store `CARoot` from the
primary. In both cases the source of the data is the primary DC.

Technically they could be different if someone modified the provider
outside of Consul, but that would break many things, so is not a
supported flow.

If these were out of sync because of ordering of events then the
secondary will soon receive an update to `primaryRoots` and everything
will be sorted out again.

agent/consul/leader_connect_ca.go

@@ -197,11 +197,18 @@ func (c *CAManager) secondarySetPrimaryRoots(newRoots structs.IndexedCARoots) {
 	c.primaryRoots = newRoots
 }
 
-func (c *CAManager) secondaryGetPrimaryRoots() structs.IndexedCARoots {
+func (c *CAManager) secondaryGetActivePrimaryCARoot() (*structs.CARoot, error) {
 	// TODO: this could be a different lock, as long as its the same lock in secondarySetPrimaryRoots
 	c.stateLock.Lock()
-	defer c.stateLock.Unlock()
-	return c.primaryRoots
+	primaryRoots := c.primaryRoots
+	c.stateLock.Unlock()
+
+	for _, root := range primaryRoots.Roots {
+		if root.ID == primaryRoots.ActiveRootID && root.Active {
+			return root, nil
+		}
+	}
+	return nil, fmt.Errorf("primary datacenter does not have an active root CA for Connect")
 }
 
 // initializeCAConfig is used to initialize the CA config if necessary
@@ -658,25 +665,17 @@ func (c *CAManager) secondaryInitializeIntermediateCA(provider ca.Provider, conf
 		}
 	}
 
-	// Determine which of the provided PRIMARY representations of roots is the
-	// active one. We'll use this as a template to generate any new root
-	// representations meant for this secondary.
-	var newActiveRoot *structs.CARoot
-	primaryRoots := c.secondaryGetPrimaryRoots()
-	for _, root := range primaryRoots.Roots {
-		if root.ID == primaryRoots.ActiveRootID && root.Active {
-			newActiveRoot = root
-			break
-		}
-	}
-	if newActiveRoot == nil {
-		return fmt.Errorf("primary datacenter does not have an active root CA for Connect")
+	newActiveRoot, err := c.secondaryGetActivePrimaryCARoot()
+	if err != nil {
+		return err
 	}
 
+	_ = storedRootID // TODO: will be removed in the next commit
+
 	// Get a signed intermediate from the primary DC if the provider
 	// hasn't been initialized yet or if the primary's root has changed.
 	needsNewIntermediate := false
-	if activeIntermediate == "" || storedRootID != primaryRoots.ActiveRootID {
+	if activeIntermediate == "" {
 		needsNewIntermediate = true
 	}