~bzr-pqm/bzr/bzr.dev

The status command

==================

The status command is used to provide a pithy listing of the changes between

two trees. Its common case is between the working tree and the basis tree, but

it can be used between any two arbitrary trees.

.. contents:: :local:

UI Overview

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Status shows several things in parallel (for the paths the user supplied mapped

across the from and to tree, and any pending merges in the to tree).

1. Single line summary of all new revisions - the pending merges and their

   parents recursively.

2. Changes to the tree shape - adds/deletes/renames.

3. Changes to versioned content - kind changes and content changes.

4. Unknown files in the to tree.

5. Files with conflicts in the to tree.

Ideal work for working tree to historical status

------------------------------------------------

We need to do the following things at a minimum:

1. Determine new revisions - the pending merges and history.

1. Retrieve the first line of the commit message for the new revisions.

1. Determine the tree differences between the two trees using the users paths

   to limit the scope, and resolving paths in the trees for any pending merges.

   We arguably don't care about tracking metadata for this - only the value of

   the tree the user commited.

1. The entire contents of directories which are versioned when showing

   unknowns.

1. Whether a given unversioned path is unknown or ignored.

1. The list conflicted paths in the tree (which match the users path

   selection?)

Expanding on the tree difference case we will need to:

1. Stat every path in working trees which is included by the users path

   selection to ascertain kind and execute bit.

1. For paths which have the same kind in both trees and have content, read

   that content or otherwise determine whether the content has changed. Using

   our hash cache from the dirstate allows us to avoid reading the file in the

   common case. There are alternative ways to achieve this - we could record

   a pointer to a revision which contained this fileid with the current content

   rather than storing the content's hash; but this seems to be a pointless 

   double-indirection unless we save enough storage in the working tree. A

   variation of this is to not record an explicit pointer but instead

   define an implicit pointer as being to the left-hand-parent tree.

Locality of reference

---------------------

- We should stat files in the same directory without reading or statting

  files in other directories. That is we should do all the statting we

  intend to do within a given directory without doing any other IO, to

  minimise pressure on the drive heads to seek.

- We should read files in the same directory without reading or writing

  files in other directories - and note this is separate to statting (file

  data is usually physically disjoint to metadata).

Scaling observations

--------------------

- The stat operation clearly involves every versioned path in the common case.

- Expanding out the users path selection in a naive manner involves reading the

  entire tree shape information for both trees and for all pending-merge trees.

  (Dirstate makes this tolerably cheap for now, but we're still scaling

  extra-linearly.)

- The amount of effort required to generate tree differences between the

  working tree and the basis tree is interesting: with a tree-like structure

  and some generatable name for child nodes we use the working tree data to

  eliminate accessing or considering subtrees regardless of historival

  age. However, if we have had to access the historical tree shape to

  perform path selection this rather reduces the win we can obtain here.

  If we can cause path expansion to not require historical shape access

  (perhaps by performing the expansion after calculating the tree

  difference for the top level of the selected path) then we can gain a

  larger win. This strongly suggests that path expansion and tree

  difference generation should be linked in terms of API.

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