Session History Modeling in HTML5, Servo & Gecko

The problems of HTML5's session history modeling. Keynote available at https://www.icloud.com/keynote/0EXRQ8g5KCa2HRc5gYZLijLMw#shistory

Transcript

1. Session History Modeling in HTML5, Servo & Gecko Samael Wang
2. HTML5 Modeling
3. HTML5 Terminology 0
4. HTML5 Terminology 1 0
5. HTML5 Terminology 1 2 3 0
6. HTML5 Terminology 1 2 3 0 §7.7.1 The session history of browsing contexts The sequence of Documents in a browsing context is its session history. Each browsing context, including nested browsing contexts, has a distinct session history. A browsing context's session history consists of a flat list of session history entries.
7. HTML5 Terminology 1 2 3 0 §7.7.1 The session history of browsing contexts The sequence of Documents in a browsing context is its session history. Each browsing context, including nested browsing contexts, has a distinct session history. A browsing context's session history consists of a flat list of session history entries. session history of top-level browsing context session history of a nested browsing context
8. HTML5 Terminology 1 2 3 0 §7.7.1 The session history of browsing contexts At any point, one of the entries in the session history is the current entry. This is the entry representing the active document of the browsing context.
9. HTML5 Terminology 1 2 3 0 §7.7.1 The session history of browsing contexts At any point, one of the entries in the session history is the current entry. This is the entry representing the active document of the browsing context. current entry / active document current entry / active document
10. HTML5 Terminology 1 2 3 4 0
11. HTML5 Terminology 2 1 3 5 4 0
12. HTML5 Terminology 2 1 3 5 4 0 §7.7.2 The History interface A Document is said to be fully active when it has a browsing context and it is the active document of that browsing context, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the Document through which it is nested is itself fully active.
13. HTML5 Terminology 2 1 3 5 4 0 §7.7.2 The History interface A Document is said to be fully active when it has a browsing context and it is the active document of that browsing context, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the Document through which it is nested is itself fully active. active fully active
14. HTML5 Terminology 2 1 3 5 4 0 active fully active §7.7.2 The History interface The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, …
15. HTML5 Terminology 2 1 3 5 4 0 active fully active §7.7.2 The History interface The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, … 0 1 2 5 joint session history
16. HTML5 Terminology 2 1 3 5 4 0 0 1 2 5 joint session history §7.7.2 The History interface To traverse the history by a delta delta, … … 2. Let specified entry be the entry in the joint session history whose index is the sum of delta and the index of the current entry of the joint session history. 3. Let specified browsing context be the browsing context of the specified entry. … 5.3. Traverse the history of the specified browsing context to the specified entry.
17. HTML5 Terminology 2 4 5 1 3 0 0 1 2 5 joint session history
18. HTML5 Terminology 2 4 5 1 3 0 0 1 2 5 joint session history §7.7.2 The History interface The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, with all the entries that are current entries in their respective session histories removed except for the current entry of the joint session history. The current entry of the joint session history is the entry that most recently became a current entry in its session history. Entries in the joint session history are ordered chronologically by the time they were added to their respective session histories.
19. 0 1 3 2 5 HTML5 Terminology 2 4 5 1 3 0 joint session history §7.7.2 The History interface The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, with all the entries that are current entries in their respective session histories removed except for the current entry of the joint session history. The current entry of the joint session history is the entry that most recently became a current entry in its session history. Entries in the joint session history are ordered chronologically by the time they were added to their respective session histories. 0 1 3 4 5 ?
20. 4 2 Defined by “A model of navigation history” Join Session Past / Future 5 1 3 0 The session past of a browsing context is the entries added before current entry. The session future is the entries added after current entry. The joint session past of a top-level browsing context is the union of session pasts of all browsing contexts of all the fully active Document objects that share that top-level browsing context. The joint session future is the union of session futures of these browsing contexts. Both are ordered chronologically by the time they were added to their respective session histories.
21. 4 2 Defined by “A model of navigation history” Join Session Past / Future 5 1 3 0 0 1 3 joint session past The session past of a browsing context is the entries added before current entry. The session future is the entries added after current entry. The joint session past of a top-level browsing context is the union of session pasts of all browsing contexts of all the fully active Document objects that share that top-level browsing context. The joint session future is the union of session futures of these browsing contexts. Both are ordered chronologically by the time they were added to their respective session histories.
22. 4 2 Defined by “A model of navigation history” Join Session Past / Future 5 1 3 0 0 1 3 joint session past 5 joint session future The session past of a browsing context is the entries added before current entry. The session future is the entries added after current entry. The joint session past of a top-level browsing context is the union of session pasts of all browsing contexts of all the fully active Document objects that share that top-level browsing context. The joint session future is the union of session futures of these browsing contexts. Both are ordered chronologically by the time they were added to their respective session histories.
23. Legend / Terminology 3 0 2 1 1 2 3 0 0 2 3 inactive current entry / active current entry / fully active Number-ordered entries Parent / child relationship 1 3 0 2 • current entry • active document • fully active document • session past / future • joint session past / future • traverse the history by a delta • traverse the history 
 (of a browsing context) Terminology
24. Problems of HTML5 Modeling https://arxiv.org/abs/1608.05444
25. 0 Traverse-then-traverse Example 1 1 2 3 4 2 1 0 1 2 3 4 2 1 history.go(1); history.go(1); history.go(2); 3 4 joint session future 3 4 joint session future
26. 0 Traverse-then-traverse Example 1 1 2 3 4 2 0 1 2 3 4 2 1 history.go(1); history.go(1); history.go(2); 3 3 4 joint session future 3 4 joint session future
27. 0 Traverse-then-traverse Example 1 1 2 3 4 2 0 1 2 3 4 2 1 history.go(1); history.go(1); history.go(2); 3 4 joint session future 3 4 joint session future
28. 0 Traverse-then-traverse Example 1 1 2 3 4 0 1 2 3 4 2 1 history.go(1); history.go(1); history.go(2); 3 4 4 joint session future 3 4 joint session future
29. 0 Traverse-then-traverse Example 1 1 2 3 4 0 1 2 3 4 1 history.go(1); history.go(1); history.go(2); 3 4 4 4 joint session future 3 4 joint session future
30. 0 Traverse-then-traverse Example 1 1 2 3 4 0 1 2 3 4 1 history.go(1); history.go(1); history.go(2); 3 4 4 4 joint session future 3 4 joint session future This counterexample is caused by the definition of ‘traverses the history by δ’ which only traverses one document’s session history. Instead, we should traverse the history of all δ documents.
31. 3 0 3 Traverse-then-traverse Example 2 1 2 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 0 1 2 5 joint session future joint session future 2 5 1 1
32. 3 0 3 Traverse-then-traverse Example 2 1 2 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 0 2 5 joint session future joint session future 2 5 2 1 1
33. 3 0 3 Traverse-then-traverse Example 2 1 2 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 0 5 joint session future joint session future 2 5 2 4 1 1
34. 0 3 Traverse-then-traverse Example 2 1 2 3 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 0 5 joint session future joint session future 2 5 2 4 4 1 1
35. 0 3 Traverse-then-traverse Example 2 1 2 3 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 5 joint session future joint session future 2 5 2 5 4 4 1
36. 0 3 Traverse-then-traverse Example 2 1 2 3 4 5 0 2 4 5 history.go(1); history.go(1); history.go(2); 5 joint session future joint session future 2 5 2 5 4 4 1 The problem this time is that the definition of ‘joint session history’ only includes the fully active documents, not all active documents.
37. 2 4 2 2 3 3 0 Symmetric Example 1 1 3 4 history.go(-1); history.go(1); 1 joint session past 4 3 3 0 1 3
38. 2 4 2 2 3 3 0 Symmetric Example 1 1 3 4 history.go(-1); history.go(1); 1 joint session past 1 3 3 0 1 3
39. 2 4 2 2 joint session future 3 3 0 Symmetric Example 1 1 3 4 history.go(-1); history.go(1); 4 3 1 3 3 0 1 3
40. 2 4 2 joint session future 3 3 0 Symmetric Example 1 1 3 4 history.go(-1); history.go(1); 4 3 3 1 3 3 0 1 3
41. 2 4 2 joint session future 3 3 0 Symmetric Example 1 1 3 4 history.go(-1); history.go(1); 4 3 3 1 3 3 0 1 3 This counterexample is caused by an asymmetry in the definition of traversal: it is defined in terms of navigating to a document d, and not navigating from a document.
42. 1 2 4 1 4 Symmetric Example 2 history.go(-1); history.go(1); 2 joint session past 0 2 3 1 4 0 2 3
43. 1 2 4 1 4 Symmetric Example 2 history.go(-1); history.go(1); 2 joint session past 0 2 3 1 0 3 2
44. 1 2 4 1 4 joint session future Symmetric Example 2 history.go(-1); history.go(1); 4 3 0 2 3 1 0 3 2
45. 1 2 4 1 4 joint session future Symmetric Example 2 history.go(-1); history.go(1); 4 3 0 2 3 0 3 3
46. 1 2 4 1 4 joint session future Symmetric Example 2 history.go(-1); history.go(1); 4 3 0 2 3 0 3 3 The definition of navigation histories allows malformed state.
47. 2 1 1 4 Malformed Examples 2 3 3 4 1 2 3 4
48. 2 1 1 4 Malformed Examples 2 3 3 4 1 2 3 4 Definition: A navigation history is well formed whenever for any a ≲ b and c ≲ d, 
 if a ∈ A and d ∈ A then d ≤ b. Translate: Documents in any session future should not be chronologically earlier than any other active documents sharing the same root browsing context.
49. Navigation-causes-malformed Example 1 0 2 3 2
50. Navigation-causes-malformed Example 1 4 0 2 3
51. Navigation-causes-malformed Example 1 4 0 2 3 The joint session future should be deleted before adding any new document.
52. Servo’s Solution
53. Remove the definition of joint session history and current entry of the joint session history, and add the following: • The session past of a browsing context is the entries of the session history added before the current entry (and does not include the current entry). • The session future of a browsing context is the entries of the session history added after the current entry (and does not include the current entry). • If an entry has a next entry in the chronologically ordered session history, it is its successor. • If an entry has a previous entry in the chronologically ordered session history, it is its predecessor. • The joint session past of a top-level browsing context is the union of all the session pasts of all browsing contexts that share that top-level browsing context. Entries in the joint session past are in decreasing chronological order of the time they were added to their respective session histories. • The joint session future of a top-level browsing context is the union of all the session futures of all browsing contexts that share that top-level browsing context. Entries in the joint session future are in increasing chronological order of the time their predecessor were added to their respective session histories.
54. To traverse the history by a delta delta, … 1. Define the entry sequence as follows: 1. If delta is a positive integer +n, … let the entry sequence be the first n entries of the joint session future. 2. If delta is a negative integer −n, … let the entry sequence be the first n entries of the joint session past. 2. A session entry is said to become active when it is a member of the entry sequence, and no session entry after it in the entry sequence has the same browsing context. 
 3. A session entry is said to stay active when it it the current entry of its browsing context, and there are no members of the entry sequence with the same browsing context. 
 4. A session entry is said to be activating when either it will become active or stay active.
 Note: the activating documents will be active after traversal has finished. 
 5. A session entry is said to be fully activating if is activating, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the session entry through which it is nested is itself fully activating. 
 Note: the fully activating documents will be fully active after traversal has finished.
55. (cont’d) For each specified entry in the entry sequence, run the following substeps 1. Let specified browsing context be the browsing context of the specified entry. 
 … 4. If the specified entry is activating but not fully activating, then set the current entry of the session history of specified browsing context to be the specified entry. 5. If the specified entry is fully activating, then traverse the history of the specified browsing context to the specified entry.
 Note: in this case, the document will be fully active, so should be loaded.
56. Yet It Ain’t Perfect
57. §7.1.1 Nested browsing contexts Certain elements (for example, iframe elements) can instantiate further browsing contexts. These elements are called browsing context containers. Each browsing context container has a nested browsing context, which is either a browsing context or null. §7.7.1 The session history of browsing contexts A browsing context's session history consists of a flat list of session history entries. Each session history entry consists, at a minimum, of a URL, and each entry may in addition have … 1 0 2 3 4 nbc1 nbc2 Who’s holding nested browsing context nbc1 & nbc2?
58. §7.3.4 Garbage collection and browsing contexts When a browsing context is to discard a Document, the user agent must run the following steps: … 5. Discard all the child browsing contexts of the Document. 6. Lose the strong reference from the Document's browsing context to the Document. When a browsing context is discarded, the strong reference from the user agent itself to the browsing context must be severed, and all the Document objects for all the entries in the browsing context's session history must be discarded as well. 1 0 2 3 4 nbc1 nbc2 Who’s holding nested browsing context nbc1 & nbc2?
59. The lifetime of session history is bound to its browsing context, not root browsing context. When a document is discarded, it essentially drops all nested session histories. Browsers with no bfcache would have no nested session history at all after root navigation if it tries to align with the spec. How long do we want to keep documents / browsing contexts alive in a user agent? Characteristics of Joint Session History
60. The lifetime of session history is bound to its browsing context, not root browsing context. When a document is discarded, it essentially drops all nested session histories. Browsers with no bfcache would have no nested session history at all after root navigation if it tries to align with the spec. How long do we want to keep documents / browsing contexts alive in a user agent? About servo: “In Servo, we don't ever unload any documents that are still in the session history, but we are working on adding this.” Characteristics of Joint Session History
61. Gecko's Implementation
62. Both Chrome & Firefox are using a tree-structured model 2 Tree-structured Model 1 0 2 3 4 nbc1 nbc2 4 1 0 3 0 nbc1 nbc2 Entries matching nested browsing contexts, but lifetime bound to root.
63. Tree-structured Model in Gecko 0
64. Tree-structured Model in Gecko 1 0
65. 3 2 Tree-structured Model in Gecko 1 0
66. 3 2 4 2 Tree-structured Model in Gecko 1 0 CloneAndReplaceChild
67. 3 2 4 2 Tree-structured Model in Gecko 1 0 5 CloneAndReplaceChild
68. 4 2 3 2 Tree-structured Model in Gecko 1 0 5 CloneAndReplaceChild
69. • Nested browsing context matching is tricky as there’s no precious way to identify an iframe. • Chrome generates a unique name for iframe with window.name or position + some extra process. • Gecko uses iframe ordering to match static iframes, and doesn’t keep dynamic iframe entries after the document is discarded (i.e. bound to bfcache). • Deletion of an iframe causes redundant parent entries. Drawbacks of Tree-structured Model
70. 4 0 Deletion of an iFrame Handling 0 0 0 3 1 1 nbc1 2 nbc2 console.log(history.length()); iframe.remove(); console.log(history.length()); Output?
71. 4 0 Deletion of an iFrame Handling 0 0 0 console.log(history.length()); iframe.remove(); console.log(history.length()); Output?
72. 4 0 Deletion of an iFrame Handling console.log(history.length()); iframe.remove(); console.log(history.length()); Output? RemoveDuplicate
73. 4 0 Deletion of an iFrame Handling console.log(history.length()); iframe.remove(); console.log(history.length()); Output? RemoveDuplicate Without removing duplicate entries with current root document, forward & backward navigation of the same document would take no effect.
74. S Deletion + Dynamic iFrame Handling
75. F1 A S Deletion + Dynamic iFrame Handling
76. F2 A F1 A S Deletion + Dynamic iFrame Handling
77. F3 A F2 A F1 A S Deletion + Dynamic iFrame Handling
78. F3 A F2 A F1 A S Deletion + Dynamic iFrame Handling var ifr = window.frameElement; var p = ifr.parentNode; p.innerHTML = '<h3>Top level page A, with replaced iframe</h3>' + '<iframe height=400 width=300 src=A_iframe_3_replaced.html class=replaced></iframe>';
79. F3 A F2 A F1 A S Deletion + Dynamic iFrame Handling var ifr = window.frameElement; var p = ifr.parentNode; p.innerHTML = '<h3>Top level page A, with replaced iframe</h3>' + '<iframe height=400 width=300 src=A_iframe_3_replaced.html class=replaced></iframe>';
80. A S Deletion + Dynamic iFrame Handling var ifr = window.frameElement; var p = ifr.parentNode; p.innerHTML = '<h3>Top level page A, with replaced iframe</h3>' + '<iframe height=400 width=300 src=A_iframe_3_replaced.html class=replaced></iframe>';
81. A S Fd Deletion + Dynamic iFrame Handling var ifr = window.frameElement; var p = ifr.parentNode; p.innerHTML = '<h3>Top level page A, with replaced iframe</h3>' + '<iframe height=400 width=300 src=A_iframe_3_replaced.html class=replaced></iframe>'; The entries of dynamic iframe 
 will later be discarded when 
 bfcache dropped.
82. Fd A S S Deletion + Dynamic iFrame Handling The entries of dynamic iframe 
 will later be discarded when 
 bfcache dropped.
83. F3 A F2 A F1 A S BFCache
84. F3 A F2 A F1 A S BFCache nsSHEntryShared Entries for the same document point to a shared instance of nsSHEntryShared (nsIBFCacheEntry)
85. Document A iframe DocShell Content Viewer Content Veiwer Document F3 F3 A F2 A F1 A S BFCache nsSHEntryShared Entries for the same document point to a shared instance of nsSHEntryShared (nsIBFCacheEntry) Root nsSHEntryShared holds the content viewer, which holds the document, which holds its frames & corresponding docshells / content viewers / sub-documents of these frames.
86. F3 A F2 A F1 A S BFCache nsSHEntryShared Nested nsSHEntryShared doesn’t hold content viewer & document. (unless you try to enable a feature which we disabled 12yrs ago)
87. Find the spec issue: https://github.com/whatwg/html/issues/ 1454 If You’re Interested
89. Backup
90. §7.7.2 The History interface To traverse the history by a delta delta, … … 2. Let specified entry be the entry in the joint session history whose index is the sum of delta and the index of the current entry of the joint session history. 3. Let specified browsing context be the browsing context of the specified entry. … 5.3. Traverse the history of the specified browsing context to the specified entry. History Traversal in HTML5
91. A navigation history H = (D, A, →, ≤, ∼) consists of: • a finite set D (the documents), • a subset A ⊆ D (the active documents), • a forest (D, →) (the child relationship), • a total order (D, ≤) (the chronological order), and • an equivalence relation (D, ∼) (the same-session equivalence). such that: • for every d there is a unique d′ ∈ A such that d ∼ d′, • for every d → e ∼ e′ we have d → e′, and • for every d → e, we have d ≤ e. Definition of Navigation History (1)
92. In such a navigation history, we define: • d0 is the unique active root document, • d ↠ e when d → e and e ∈ A (the active child relationship), • FA = {d | d0 ↠∗ d} (the fully active documents), • d ≲ e whenever d ∼ e and d < e, • the session future of d is {e | d ≲ e}, • the session past of d is {e | d ≳ e}, • the joint session future is {e | ∃d ∈ FA. d ≲ e}, • the joint session past is {e | ∃d ∈ FA. d ≳ e} Definition of Navigation History (2)