Main frame's unique name can always be null.

third_party/WebKit/Source/core/page/FrameTree.cpp

 /*
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 60 matching lines @@
     if (m_name == name)
         return;
 
     m_name = name;
 
     // https://crbug.com/607205: Make sure m_uniqueName doesn't change after
     // initial navigation - session history depends on this.
     if (toLocalFrame(m_thisFrame)->loader().stateMachine()->committedFirstRealDocumentLoad())
         return;
 
-    // Remove our old frame name so it's not considered in calculateUniqueNameForChildFrame
-    // and appendUniqueSuffix calls below.
+    // Leave main frame's unique name set to a null string.
+    if (!parent())
+        return;
+
+    // Remove our old frame name so it's not considered in
+    // calculateUniqueNameForChildFrame call below.
     m_uniqueName = AtomicString();
 
     // Calculate a new unique name based on inputs.
-    if (parent()) {
-        setUniqueName(
-            parent()->tree().calculateUniqueNameForChildFrame(m_thisFrame, name, nullAtom));
-    } else if (name.isEmpty() || !uniqueNameExists(name)) {
-        // Only main frame can have an empty unique name, so for main frames
-        // emptiness guarantees uniquness.
-        setUniqueName(name);
-    } else {
-        setUniqueName(appendUniqueSuffix(name, "<!--framePosition"));
-    }
+    setUniqueName(
+        parent()->tree().calculateUniqueNameForChildFrame(m_thisFrame, name, nullAtom));
 }
 
 void FrameTree::setPrecalculatedName(const AtomicString& name, const AtomicString& uniqueName)
 {
     m_name = name;
 
-    // Non-main frames should have a non-empty unique name.
-    DCHECK(!parent() || !uniqueName.isEmpty());
+    if (parent()) {
+        // Non-main frames should have a non-empty unique name.
+        DCHECK(!uniqueName.isEmpty());
+    } else {
+        // Unique name of main frames should always stay empty.
+        DCHECK(uniqueName.isEmpty());
+    }
 
     // TODO(lukasza): We would like to assert uniqueness below (i.e. by calling
     // setUniqueName), but
     // 1) uniqueness is currently violated by provisional/old frame pairs.
     // 2) there is an unresolved race between 2 OOPIFs, that can result in a
     //    non-unique |uniqueName| - see https://crbug.com/558680#c14.
     m_uniqueName = uniqueName;
 }
 
 Frame* FrameTree::parent() const
@@ skipping 192 matching lines @@
 
     // Description of the current unique name format
     // ---------------------------------------------
     //
     // Changing the format of unique name is undesirable, because it breaks
     // backcompatibility of session history (which stores unique names
     // generated in the past on user's disk).  This incremental,
     // backcompatibility-aware approach has resulted so far in the following
     // rather baroque format... :
     //
-    //   uniqueName ::= <assignedName> | <generatedName>
+    //   uniqueName ::= <nullForMainFrame> | <assignedName> | <generatedName>
     //                 (generatedName is used if assignedName is
-    //                 1) non-unique / conflicts with other frame's unique name or
-    //                 2) assignedName is empty for a non-main frame)
+    //                 non-unique / conflicts with other frame's unique name.
     //
     //   assignedName ::= value of iframe's name attribute
     //                 or value assigned to window.name (*before* the first
     //                    real commit - afterwards unique name stays immutable).
     //
     //   generatedName ::= oldGeneratedName newUniqueSuffix?
     //                  (newUniqueSuffix is only present if oldGeneratedName was
     //                  not unique after all)
     //
     //   oldGeneratedName ::= "<!--framePath //" ancestorChain "/<!--frame" childCount "-->-->"
-    //                  (main frame is special - oldGeneratedName for main frame
-    //                  is always the frame's assignedName;  oldGeneratedName is
-    //                  generated by generateUniqueNameCandidate method).
+    //                  (oldGeneratedName is generated by generateUniqueNameCandidate method).
     //
     //   childCount ::= current number of siblings
     //
     //   ancestorChain ::= concatenated unique names of ancestor chain,
     //                     terminated on the first ancestor (if any) starting with "<!--framePath"
     //                     each ancestor's unique name is separated by "/" character
     //   ancestorChain example1: "grandparent/parent"
     //  (ancestor's unique names :   #1--^   | #2-^ )
     //   ancestorChain example2: "<!--framePath //foo/bar/<!--frame42-->-->/blah/foobar"
     //  (ancestor's unique names :        ^--#1--^                         | #2 | #3-^ )
     //
     //   newUniqueSuffix ::= "<!--framePosition" framePosition "/" retryNumber "-->"
     //
     //   framePosition ::= "-" numberOfSiblingsBeforeChild [ framePosition-forParent? ]
-    //                   | <empty string for main frame>
     //
     //   retryNumber ::= smallest non-negative integer resulting in unique name
 }
 
 void FrameTree::setUniqueName(const AtomicString& uniqueName)
 {
-    // Main frame is the only frame that can have an empty unique name.
-    if (parent()) {
-        DCHECK(!uniqueName.isEmpty() && !uniqueNameExists(uniqueName));
-    } else {
-        DCHECK(uniqueName.isEmpty() || !uniqueNameExists(uniqueName));
-    }
-
+    DCHECK(parent()); // Only subframes can have a custom-tailored unique name.

[dcheng, 2016/09/15 06:33:23]

custom-tailored => non-empty

[Łukasz Anforowicz, 2016/09/15 15:55:18]

Done + expanded the comment a bit more.

+    DCHECK(!uniqueName.isEmpty() && !uniqueNameExists(uniqueName));
     m_uniqueName = uniqueName;
 }
 
 Frame* FrameTree::scopedChild(unsigned index) const
 {
     unsigned scopedIndex = 0;
     for (Frame* child = firstChild(); child; child = child->tree().nextSibling()) {
         if (child->client()->inShadowTree())
             continue;
         if (scopedIndex == index)
@@ skipping 226 matching lines @@
 {
     if (!frame) {
         printf("Null input frame\n");
         return;
     }
 
     printFrames(frame->tree().top(), frame, 0);
 }
 
 #endif