[FAB-6746] Fix modification of root config group

A recent change was made to the configtx processing which causes a panic
when the path of a config element is empty.  This case can never happen
for any config element, except for the very root config element.  This
CR modifies the code to simply not traverse the policy manager selection
code if the path is empty, as this indicates the element is part of the
root policy manager.

Change-Id: Ibc73ad155a51b79051c83a84cd8d615d432506e2
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

common/configtx/update.go

@@ -176,13 +176,11 @@ func (cm *configManager) policyForItem(item comparable) (policies.Policy, bool)
 	logger.Debugf("Getting policy for item %s with mod_policy %s", item.key, modPolicy)
 
 	// If the mod_policy path is relative, get the right manager for the context
-	// if the mod_policy path is absolute (starts with /) evaluate at the root
-	if len(modPolicy) > 0 && modPolicy[0] != policies.PathSeparator[0] {
-		// path should always be at least length 1, panic if not
-		if len(item.path) == 0 {
-			logger.Panicf("Empty path for item %s", item.key)
-		}
+	// If the item has a zero length path, it is the root group, use the base policy manager
+	// if the mod_policy path is absolute (starts with /) also use the base policy manager
+	if len(modPolicy) > 0 && modPolicy[0] != policies.PathSeparator[0] && len(item.path) != 0 {
 		var ok bool
+
 		manager, ok = manager.Manager(item.path[1:])
 		if !ok {
 			logger.Debugf("Could not find manager at path: %v", item.path[1:])

common/configtx/update_test.go

@@ -196,17 +196,16 @@ func TestPolicyForItem(t *testing.T) {
 		assert.Equal(t, policy, fooPolicy, "Should have found relative foo policy for foo group")
 	})
 
-	t.Run("Empty item path", func(t *testing.T) {
-		policyForItemEmptyPath := func() {
-			cm.policyForItem(comparable{
-				key:  "foo",
-				path: []string{},
-				ConfigGroup: &cb.ConfigGroup{
-					ModPolicy: "foo",
-				},
-			})
-		}
-		assert.Panics(t, policyForItemEmptyPath)
+	t.Run("Root group manager", func(t *testing.T) {
+		policy, ok := cm.policyForItem(comparable{
+			path: []string{},
+			key:  "root",
+			ConfigGroup: &cb.ConfigGroup{
+				ModPolicy: "rootPolicy",
+			},
+		})
+		assert.True(t, ok)
+		assert.Equal(t, policy, rootPolicy, "Should have found relative policy off the root manager")
 	})
 }