notes/moves - trunk - niceSOFT/subversion

notes/moves
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Julian Foad
Update wiki links to point to the new(ish) Confluence Wiki.
24 сен 2019, 18:54
24 сен 2019, 18:54d6dbfd8

 =======================
 Moves in Subversion 1.8
 =======================

This file purposefully talks about 'moves' rather than 'renames'.
This isn't about true renames as requested in issue #898.
Rather, we keep the add+delete concept while trying to make moves behave
more in a way that one would expect if true renames were implemented,
as requested in issue #3631. See also the umbrella issue #3630.

So far the changes only cover local (client-side) moves in the working copy.
We reuse as much existing code as possible, so new functionality is
implemented as part of existing copy/delete code paths unless doing
so is not feasible.

One significant change from how copies work is that moves are tracked
both ways, i.e. one can locate the add-half of a move if given the
delete-half, and locate the delete-half if given the add-half.

The goals are:

  - Improve the behaviour for tree-conflicts involving a local move.
      A "local move vs. edit" tree conflict should be automatically resolved.
      Any tree conflict involving a local move should clearly indicate
      so in its description, saying 'local move' instead of 'local delete'
      or 'local add'.

  - Prepare the client to be able to use the editor-v2 rename interfaces
    when talking to the server.


Notes regarding specific layers of Subversion follow below.


== wc.db ==

The following columns in the NODES table are used to differentiate
moves from copies:

  /* Boolean value, specifying if this node was moved here (rather than just
     copied). This is set on all the nodes in the moved tree.  The source of
     the move is implied by a different node with a moved_to column pointing
     at the root node of the moved tree. */
  moved_here  INTEGER,

  /* If the underlying node was moved away (rather than just deleted), this
     specifies the local_relpath of where the node was moved to.
     This is set only on the root of a move, and is NULL for all children.

     The op-depth of the moved-to node is not recorded. A moved_to path
     always points at a node within the highest op-depth layer at the
     destination. This invariant must be maintained by operations which
     change existing move information. */
  moved_to  TEXT,

Many queries were added or changed to use these columns.

== libsvn_wc ==

In the internal wc_db API, the _scan_addition() and _scan_deletion()
interfaces were extended to make use of new DB queries to differentiate
moved nodes from copied, added, and deleted nodes.

Two functions were built on top of the wc_db API and added to the
private libsvn_wc API:
  svn_wc__node_was_moved_away() provides, for a given local_abspath:
    - the moved_to abspath within the working copy
    - the abspath of the op-root of the copy operation that created
      the node at the moved_to abspath
  svn_wc__node_was_moved_here() provides, for a given local_abspath:
    - the moved_from abspath within the working copy
    - the abspath of the op-root of the delete operation that deleted
      the node at the moved_from abspath

More API changes might be needed (TBD).
In particular, scan_deletion may need to return a list of moves
in the multi-layer case (https://cwiki.apache.org/confluence/display/SVN/MultiLayerMoves)

We might require a working copy upgrade when going from 1.7 to 1.8,
and only allow new move functionality to be used with 1.8 working copies.
 

== libsvn_client ==

This layer already uses 1.7 and earlier svn_wc move APIs. For callers
of such APIs, changes will hopefully be fairly transparent apart from
changes that enhance behaviour of move operations.

Interfaces which have changed behaviour:

 svn_client_commit: Commit will refuse to commit anything if only one
   half of a move appears in the commit target list, or if only one half of
   a move is picked up by recursion.

 svn_client_revert: The behaviour of this API is not changed, but it
   is worth noting how it behaves for moves:
   - If both halves of a move are among the revert targets (either by
     virtue of being listed explicitly, or by being picked up during
     recursion), the entire move is reverted.
   - If only one half of a move is among the revert targets, the other
     half will be transformed into a normal copy or delete.
     See http://svn.haxx.se/dev/archive-2011-08/0239.shtml for rationale.

 - svn_client_status: Status provides the moved-to abspath for a moved-away
     nodes, and the moved-from abspath for a moved-here node.
     Note that, mostly due to performance reasons, only information about
     roots of moves is provided. Children of moved nodes aren't marked as such.

 - svn_client_info: Like status, except that it also provides move
     information about children of moved nodes.

 - svn_client_patch: Patch uses move information to apply changes to files
     which have been moved locally.

 - svn_client_update/svn_client_merge: Update and Merge use move
    information to auto-resolve the "local move vs. incoming edit"
    tree conflict scenario.

Interfaces which have not changed behaviour yet but might change in 1.8.0:

 - svn_client_update/svn_client_merge: Update and Merge might use move
    information to auto-resolve some additional tree conflict scenarios.

 - diff: Diff might use move information to generate 'rename from' headers
    when the --git option is used. (A related problem is making diff use
    correct copyfrom in repos-repos diffs, which is not trivial.)

Several public APIs may still be bumped as their behaviour changes.

For backwards compatibility, APIs released prior to 1.8 will continue
to treat moves just like 1.7 and earlier releases did. (However, see
also the note on working copy upgrades above, which might affect to
what degree the APIs need to stay compatible.)


== svn ==

The svn client presents moves similar to but distinct from copy operations. 

svn status shows move roots:

$ svn status
A  +    foo
        > moved from 'bar'
D       bar
        > moved to 'foo'
$

svn info shows additional Moved-To: and Moved-From: lines for any moved node.