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

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

For full information on how Tor is supposed to work, look at the files in
https://gitweb.torproject.org/torspec.git/tree

For an explanation of how to change Tor's design to work differently, look at
https://gitweb.torproject.org/torspec.git/blob_plain/HEAD:/proposals/001-process.txt

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

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

We talk about Tor on the tor-talk mailing list.  Design proposals and
discussion belong on the tor-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 supported series
that has that bug.  (As of June 2013, we're supporting 0.2.3 and later.) 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 99999; 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. To verify the format of the changes file,
you can use "make check-changes".

When we go to make a release, we will concatenate all the entries
in changes to make a draft changelog, and clear the directory. We'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.

Jenkins
~~~~~~~

https://jenkins.torproject.org

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 lcov for unit test coverage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Lcov is a utility that generates pretty HTML reports of test code coverage.
To generate such a report:

-----
   ./configure --enable-coverage
   make
   make coverage-html
   $BROWSER ./coverage_html/index.html
-----

This will run the tor unit test suite `./src/test/test` and generate the HTML
coverage code report under the directory ./coverage_html/. To change the
output directory, use `make coverage-html HTML_COVER_DIR=./funky_new_cov_dir`.

Coverage diffs using lcov are not currently implemented, but are being
investigated (as of July 2014).

Running the unit tests
~~~~~~~~~~~~~~~~~~~~~~

To quickly run all tests:
-----
   make check
-----

To run unit tests only:
-----
   make test
-----

To selectively run just some tests (the following can be combined
arbitrarily):
-----
   ./src/test/test <name_of_test> [<name of test 2>] ...
   ./src/test/test <prefix_of_name_of_test>.. [<prefix_of_name_of_test2>..] ...
   ./src/test/test :<name_of_excluded_test> [:<name_of_excluded_test2]...
-----

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

-----
   ./configure --enable-coverage
   make
   make check
   mkdir coverage-output
   ./scripts/test/coverage coverage-output
-----

(On OSX, you'll need to start with "--enable-coverage CC=clang".)

Then, look at the .gcov files in coverage-output.  '-' 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.

If that doesn't work:
   * Try configuring Tor with --disable-gcc-hardening
   * You might need to run 'make clean' after you run './configure'.

If you make changes to Tor and want to get another set of coverage results,
you can run "make reset-gcov" to clear the intermediary gcov output.

If you have two different "coverage-output" directories, and you want to see
a meaningful diff between them, you can run:

-----
   ./scripts/test/cov-diff coverage-output1 coverage-output2 | less
-----

In this diff, any lines that were visited at least once will have coverage
"1".  This lets you inspect what you (probably) really want to know: which
untested lines were changed?  Are there any new untested lines?

Running integration tests
~~~~~~~~~~~~~~~~~~~~~~~~~

We have the beginnings of a set of scripts to run integration tests using
Chutney. To try them, set CHUTNEY_PATH to your chutney source directory, and
run "make test-network".

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 run "make check-docs" to see whether all new options are on
     the manpage?
   - 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://www.torproject.org/docs/faq#LogLevel

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.

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).

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.

Putting out a new release
-------------------------

Here are the steps Roger takes when putting out a new Tor release:

1) Use it for a while, as a client, as a relay, as a hidden service,
and as a directory authority. See if it has any obvious bugs, and
resolve those.

1.5) As applicable, merge the maint-X branch into the release-X branch.

2) Gather the changes/* files into a changelog entry, rewriting many
of them and reordering to focus on what users and funders would find
interesting and understandable.

   2.1) Make sure that everything that wants a bug number has one.
        Make sure that everything which is a bugfix says what version
        it was a bugfix on.
   2.2) Concatenate them.
   2.3) Sort them by section. Within each section, sort by "version it's
        a bugfix on", else by numerical ticket order.

   2.4) Clean them up:

   Standard idioms:
     "Fixes bug 9999; bugfix on 0.3.3.3-alpha."

   One space after a period.

   Make stuff very terse

   Make sure each section name ends with a colon

   Describe the user-visible problem right away

   Mention relevant config options by name.  If they're rare or unusual,
   remind people what they're for

   Avoid starting lines with open-paren

   Present and imperative tense: not past.

   'Relays', not 'servers' or 'nodes' or 'Tor relays'.

   "Stop FOOing", not "Fix a bug where we would FOO".

   Try not to let any given section be longer than about a page. Break up
   long sections into subsections by some sort of common subtopic. This
   guideline is especially important when organizing Release Notes for
   new stable releases.

   If a given changes stanza showed up in a different release (e.g.
   maint-0.2.1), be sure to make the stanzas identical (so people can
   distinguish if these are the same change).

   2.5) Merge them in.

   2.6) Clean everything one last time.

   2.7) Run ./scripts/maint/format_changelog.py to make it prettier.

3) Compose a short release blurb to highlight the user-facing
changes. Insert said release blurb into the ChangeLog stanza. If it's
a stable release, add it to the ReleaseNotes file too. If we're adding
to a release-0.2.x branch, manually commit the changelogs to the later
git branches too.

4) Bump the version number in configure.ac and rebuild.  Then run
   "make update-versions".

5) Make dist, put the tarball up somewhere, and tell #tor about it. Wait
a while to see if anybody has problems building it. Try to get Sebastian
or somebody to try building it on Windows.

6) Get at least two of weasel/arma/sebastian to put the new version number
in their approved versions list.

7) Sign the tarball, then sign and push the git tag:
  gpg -ba <the_tarball>
  git tag -u <keyid> tor-0.2.x.y-status
  git push origin tag tor-0.2.x.y-status

8a) scp the tarball and its sig to the dist website, i.e.
/srv/dist-master.torproject.org/htdocs/ on dist-master. When you want
it to go live, you run "static-update-component dist.torproject.org"
on dist-master.

8b) Edit "include/versions.wmi" and "Makefile" to note the new version.

9) Email the packagers (cc'ing tor-assistants) that a new tarball is up.
   The current list of packagers is:
       {weasel,gk,mikeperry} at torproject dot org
       {blueness} at gentoo dot org
       {paul} at invizbox dot io
       {ondrej.mikle} at gmail dot com
       {lfleischer} at archlinux dot org

10) Add the version number to Trac.  To do this, go to Trac, log in,
select "Admin" near the top of the screen, then select "Versions" from
the menu on the left.  At the right, there will be an "Add version"
box.  By convention, we enter the version in the form "Tor:
0.2.2.23-alpha" (or whatever the version is), and we select the date as
the date in the ChangeLog.

11) Forward-port the ChangeLog.

12) Wait up to a day or two (for a development release), or until most
packages are up (for a stable release), and mail the release blurb and
changelog to tor-talk or tor-announce.

  (We might be moving to faster announcements, but don't announce until
  the website is at least updated.)

13) If it's a stable release, bump the version number in the maint-x.y.z
    branch to "newversion-dev", and do a "merge -s ours" merge to avoid
    taking that change into master.  Do a similar 'merge -s theirs'
    merge to get the change (and only that change) into release.  (Some
    of the build scripts require that maint merge cleanly into release.)