Hacking Tor: An Incomplete Guide
================================

Getting started
---------------

For full information on how Tor is supposed to work, look at the files in
doc/spec/ .

For an explanation of how to change Tor's design to work differently, look at
doc/spec/proposals/001-process.txt .

For the latest version of the code, get a copy of git, and

   git clone git://git.torproject.org/git/tor .

We talk about Tor on the or-talk mailing list.  Design proposals and
discussion belong on the or-dev mailing list.  We hang around on
irc.oftc.net, with general discussion happening on #tor and development
happening on #tor-dev.

How we use Git branches
-----------------------

Each main development series (like 0.2.1, 0.2.2, etc) has its main work
applied to a single branch.  At most one series can be the development series
at a time; all other series are maintenance series that get bug-fixes only.
The development series is built in a git branch called "master"; the
maintenance series are built in branches called "maint-0.2.0", "maint-0.2.1",
and so on.  We regularly merge the active maint branches forward.

For all series except the development series, we also have a "release" branch
(as in "release-0.2.1").  The release series is based on the corresponding
maintenance series, except that it deliberately lags the maint series for
most of its patches, so that bugfix patches are not typically included in a
maintenance release until they've been tested for a while in a development
release.  Occasionally, we'll merge an urgent bugfix into the release branch
before it gets merged into maint, but that's rare.

If you're working on a bugfix for a bug that occurs in a particular version,
base your bugfix branch on the "maint" branch for the first _actively
developed_ series that has that bug.  (Right now, that's 0.2.1.)  If you're
working on a new feature, base it on the master branch.


How we log changes
------------------

When you do a commit that needs a ChangeLog entry, add a new file to
the "changes" toplevel subdirectory.  It should have the format of a
one-entry changelog section from the current ChangeLog file, as in

  o Major bugfixes:
    - Fix a potential buffer overflow. Fixes bug 9999; bugfix on
      0.3.1.4-beta.

To write a changes file, first categorize the change.  Some common categories
are: Minor bugfixes, Major bugfixes, Minor features, Major features, Code
simplifications and refactoring.  Then say what the change does.  If
it's a bugfix, mention what bug it fixes and when the bug was
introduced.  To find out which Git tag the change was introduced in,
you can use "git describe --contains <sha1 of commit>".

If at all possible, try to create this file in the same commit where
you are making the change.  Please give it a distinctive name that no
other branch will use for the lifetime of your change.

When Roger goes to make a release, he will concatenate all the entries
in changes to make a draft changelog, and clear the directory.  He'll
then edit the draft changelog into a nice readable format.

What needs a changes file?::
   A not-exhaustive list: Anything that might change user-visible
   behavior. Anything that changes internals, documentation, or the build
   system enough that somebody could notice.  Big or interesting code
   rewrites.  Anything about which somebody might plausibly wonder "when
   did that happen, and/or why did we do that" 6 months down the line.

Why use changes files instead of Git commit messages?::
   Git commit messages are written for developers, not users, and they
   are nigh-impossible to revise after the fact.

Why use changes files instead of entries in the ChangeLog?::
   Having every single commit touch the ChangeLog file tended to create
   zillions of merge conflicts.

Useful tools
------------

These aren't strictly necessary for hacking on Tor, but they can help track
down bugs.

The buildbot
~~~~~~~~~~~~

https://buildbot.vidalia-project.net/one_line_per_build

Dmalloc
~~~~~~~

The dmalloc library will keep track of memory allocation, so you can find out
if we're leaking memory, doing any double-frees, or so on.

  dmalloc -l ~/dmalloc.log
  (run the commands it tells you)
  ./configure --with-dmalloc

Valgrind
~~~~~~~~

valgrind --leak-check=yes --error-limit=no --show-reachable=yes src/or/tor

(Note that if you get a zillion openssl warnings, you will also need to
pass --undef-value-errors=no to valgrind, or rebuild your openssl
with -DPURIFY.)

Running gcov for unit test coverage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-----
  make clean
  make CFLAGS='-g -fprofile-arcs -ftest-coverage'
  ./src/test/test
  cd src/common; gcov *.[ch]
  cd ../or; gcov *.[ch]
-----

Then, look at the .gcov files.  '-' before a line means that the
compiler generated  no code for that line.  '######' means that the
line was never reached.  Lines with numbers were called that number
of times.

Profiling Tor with oprofile
~~~~~~~~~~~~~~~~~~~~~~~~~~~

The oprofile tool runs (on Linux only!) to tell you what functions Tor is
spending its CPU time in, so we can identify berformance pottlenecks.

Here are some basic instructions

 - Build tor with debugging symbols (you probably already have, unless
   you messed with CFLAGS during the build process).
 - Build all the libraries you care about with debugging symbols
   (probably you only care about libssl, maybe zlib and Libevent).
 - Copy this tor to a new directory
 - Copy all the libraries it uses to that dir too (ldd ./tor will
   tell you)
 - Set LD_LIBRARY_PATH to include that dir.  ldd ./tor should now
   show you it's using the libs in that dir
 - Run that tor
 - Reset oprofiles counters/start it
   * "opcontrol --reset; opcontrol --start", if Nick remembers right.
 - After a while, have it dump the stats on tor and all the libs
   in that dir you created.
   * "opcontrol --dump;"
   * "opreport -l that_dir/*"
 - Profit


Coding conventions
------------------

Patch checklist
~~~~~~~~~~~~~~~

If possible, send your patch as one of these (in descending order of
preference)

   - A git branch we can pull from
   - Patches generated by git format-patch
   - A unified diff

Did you remember...

   - To build your code while configured with --enable-gcc-warnings?
   - To run "make check-spaces" on your code?
   - To write unit tests, as possible?
   - To base your code on the appropriate branch?
   - To include a file in the "changes" directory as appropriate?

Whitespace and C conformance
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Invoke "make check-spaces" from time to time, so it can tell you about
deviations from our C whitespace style.  Generally, we use:

    - Unix-style line endings
    - K&R-style indentation
    - No space before newlines
    - A blank line at the end of each file
    - Never more than one blank line in a row
    - Always spaces, never tabs
    - No more than 79-columns per line.
    - Two spaces per indent.
    - A space between control keywords and their corresponding paren
      "if (x)", "while (x)", and "switch (x)", never "if(x)", "while(x)", or
      "switch(x)".
    - A space between anything and an open brace.
    - No space between a function name and an opening paren. "puts(x)", not
      "puts (x)".
    - Function declarations at the start of the line.

We try hard to build without warnings everywhere.  In particular, if you're
using gcc, you should invoke the configure script with the option
"--enable-gcc-warnings".  This will give a bunch of extra warning flags to
the compiler, and help us find divergences from our preferred C style.

Getting emacs to edit Tor source properly
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Nick likes to put the following snippet in his .emacs file:

-----
    (add-hook 'c-mode-hook
          (lambda ()
            (font-lock-mode 1)
            (set-variable 'show-trailing-whitespace t)

            (let ((fname (expand-file-name (buffer-file-name))))
              (cond
               ((string-match "^/home/nickm/src/libevent" fname)
                (set-variable 'indent-tabs-mode t)
                (set-variable 'c-basic-offset 4)
                (set-variable 'tab-width 4))
               ((string-match "^/home/nickm/src/tor" fname)
                (set-variable 'indent-tabs-mode nil)
                (set-variable 'c-basic-offset 2))
               ((string-match "^/home/nickm/src/openssl" fname)
                (set-variable 'indent-tabs-mode t)
                (set-variable 'c-basic-offset 8)
                (set-variable 'tab-width 8))
            ))))
-----

You'll note that it defaults to showing all trailing whitespace.  The "cond"
test detects whether the file is one of a few C free software projects that I
often edit, and sets up the indentation level and tab preferences to match
what they want.

If you want to try this out, you'll need to change the filename regex
patterns to match where you keep your Tor files.

If you use emacs for editing Tor and nothing else, you could always just say:

-----
   (add-hook 'c-mode-hook
          (lambda ()
            (font-lock-mode 1)
            (set-variable 'show-trailing-whitespace t)
            (set-variable 'indent-tabs-mode nil)
            (set-variable 'c-basic-offset 2)))
-----

There is probably a better way to do this.  No, we are probably not going
to clutter the files with emacs stuff.


Functions to use
~~~~~~~~~~~~~~~~

We have some wrapper functions like tor_malloc, tor_free, tor_strdup, and
tor_gettimeofday; use them instead of their generic equivalents.  (They
always succeed or exit.)

You can get a full list of the compatibility functions that Tor provides by
looking through src/common/util.h and src/common/compat.h.  You can see the
available containers in src/common/containers.h.  You should probably
familiarize yourself with these modules before you write too much code, or
else you'll wind up reinventing the wheel.

Use 'INLINE' instead of 'inline', so that we work properly on Windows.

Calling and naming conventions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Whenever possible, functions should return -1 on error and 0 on success.

For multi-word identifiers, use lowercase words combined with
underscores. (e.g., "multi_word_identifier").  Use ALL_CAPS for macros and
constants.

Typenames should end with "_t".

Function names should be prefixed with a module name or object name.  (In
general, code to manipulate an object should be a module with the same name
as the object, so it's hard to tell which convention is used.)

Functions that do things should have imperative-verb names
(e.g. buffer_clear, buffer_resize); functions that return booleans should
have predicate names (e.g. buffer_is_empty, buffer_needs_resizing).

If you find that you have four or more possible return code values, it's
probably time to create an enum.  If you find that you are passing three or
more flags to a function, it's probably time to create a flags argument that
takes a bitfield.

What To Optimize
~~~~~~~~~~~~~~~~

Don't optimize anything if it's not in the critical path.  Right now, the
critical path seems to be AES, logging, and the network itself.  Feel free to
do your own profiling to determine otherwise.

Log conventions
~~~~~~~~~~~~~~~

https://wiki.torproject.org/noreply/TheOnionRouter/TorFAQ#LogLevels

No error or warning messages should be expected during normal OR or OP
operation.

If a library function is currently called such that failure always means ERR,
then the library function should log WARN and let the caller log ERR.

[XXX Proposed convention: every message of severity INFO or higher should
either (A) be intelligible to end-users who don't know the Tor source; or (B)
somehow inform the end-users that they aren't expected to understand the
message (perhaps with a string like "internal error").  Option (A) is to be
preferred to option (B). -NM]

Doxygen
~~~~~~~~

We use the 'doxygen' utility to generate documentation from our
source code. Here's how to use it:

  1. Begin every file that should be documented with
         /**
          * \file filename.c
          * \brief Short description of the file.
          **/

     (Doxygen will recognize any comment beginning with /** as special.)

  2. Before any function, structure, #define, or variable you want to
     document, add a comment of the form:

        /** Describe the function's actions in imperative sentences.
         *
         * Use blank lines for paragraph breaks
         *   - and
         *   - hyphens
         *   - for
         *   - lists.
         *
         * Write <b>argument_names</b> in boldface.
         *
         * \code
         *     place_example_code();
         *     between_code_and_endcode_commands();
         * \endcode
         */

  3. Make sure to escape the characters "<", ">", "\", "%" and "#" as "\<",
     "\>", "\\", "\%", and "\#".

  4. To document structure members, you can use two forms:

       struct foo {
         /** You can put the comment before an element; */
         int a;
         int b; /**< Or use the less-than symbol to put the comment
                 * after the element. */
       };

  5. To generate documentation from the Tor source code, type:

     $ doxygen -g

     To generate a file called 'Doxyfile'.  Edit that file and run
     'doxygen' to generate the API documentation.

  6. See the Doxygen manual for more information; this summary just
     scratches the surface.

Doxygen comment conventions
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Say what functions do as a series of one or more imperative sentences, as
though you were telling somebody how to be the function.  In other words, DO
NOT say:

     /** The strtol function parses a number.
      *
      * nptr -- the string to parse.  It can include whitespace.
      * endptr -- a string pointer to hold the first thing that is not part
      *    of the number, if present.
      * base -- the numeric base.
      * returns: the resulting number.
      */
     long strtol(const char *nptr, char **nptr, int base);

Instead, please DO say:

     /** Parse a number in radix <b>base</b> from the string <b>nptr</b>,
      * and return the result.  Skip all leading whitespace.  If
      * <b>endptr</b> is not NULL, set *<b>endptr</b> to the first character
      * after the number parsed.
      **/
     long strtol(const char *nptr, char **nptr, int base);

Doxygen comments are the contract in our abstraction-by-contract world: if
the functions that call your function rely on it doing something, then your
function should mention that it does that something in the documentation.  If
you rely on a function doing something beyond what is in its documentation,
then you should watch out, or it might do something else later.


