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-*- 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...]
###GJS: the APIs should use streams so that we can decompress as the
stream is being read. the diff library will need a callback of some
kind to perform the rewind, which will effectively just close and
reopen the stream. if it rewinds *multiple* times, then we may want
to cache the decompressed version of the file. I'll
investigate. Given our metadata/base-text storage system, I suspect
it will be very easy to cache decompressed copies for a while.
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.
#####XBC Do please note that this suggests yet another instance of
pure polymorphism coded in C. This runs contrary to the
developer sanity requirement.
###GJS: agreed with XBC.
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!).
###GJS: working copy file counts are unbounded, so we need to be
careful about keeping "all" stat results in memory. I'll certainly
keep this in mind, however.
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)
###GJS: euh... aren't we axing this? who has *requested* this?
- in-'tree root' metadata storage (working copy central)
- detached metadata storage (user-central)
- in $HOME/.subversion/
- in arbitrary location (e.g. $HOME is a (slow) NFS mount, and we
want the metadata on a local drive, such as /var/...)
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).
###GJS: note that we are talking about both metadata AND base-text
content. (and yeah, optional and compresses base-texts can be done
during this rewrite) Also note that we might be able to share
base-text content across working copies if they are all keyed by
the MD5 hash into storage directories (under the user-central model)
###GJS: I don't think vtables are needed here. This is simply altering
the base location, not a whole new implementation. My plan is to
default to the "tree root" model with a .svn subdirectory. If a
.svn subdir is not found, then we fall back to looking in the
$HOME/.subversion/ directory (some subdir under there). If we
*still* don't find it, then some config options will point us to
the metadata/base-text location.
###GJS: my plan is to upgrade the working copy if we find a pre-1.6
working copy. all the data will be lifted from the multiple .svn
subdirectories, and relocated to the "proper" storage location.
This will be a non-reversable upgrade, and will preclude pre-1.6
clients from using that working copy again.
Note: because of the "destructive" nature of this upgrade, and the
expected duration, we may want to require the user to perform an
explicit action in order to complete the upgrade. However, 1.6 will
not be able to *modify* wc-1.0 metadata -- just read it in order to
upgrade it to the new storage system.
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'.
###GJS: agreed. as much as possible, when I (re)implement the old APIs
in terms of the new APIs, then I'll write a whitebox test. we'll
see how long I keep that up :-P
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]
#####XBC I submit this is a server-side feature that the client
(i.e. the WC library) should not know about.
* 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)
#####XBC This is really required of all our module public APIs.
- 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.
#####XBC Such flexibility will bring the WC to the kind of
purgatory the RA layers are in. We promise feature and semantics
parity between them, and the result is that even a small change
in that layer requires knowledge of three different protocols
and four different implementations.
Given the assumption of 'little code duplication', the choice for
having several modules which implement the same API (vtable) is
justifiable.
###GJS: disagree. I plan to have just one library (libsvn_wc2?) and
will probably have no vtables. there is very little need for
distinct implementations, as far as I can tell.
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.
Basic Storage Mechanics
-----------------------
All metadata will be stored into a single SQLite database. This
includes all of the "entry" fields *and* all of the properties
attached to the files/directories. SQLite transactions will be used
rather than the "loggy" mechanics of wc-1.0.
###GJS: note that atomicity across the sqlite database and the content
of the ACTUAL tree is freakin' difficult. idea to test: metadata
says "not sure of ACTUAL", and when ops complete successfully, then
we clear the flag. during any future operation, if the flag is
present, then we approach the ACTUAL with extreme prejudice. also
note that we can batch clearing of the flags as an optimistic
efficiency approach (since if we batch 100 and the last fails, then
the other 99 will be slower until the wc-ng determines the ACTUAL
is in fine shape and clears the flag for future operations).
###GJS: be wary of sqlite commit performance (based on some of my
prior experience with it). must have timing/debugging around the
commit operations. may need to use various transaction isolations
and/or batching of commits to get proper performance. thus, profile
output capability is mandatory to determine if we have issues, and
where they occur.
Base text data will be stored in a multi-level directory structure,
keyed/named by the MD5 of the file. The database will record
appropriate mappings, content/compression types, and refcounts for the
base text files (for the shared case).
Metadata Schemas
----------------
BASE schema
name string
kind enum
checksum string ### string should indicate what kind of checksum?
props blob ### serialized props. maybe keep unserialized?
### maybe some props should be broken out?
children [string] ### someting more than array-of-string?
cmt_rev integer64
cmt_date integer64 ### is there a time type?
cmt_author string ### normalize? (besides utf8, what does "normalize" mean?)
absent boolean ### really? maybe state instead, such as "omitted"?
revision integer64
url string ### should be computed value?
repos_url string ### shared, per-dir?
repos_uuid string ### shared, per-dir?
### lock information? in this table, or a separate locks table?
### KFF: in this table, I think. It's rare to tell Subversion "Show
### me everything that's locked", but it's common to say "Tell me if
### this particular path [or set of paths] is locked".
###
### depth?
WORKING schema
name string ###
kind enum
checksum string
props blob ### serialized props. maybe keep unserialized?
### maybe some props should be broken out?
children [string] ### someting more than array-of-string?
### following are set when a copyfrom occurs. do we really need these?
cmt_rev integer64
cmt_date integer64 ### is there a time type?
cmt_author string ### normalize?
deleted boolean
copyfrom_url string
copyfrom_rev integer64
conflict_old string ### conflict data goes on WORKING, right?
conflict_new string
conflict_wrk string
prejfile string
changelist string
### KFF: With changelists, it's the opposite of locks: one
### frequently says "show me all the files in changelist X"; I'm
### not sure how frequently we need to say "show me all the
### changelists this file is part of". But anyway, the need to
### discover all the files in changelist X implies that it might be
### better to make a changelist table?
### how to represent renames?
### repos_url, repos_uuid: combined working copies, and/or a foreign
### repos brought in and about to be linked via svn:externals. ??
### depth?
ACTUAL schema (what we know about the ACTUAL tree)
name string
### cache the checksum? probably unreliable, but could be a marker
### for "we've already noted the file is different from WORKING"
### cache the kind? assume that kind, then on exception do extended
### processing to figure out the "right" thing.
lastmod integer64 ### is there a time type?
### note that we can't really hold any/much data here since ACTUAL
could change at any point in time. anything stored here should
simply be optimization indicators on how to approach the ACTUAL
tree (i.e. what assumptions to make).
BASETEXT schema
checksum string ### KFF: how is this different from the
### checksum in the "BASE" schema above?
present enum ### something like: yes, no, cached, notyet
refcount integer
compressed boolean ### enum for various storage styles?
### learn from GIT storage? can we somehow compress across this
entire storage heirarchy? (eg. use history and deltas to
reassemble fulltexts from line-of-descent)
Note that 'children' applies only to kind==DIRECTORY
### break down all modification operations to things that operate on a
small/fixed set of rows. if a large sequence of operations fails,
then it can leave the system in reparable state, since most were
performed. note that ACTUAL can change at any time, thus all mods
should be able to compensate for ACTUAL being something
unexpected. thus, the transformative operations should be able to
fail in such a way as to leave ACTUAL pretty bunged up.
### maybe use handles to refer to files/dirs? take input pathname,
convert to native charset, and return a handle for that. same
handle across varous schemas.
note: could be very handy, as I'm thinking abspath for all names,
which gets to be pretty wordy for large working copies.
note: with handles, a file entry could have "parent dir" cheaply,
and from that, we could derive repos_url/uuid from some dir table.
### probably want to special-case the checksum and BASETEXT entry for
the "empty file"
Code Organization
-----------------
libsvn_wc/wc_db.h (symbols: svn_wc__db_*)
Storage subsystem for the WC metadata/base-text information.
This is a private API, and the rest of the WC will be rebuilt
on top of this.
This code deals with storage, and transactional modifications
of the data.
Other sections
==============
remain to be done