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Aventures with the dump file format

From: Edmund Horner <edmund_at_chrysophylax.cjb.net>
Date: 2003-12-08 11:17:02 CET

Hi all,

Warning: this rambling e-mail is "informative": it's just informing you
of my first-time experiences with the Subversion code. Any comments
(+ive or -ive) on what I've done would be gladly received.

Last night I began to write a dump file filter (in the vein of
svndumpfilter), whose most exciting feature is that it doesn't make any
use of the Subversion libraries. That's because I couldn't get
Subversion to compile at the time.

Writing dump file parsing from scratch, according to
notes/fs_dumprestore.txt, was surprisingly satisfying, just as is
climbing a large hill "because its there".

I. A strange error which doesn't seem to cause any harm.

The first test for the filter was to give the same output,
byte-for-byte, as the input. That was acheived, except for one problem:
r1 of the test input (generated by svnadmin) had a Prop-length that was
5 bytes smaller than its Content-length! (There was in fact
Content-length bytes worth of props, incl. "PROPS-END\n".) Now, as I
understand it, revision records don't have any text content.

Strangly enough, svnadmin had no trouble loading this file. I assume it
ignored Prop-length and used Content-length instead?

II. Inadvertantly removing properties.

The purpose for my filter is to retroactively add "svn:eol-style:
native" to file paths matching *.c, etc. From looking at
libsvn_repos/dump.c, I take it properties are only dumped if the file is
new, or if they are changed. And in load.c I learned that whenever node
properties are read from a dumpfile, existing properties for the node
are cleared first.

So the problem is: if I change the dumpfile to have an extra property
for a node, then I will also need to know the existing properties so
they can be specified as well. Unless a node is being added or
properties are changed, the properties aren't in the dumpfile.

That is, if I add "svn:eol-style" to a node that isn't new, then I will
effectively be wiping any properties set in earlier revisions.

In practice this doesn't seem to have been a problem, since I only add
"svn:eol-style" when a node is added, in which case I append it to any
other properties that exist when it's added.

Another point I'm not clear on is the difference between having no prop
block in the dumpfile, and having a block that is simply "PROPS-END\n"
-- do they both cause existing props for the node to be cleared?

III. Asking svnadmin to always dump properties for nodes.

Now that Subversion is compiling (for the first time, yay!), I have had
a little play with svnadmin, and added an option called --all-props.
This option causes svnadmin to always include the properties for nodes
when dumping, even if they haven't changed.

Admittedly it's a pretty obscure feature, but I think it does mean a
dump filter can change node properties with impunity.

Edmund Horner.

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Received on Mon Dec 8 11:18:35 2003

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