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From Stefan Fuhrmann <stefan.fuhrm...@wandisco.com>
Subject Re: svn commit: r1604421 - in /subversion/trunk: subversion/libsvn_fs_fs/ subversion/tests/cmdline/ subversion/tests/libsvn_fs_fs/ tools/server-side/svnfsfs/
Date Thu, 26 Jun 2014 15:08:38 GMT
On Thu, Jun 26, 2014 at 11:52 AM, Ivan Zhakov <ivan@visualsvn.com> wrote:

> On 25 June 2014 22:11, Daniel Shahaf <d.s@daniel.shahaf.name> wrote:
> > Ivan Zhakov wrote on Wed, Jun 25, 2014 at 20:03:11 +0400:
> >> On 25 June 2014 19:54, Stefan Fuhrmann <stefan.fuhrmann@wandisco.com>
> wrote:
> >> > On Wed, Jun 25, 2014 at 5:46 PM, Ivan Zhakov <ivan@visualsvn.com>
> wrote:
> >> >>
> >> >> On 25 June 2014 19:24, Stefan Fuhrmann <stefan.fuhrmann@wandisco.com
> >
> >> >> wrote:
> >> >> > On Wed, Jun 25, 2014 at 4:57 PM, Ivan Zhakov <ivan@visualsvn.com>
> wrote:
> >> >> >>
> >> >> >> On 21 June 2014 17:15,  <stefan2@apache.org> wrote:
> >> >> >> > Author: stefan2
> >> >> >> > Date: Sat Jun 21 15:15:30 2014
> >> >> >> > New Revision: 1604421
> >> >> >> >
> >> >> >> > URL: http://svn.apache.org/r1604421
> >> >> >> > Log:
> >> >> >> > Append index data to the rev data file instead of using
separate
> >> >> >> > files.
>


> >> >> >> > >
> >==============================================================================
> >> >> >> >--- subversion/trunk/subversion/libsvn_fs_fs/fs_fs.c (original)
> >> >> >> >+++ subversion/trunk/subversion/libsvn_fs_fs/fs_fs.c Sat
Jun 21
> >> >> >> > 15:15:30
> >> >> >> > 2014
> >> >> >> >@@ -1449,15 +1449,35 @@ write_revision_zero(svn_fs_t *fs)
> >> >> >> >+    SVN_ERR(svn_io_file_create_binary(path_revision_zero,
> >> >> >> >+                  "PLAIN\nEND\nENDREP\n"
> >> >> >> >+                  "id: 0.0.r0/2\n"
> >> >> >> >+                  "type: dir\n"
> >> >> >> >+                  "count: 0\n"
> >> >> >> >+                  "text: 0 3 4 4 "
> >> >> >> >+                  "2d2977d1c96f487abe4a1e202dd03b4e\n"
> >> >> >> >+                  "cpath: /\n"
> >> >> >> >+                  "\n\n"
> >> >> >> >+
> >> >> >> >+                  /* L2P index */
> >> >> >> >+                  "\0\x80\x40"          /* rev 0, 8k
entries
> per page
> >> >> >> > */
> >> >> >> >+                  "\1\1\1"              /* 1 rev, 1 page,
1
> page in
> >> >> >> > 1st
> >> >> >> > rev */
> >> >> >> >+                  "\6\4"                /* page size:
bytes,
> count */
> >> >> >> >+                  "\0\xd6\1\xb1\1\x21"  /* phys offsets
+ 1 */
> >> >> >> >+
> >> >> >> >+                  /* P2L index */
> >> >> >> >+                  "\0\x6b"              /* start rev,
rev file
> size
> >> >> >> > */
> >> >> >> >+                  "\x80\x80\4\1\x1D"    /* 64k pages,
1 page
> using 29
> >> >> >> > bytes */
> >> >> >> >+                  "\0"                  /* offset entry
0 page
> 1 */
> >> >> >> >+                                        /* len, item
& type,
> rev,
> >> >> >> > checksum */
> >> >> >> >+                  "\x11\x34\0\xf5\xd6\x8c\x81\x06"
> >> >> >> >+                  "\x59\x09\0\xc8\xfc\xf6\x81\x04"
> >> >> >> >+                  "\1\x0d\0\x9d\x9e\xa9\x94\x0f"
> >> >> >> >+                  "\x95\xff\3\x1b\0\0"  /* last entry
fills up
> 64k
> >> >> >> > page
> >> >> >> > */
> >> >> >> >+
> >> >> >> >+                  /* Footer */
> >> >> >> >+                  "107 121\7",
> >> >> >> >+                  107 + 14 + 38 + 7 + 1, fs->pool));
> >> >> >> This constant makes code unreadable, unmaintainable and very
error
> >> >> >> prone.
> > ...
> >> I saw it, but it doesn't make code easier to maintain.
> >
> > Ivan, that's the third time in as many emails that you say "the new code
> > is hard to maintain".  Could you please explain *how* you find it hard
> > to maintain?
> >
> > Anyway, that's just me guessing.  Could you please clarify what exactly
> > makes the code unmaintainable in your opinion?
> >
> In my opinion 'code maintainability' is how easy or hard to make
> change and review the code. This magic constant with a lot 7b encoded
> numbers are very hard to review and modify if needed in future. Even
> Stefan2 mentioned that he made slight mistakes when changing that
> constant that was caught by test suite. But I don't assume that code
> that passes all test suite doesn't have bugs, while Stefan2 seems to
> assume. I bet that nobody reviewed "\x11\x34\0\xf5\xd6\x8c\x81\x06"
> constant in the code above. Even I didn't, because I think it's a
> waste of time and proper code should be committed or change reverted.
>

Hi Ivan,

I see three alternative ways to code that function

1. As hard coded string / byte sequence (current implementation).
  Cons:
* Hard to write, hard to review by just looking at it (applies to time
  until initial release only).
  Pros:
* Explicitly coded as constant, deterring people from changing it.
* Independent of other code, i.e. unintended changes to the format /
  encoding generated by the normal code usually become apparent
  when running the test suite.

2. Use our txn code to write r0. This should be simple and might
  at most require some special ID handling.
  Cons:
* Generating incompatible r0s is likely not caught by our test suite
  (assuming that reader and writer functions are in sync). Basically
  all the risk that comes with dynamically generating a "constant".
* Test cases must make sure our backward compat repo creation
  options create repos that can actually be used by old releases.
  (we might want explicit test for that anyway, though)
  Pros:
* No or very little special code for r0 (not sure, yet).
* Format / encoding changes don't require new r0 templates.

3. Write code to "stitch" r0 together, e.g. string_add(md5("END\n")).
  Cons:
* No more robust than 1. but requires much more code.
* May _look_ easy to understand but an actual offline review is still hard.
  Pros:
* Widely independent of other code, although not as much as 1.

Do you generally agree with the range of options and their assessment?
Which one would you pick and why?

I'd be fine with switching to option 2 as long as everyone understands
the implications.

-- Stefan^2.

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