* subversion/libsvn_repos/repos.c

[svn.git] / notes / wc-ng-design

                                                                -*- Text -*-

Content
=======

 * Context
 * Requirements
 * Nice-to-have's
 * Non-goals
 * Open items / discussion points
 * Problems in wc-1.0
 * Possible solutions
 * Prerequisites for a good wc implementation
 * Modularization
 * Implementation proposals for
   - metadata storage/access abstraction
   - BASE tree storage/access abstraction
   - WORKING tree storage/access abstraction
   - TARGET & MERGE-END tree storage/access abstraction
   - transactional manipulation API proposal
   - delta-application algorithm
      (in light of metadata, tree and textual conflicts)
   - 


Context
=======

The working copy library has traditionally been a complex piece of
machinery and libsvn_wc-1.0 (wc-1.0 hereafter) was more a result of
evolution than it was a result of design.  This can't be said to be
anybody's fault as much as it was unawareness of the developers at
the time with the problem(s) inherent to versioning trees instead of
files (as was the usual context within CVS).  As a result, the WC
has been one of the most fragile areas of the Subversion versioning
model.

The wc is where a large number of issues come together which can
be considered separate issues in the remainder of the system, or
don't have any effect on the rest of the system at all.  The following
things come to mind:

 * Different behaviours required by different use-cases (users)
   For example: some users want mtime's at checkout time
     to be the checkout time, some want it to be the historical
     value at check-in time (and others want different variants).
 * Different filesystems behave differently, yet Subversion
   is a cross platform tool and tries to behave the same on all
   filesystems (timestamp resolution may be an example of this).

When considering the wc-1.0 design, one finds that there are a lot
of situations where the exact state of the versioned tree isn't
defined.  When explicitly considering which trees relate to the
working copy at one time or another, the following trees can be
found:

 * BASE: The tree as it was in unmodified form
 * WORKING: The tree as it is in modified form, based on the
     administrative information recorded by the transforming
     'svn ..' commands
     Note: This tree will -as far as text bases goes- generally
           overlap with BASE, but isn't required to;
           e.g. "add-with-history"
 * ACTUAL: The tree as it is in modified form on the local disk.
    This tree may differ from WORKING when having been modified
    with non-Subversion transforming commands (such as plain 'rm').

In the context of the 'svn update' command:

 * BASE-TARGET: The tree to which BASE is being updated and for
     which the changes w.r.t. BASE are integrated into
     WORKING and ACTUAL
 * WORKING-TARGET, ACTUAL-TARGET: Trees in which the above mentioned
     changes have been integrated, but which haven't "gone live" yet;
     these trees generally represent "in transition" or "intermediary"
     state with the intent to become the final tree.

Additionally, three more trees may be related to the working copy
when considering the 'svn merge' command:

 * START: The tree used as the base state for the 'merge' command
 * END: The tree used as the ending state for the 'merge' command
    The difference between these trees will be merged into the
    WORKING and ACTUAL trees.

In the following example 10 == START and 15 == END:
  $ svn merge -r10:15 http://svn.example.com/svn/ .

Please note that the WORKING-TARGET and ACTUAL-TARGET trees also
apply to 'svn merge' as they can result in 'add with history' schedules,
which will place text bases in the WORKING-TARGET tree.  Also note
that -since merge is by definition an 'edit' operation- the BASE and
BASE-TARGET trees are not concerned with a merge.

###EHU: To which trees do BASE and TARGET refer when we're in a subdir
of a replaced tree? And which trees do they refer to in a subdir of
a replaced tree which itself is replaced? (Preliminary answer: the
base in a replaced subdir should probably be the base as defined by
the parent which got copied in, not the base as was deleted, because
otherwise it won't be possible to delete files from the replaced subdir:
there would be no way to express a deletion against the new dir.)



Requirements
============

 * Developer sanity
   From this requirement, a number of additional ones follow:
    - Very explicit tree state management; clear difference between
      each of the 5 states we may be looking at
    - It must be "fun" to code wc-ng enhancements
 * Speed
   (Note: a trade off may be required for 'checkout' vs 'status' speed)
 * Cross-node-type working copy changes
 * Flexibility
   The model should make it easy to support
     - central vs local metadata storage
     - Last modified timestamp behaviours
     - .svn-less working copy subtrees
     - different file-changed detection schemes
        (e.g. full tree scan as in wc-1.0 as well as 'p4 edit')
 * Graceful (defined) fallback for non-supported operations
   When a checkout tries to create a symlink on an OS which supports
   them, on a filesystem which doesn't, we should cope without
   canceling the complete checkout.  Same for marking metadata read-only.
 * Gracefully handle symlinks in relation to any special-handling of
   files (don't special-handle symlinks!)
 * Clear/reparable tree state
   Other than our current loggy system, I mean here: "there is a command
   by which the user can restart the command he/she last issued and
   Subversion will help complete that command", which differs from our
   loggy system in the way that it will return the working copy to a
   defined (but to the user unknown) state.
 * Transactional/ repairable tree state (with which I mean something
   which achieves the same as our loggy system, but better).
 * Case sensitive filesystem aware / resilient
 * Working copy stability; a number of scenario's with switch and
   update obstructions used to leave the working copy unrecoverable
 * Client side 'true renames' support where one side can't be committed
   without the other (relates to issue #876)
 * Change detection should become entirely internal to libsvn_wc (referring
   to the fact that libsvn_client currently calls svn_wait_for_timestamps()),
   even though under 'use-commit-times=yes', this waiting is
   completely useless.
 * Last-modified recording as a preparation for solving issue #1256 and
   as defined in this mail, also linked from the issue:
   http://svn.haxx.se/dev/archive-2006-10/0193.shtml
 * Representing "this node is part of a replaced-with-history tree and
   I'm *not* in the replacement tree" as well as "... and I'm deleted
   from the replacement tree" [issues #1962 and #2690]


Would-be-very-nice-to-have's
============================

 * Multiple users with a single working copy (aka shared working copy)
 * Ending up with an implementation which can use current WCs
   (without conversion)
 * Working copies/ metadata storages without local storage of text-bases
   (other than a few cached ones)


Non-goals
=========

 * Off-line commits
 * Distributed VC

Open items / discussion points
==============================

 * Files changed during the window "sent as part of commit" to
    "post commit wc processing"; these are currently explicitly
    supported. Do we want to keep this support (at the cost of speed)?
 * Single working copy lock. Should we have one lock which locks the
    entire working copy, disabling any parallel actions on disjoint
    parts of the working copy?
 * Meta data physical read-only marking (as in wc-1.0). Is it still
    required, or should it become advisory (ie ignore errors on failure)?
 * Is issue #1599 a real use-case we need to address?
    (Loosing and regaining authz access with updates in between)


Problems in wc-1.0
==================

 * There's no way to clear unused parts of the entries cache
 * The code is littered with path calculations in order
   to access different parts of the working copy (incl. admin areas)
 * The code is littered with direct accesses to both wc files and
   admin area files
 * It's not always clear at which time log files are being processed
   (ie transactions are being committed), meaning it's not always
   clear at which version of a tree one is looking at: the pre or post
   transformation versions...
 * There's no support for nested transactions (even though some
   functions want to start a new transaction, regardless whether one
   was already started)
 * It's very hard to determine when an action needs to be written
   to a transaction or needs to be executed directly
 * All code assumes local access to admin (meta)data
 * The transaction system contains non-runnable commands
 * It's possible to generate combinations of commands, each of which
   is runnable, but the series isn't
 * Long if() blocks to sort through all possible states of
   WORKING, ACTUAL and BASE, without calling it that.
 * Large if() blocks dealing with the difference between file and
   directory nodes
 * Many special-handling if()s for svn:special files
 * Manipulation of paths, URLs and base-text paths in 1 function
 * 'Switchedness' of subdirectories has to be derived from the
   URLs of the parent and the child, but copied nodes also have
   non-parent-child source URLs... (confusing)
 * Duplication of data: a 'copied' boolean and a 'copy_source' URL field
 * Checkouts fail when checking out files of different casing to a case
   insensitive filesystem
 * Checkouts fail when marking working copy admin data as read-only
   is a non-supported FS operation (VFAT or Samba mounts on Linux have
   this behaviour)
 * Obstructed updates leave operations half done; in case of a switch,
   it's not always possible to switch back (because the switch itself
   may have left now-unversioned items behind)
 * Directories which have their own children merged into them (which happens
   when merging a directory-add) won't correctly fold the children into
   schedule==normal, but instead leave them as schedule==add, resulting in
   a double commit (through HTTP, other RA layers fold the double add, but
   that's not the point) [see issue #1962]
 * transaction files (ie log files) are XML files, requiring correct
   encoding of characters and other values; given the short expected
   life-time of a log file and the fact that we're almost completely sure
   the log file is going to be read by the WC library anyway (no interchange
   problems), this is a waste of processing time
 * No strict separation between public and internal APIs: many public
   APIs also used internally, growing arguments which *should* only
   matter for internal use


Possible solutions
==================

Developer sanity
----------------
Strict separation between modules should help keep code focused at one
task.  Probably some of the required user-specific behaviours can (and
should) be hidden behind vtables; for example: setting the file stamp
to the commit time, last recorded time or leaving it at the current time
should be abstracted from.

Access to 'text bases' is another one of these areas: most routines in
wc-1.0 don't actually need access to a file (a stream would be fine as
well), but since the files are there, availability is assumed.
When abstracting all access into streams, the actual administration of
the BASE tree can be abstracted from: for all we know the 'tree storage
module' may be reading the stream directly off the repository server.
[The only module in wc-1.0 which *requires* access to the files is
the diff/merge library, because it rewinds to the start of the file
during its processing; an operation not supported by streams... and even
then, if these routines are passed file handles, they'll be quite
happy, meaning they still don't need to know where the text base /
source file is...]

In order to keep developers sane, it should be extremely clear at any
one time - when operating on a tree - which tree is being operated upon.

One way to prevent the lengthy 'if()' blocks currently in wc-1.0, would be
to design a dispatch mechanism based on the path-state in WORKING/BASE and the
required transformation, dispatching to (small) functions which perform
solely that specific task.


Speed
-----
wc-1.0 assumes the WORKING tree and the ACTUAL tree match, but then
goes out of its way to assure they actually do when deemed important.
The result is a library which calls stat() a lot more often than need be.

One of the possible improvements would be to make wc-ng read all of
the ACTUAL state (concentrated in one place, using apr_stat()), keeping
it around as long as required, matching it with the WORKING state before
operating on either (not only when deemed important!).

Working from the ACTUAL tree will also prove to be a step toward clarity
regarding the exact tree which is being operated upon.

[This suggestion from wc-improvements also applies to wc-ng:]
Most operations are I/O bound and have CPU to spare.  Consider the virtue
of compressed text bases in order to reduce the amount of I/O required.

Another idea to reduce I/O is to eliminate atomic-rename-into-place for
the metadata part of the working copy: if a file is completely written,
store the name of the base-text/prop-text in the entries file, which gets
rewritten on most wc-transformations anyway.


Cross node type change representation
-------------------------------------
####EHU To be done

Flexibility of metadata storage
-------------------------------
There are 3 known models for storing metadata as requested by different
groups of users:

 - in-subtree metadata storage (.svn subdir model, as in wc-1.0)
 - in-'tree root' metadata storage (working copy central)
 - detached metadata storage (user-central)

A solution to implementing each of these behaviours in order to satisfy
the wide range of use-cases they solve, would be to define a module
interface and implement this interface three times (possibly using vtables).

Note that using within-module vtables should be less problematic than our
post-1.0 experiences with public vtables (such as the ra-layer vtable):
implementation details are allowed to differ between releases (even patch
releases).


Transaction duration / memory management
----------------------------------------
The current pool-based memory management system is very good at managing
memory in a transaction-based processing model.  In the wc library, a
'transaction' often spans more than one call into the library.  We either
need a sane way to handle this kind of situation using pools, or we may
need a different memory management strategy in wc-ng.

Working copy stability
----------------------
In light of obstructed updates it may not always be desirable to be able
to resume the current operation (as currently is the case): in some cases
the user may want to abort the operation, in other cases the user may
want to resolve the obstruction before re-executing the operation.

The solution to this problem could be 'atomic updates': receiving the
full working copy transformation, verifying prerequisites, creating
replacement files and directories and when all that succeeds, update
the working copy.

Full workin' copy unit tests:
Exactly because the working copy is such an important part of the
Subversion experience *and* because of the 'reputation' of wc-1.0,
we need a way to ensure wc-ng completely performs according to our
expectations.  *The* way to ensure we're able to test the most contrived
edge-cases is to develop a full unit testing test-suite while developing
wc-ng.  This will both be a measure to ensure working copy stability
as well as developer sanity: in the early stages of the wc-ng develop-
ment process, we'll be able to assess how well the design holds up
under more difficult 'weather'.

Transactional updates
---------------------

.. where 'update' is meant as 'user command', not 'svn update' per se.

When applied to files, this can be summarized as:

 * Receive transformations (update, delete, add) from
   the server,


Prerequisites for a good wc implementation
==========================================

These prerequisites are to be addressed, either as definitions
in this document, or elsewhere in the subversion (source) tree:
 * Well defined behaviour for cross-node type updates/merges/..
   (tree conflicts in particular)
 * Well defined behaviour for special file handling
 * Well defined behaviour for operations on locally missing items
     (see issue #1082)
 * Well defined change detection scheme for each of the different
     last-modified handling strategies
 * No special handling of symlinks: they are first class versioned objects
 * Well defined behaviour for property changes on updates/merges/...
   (this is a problem which may resemble tree conflicts!),
   including 'svn:' special properties
 * File name manipulation routines (availability)
 * File name comparison routines (!) (availability; which compensate
     for the different ways Unicode characters can be represented
     [re: NFC/NFD Unicode issue])
 * URL manipulation routines (availability)
 * URL comparison routines (availability; which compensate for
     different ways the same URL can be encoded; see issue #2490)
 * Modularization
 * Agree on a UI to pull in other parts of the same repository
   (NOT svn:externals) [relates to issue #1167]
 * Agree on behaviour for update on moved items (relates to issue #1736)
 * Case-sensitivity detection code to probe working copy filesystem


Modularization
==============

Strict separation must be applied to a number of modules which can be
recognised.  This will help prevent spaghetti code as in wc-1.0 where
one piece of code manipulates paths to a working copy file, its URL
*and* the path to the base file.

For now, these APIs can be separated:

 - the public API (presumably not to be used by any internal
     processing, but presents functionality to working copy users)
 - tree administration API (required for BASE, TARGET and WORKING)
     Admins which files are part of the tree, which ones map to
     which repositories and which textbase / propbase files belong
     to which local files. [should provide checkpointing functionality
     for use with transactional tree modifications API]
 - tree access API (required for BASE, WORKING, TARGET and ACTUAL)
     Gives access to the content of the nodes in a tree
       - props
       - text bases (for files)
       - child nodes (for directories)
 - transactional tree modifications API (applicable to all trees,
     ###EHU do we provide the same interface to BASE/WORKING as for ACTUAL?)
 - tree transformation (required for update/switch/merge updating
     BASE, WORKING and ACTUAL), meaning all of tree changes, file
     changes and metadata changes
 - Working-copy changedness detection API
 - Metadata access API (used by tree administration module(s))
 - Event hooks (in order to be able to implement different
   timestamp-setting strategies and possibly more)

These APIs will be implemented by these (currently known) modules:

 - tree administration
   * wc_adm
 - tree access
   * wc_acc
 - transactional tree modifications
   * wc_log
 - tree transformation
   * wc_trans
 - working copy changedness detection
   wc_detect vtable-based API implemented by these modules:
     * tree crawler ('inspired' by wc-1.0)
     * tree marker (inspired by 'p4 edit')
 - metadata access API
   wc_macc vtable-based API implemented by these modules:
     * tree spread ('inspired' by wc-1.0)
     * tree root (storing all metadata in the tree root (think darcs))
     * central depot (storing 'somewhere' locally, possibly $HOME)
        this central store would open up the possibility to share
        text bases/prop bases across checkouts
     * non-local (retrieving all text and prop-bases from the server,
        except for a number of cached ones) ###EHU: maybe this is
        orthogonal to the question where metadata is stored: in all
        situations, you *could* choose not to keep local copies
 - Event hooks for the union of all paths in (BASE, WORKING)
   wc_hook event based single-callback API
   for e.g. these events:
        + props updated
        + base text updated
        + wc file updated
        + update completed
        + lock acquired
        + lock released
       (+ lock can't be acquired [in order to 'unprotect'
           svn:needs-lock protected files which have been removed
           from the repository?])
   to be implemented by these modules:
     * use-commit-times
     * versioned-mtimes
     * versioned-execute-perm
     * versioned-other-unix-perms
    (* versioned-windows-perms?)
     * needs-lock-updater

Justification for the large number of modules, with a modest number
of different APIs is that the problem is really quite complex as shown
earlier in this document.

Over the years, a large number of use cases have developed around
Subversion where different user groups have shown very valid use cases
for conflicting behaviours.  Presumably, most of these we want to
retain.  Some of the unimplemented ones have open issues indicating
there's at least an active interest.  In order to prevent locking out
some of the current use cases adding support for the open issues, we
need a flexible modularized model.  This model will also prevent that
we'll end up duplicating lots of code to support the different use cases.

Given the assumption of 'little code duplication', the choice for
having several modules which implement the same API (vtable) is
justifiable.


Implementation proposals
========================

Classification of svn_wc_entry_t fields to BASE/WORKING
-------------------------------------------------------

[Note: This section is mainly to clarify the difference between the BASE
and WORKING trees, it's not here to mean that we actually need all these
fields in wc-ng!]

Here are the mappings of all fields from svn_wc_entry_t to the BASE and
WORKING trees:

 +-------------------------------+------+---------+
 |       svn_wc_entry_t          | BASE | WORKING |
 +-------------------------------+------+---------+
 | name                          |  x   |    x (1)|
 | revision                      |  x   |    x (2)|
 | url                           |  x   |    x (2)|
 | repos                         |  x   |    x (3)|
 | uuid                          |  x   |    x (3)|
 | kind                          |  x   |    x    |
 | absent                        |  x   |         |
 | copyfrom_url                  |      |    x    |
 | copyfrom_rev                  |      |    x    |
 | conflict_old                  |      |    x    |
 | conflict_new                  |      |    x    |
 | conflict_wrk                  |      |    x    |
 | prejfile                      |      |    x    |
 | text_time                     |      |    =    |
 | prop_time                     |      |    =    |
 | checksum                      |  x   |    x (2)|
 | cmt_rev                       |  x   |    x (2)|
 | cmt_date                      |  x   |    x (2)|
 | cmt_author                    |  x   |    x (2)|
 | lock_token                    |  x(6)|         |
 | lock_owner                    |  x   |         |
 | lock_comment                  |  x   |         |
 | lock_creation_date            |  x   |         |
 | has_props                     |  x   |    x (4)|
 | has_prop_mods                 |      |    =    |
 | cachable_props                |  x(5)|    x (4)|
 | present_props                 |  x   |    x (4)|
 | changelist                    |      |    x    |
 | working_size                  |      |    =    |
 | keep_local                    |      |    =    |
 | depth                         |  x   |    x    |
 | schedule                      |      |         |
 | copied                        |      |         |
 | deleted                       |      |         |
 | incomplete                    |      |         |
 +-------------------------------+------+---------+

(1) if this one differs from BASE, it must point to the source of a rename
(2) for an add-with-history
(3) or can we assume single-repository working copies?
(4) can differ from BASE for add-with-history
(5) why is this a field at all; can't the WC code know?
(6) locks apply to in-repository paths, hence BASE

The fields marked with '=' are implementation details of internal detection
mechanisms, which means they don't belong in the public interface.

Fields with no check are to become obsolete. 'schedule', 'copied' and
'deleted' can be deducded from the difference between the BASE and WORKING
or WORKING and ACTUAL trees.  'incomplete' should become obsolete when the
goal of 'atomic updates' can be realised, in which case the tree can't be
in an incomplete yet locked state.  This would also invalidate issue #1879.


Other sections
--------------
 remain to be done