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Fix for CVS Keyword Expansion Bug

October 28th, 2005 by rahul

This bug and a proposed patch was submitted to the CVS community by me. It ran into religious hurdle w.r.t keyword storage in SCM repository, it has not yet been accepted by the CVS community but nevertheless has been available for download. Here is the original submission by me:

The cvs server will automatically (or, more accurately, as part of the update run that automatically happens after a commit) expand valid keyword strings in a text file as part of the commit operation. In other words it is the responsibility of the ‘server-side’ of cvs to fill in the appropriate values for the RCS keywords.

Currently where this breaks down is when the end-user modifies the content of the RCS keywords. For example, let’s say a file contains:

File : foo
Line 1: $Id$

On the initial commit, the server returns the file (if keyword expansion is not turned off) and expands Id a –

File: foo
$Id: id,v 1.1 2005/10/28 18:39:46 userX Exp $

If the user accidentally or intentionally changes just the expanded value (no other change) on the client-side to:

File: foo (revision 1.1 in user sandbox)
$Id: id,v 1.9 3005/10/28 18:39:46 userYYY Exp $

and then does a commit, the CVS server as it stands today will be happy to commit a new version but as part of the update run that automatically happens after a commit, will re-expand the keyword and the CVS client will have the following file in the sandbox :

File: foo
$Id: id,v 1.2 2005/10/28 18:46:18 userX Exp $

In other words the user’s intent of modifying the keyword content is not honored. This make sense as keyword expansion is owned by the server and a client can not wily-nilly change the keyword content (between the two $, with the exception of $Log$ keyword of-course)

However, this also causes spurious revisions to be created in the CVS repository and this revision is not what the end user expects. The revision metadata within the $..$ keyword expansion is controlled by the server and as seen in the above example, the user can’t change it even though his attempt to do so results in a new revision being committed.

We have come across large CVS repositories where lots of such spurious revisions (no content change except keyword) exist on several files. This causes problems downstream when for example doing branch merges, as differences on just keyword expansion can cause the file to be treated as changed. Creating new revisions is an important step. It has consequence on downstream tools. Per-user stats (how may commits by a given user, for example) are impacted when revisions are created. One way to deal with this problem is to turn off keyword expansion (as available in 1-12-XX feature tree or use -ko for every file that has keywords), but that is a sledgehammer approach that denies the benefits of RCS keywords to the end users.

The patch (off the 1-11-21 tree) that we have attached addresses this bug/mis-feature. It ensures keyword expansion will not cause spurious revisions that differ just in keyword content that the CVS server inserts between $..$ markers. With the patch, a CVS user can still use keyword expansion as expected, but during the file classification phase of a CVS commit, the patch kicks in and triggers a keyword-sensitive diff algorithm, that we have developed, which will ignore RCS/CVS keyword differences during this phase. This reduces spurious revision creation. The patch deals with all keywords except the logmsgs after a $Log$ keyword.

The patch uses a space-time efficient diff’ing algorithm that does not require the entire file to be read into memory to do the keyword canonicalization. It operates on a block at a time. The algorithm does not need to make multiple passes. As described in the src comments, the algorithm is implemented as a finite state machine that tracks the state across block comparisons and can deal with keywords straddled across block boundaries etc.

Right now, the patch is turned on by default. This behavior can be controlled via a new option that we have added to CVSROOT/config: “IgnoreKeywordDiffsAtCommit”. If set to “no” it will switch back to the old semantics, just in case someone needs that. The patch also builds a new executable “kwdiff” that can be used for comparing checked-out files such that keyword differences can be ignored. This is useful when comparing files between two users each with their own sand-box. It is also used for unit testing the implementation(see below).

As part of the patch, we have added new tests to We have also fixed testcases that were expecting the old behavior (spurious revisions). To test the algorithm for every possible corner case, we have also devised a unit test that generates millions of test file permutations. These are perl based and we didn’t integrate them into as it would change the standalone nature of the script. Memory tools like valgrind have been run on the patch to ensure there are no memory errors.

In the future, it may make more sense to create a unit test subdir for testing components of cvs (as opposed to the full cvs executable), in which case the kwdiff unit tests could be checked in there.

This patch enables the keyword sensitive diff algorithm only during commits, but the CVS community may want to consider it for updates, especially to reduce unnecessary keyword related conflicts during branch merges. Given how the code is structured it is relatively easy to turn on and off the keyword canonicalization algorithm for other cases beyond commits. Today CVS reads the entire RCS file into memory for many operation. This increases the footprint of the CVS process and can be a scalability bottleneck. The file chunk management code in kwfilecmp.c shows that it is possible to operate on a block basis, thus bounding the footprint of the cvs server process, regardless of the size of the file being processed. Perhaps, in the future, this module could be re-used for eliminating the need to read in the entire file into memory for many CVS operations.

The patch has been developed off the 1.11.21 branch. It can easily be enhanced for the feature branch to take into account local keyword aliases. This will require changes to a single function ( is_valid_keyword).

Attached bzip2 file contains the kwdiff.patch (actual patch) and the kwdiff-unit-test/ subdir.

We hope the CVS community will find the patch useful.

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