# Developer’s Guide

This guide is targeted to people that want to write new features or fix bugs in rmlint.

## Bugs

Please use the issue tracker to post and discuss bugs and features:

> https://github.com/sahib/rmlint/issues

## Philosophy

We try to adhere to some principles when adding features:

- Try to stay compatible to standard unix’ tools and ideas.

- Try to stay out of the users way and never be interactive.

- Try to make scripting as easy as possible.

- Never make `rmlint` modify the filesystem itself, only produce output to let the user easily do it.

Also keep this in mind, if you want to make a feature request.

## Making contributions

The code is hosted on GitHub, therefore our preferred way of receiving patches is using GitHub’s pull requests (normal git pull requests are okay too of course).

Note

`origin/master` should always contain working software. Base your patches and pull requests always on `origin/develop`.

Here’s a short step-by-step:

1. Fork it.

2. Create a branch from develop. (`git checkout develop && git checkout -b my_feature`)

3. Commit your changes. (`git commit -am "Fixed it all."`)

4. Check if your commit message is good. (If not: `git commit --amend`)

5. Push to the branch (`git push origin my_feature`)

6. Open a Pull Request against the develop branch.

7. Enjoy a refreshing Tea and wait until we get back to you.

Here are some other things to check before submitting your contribution:

- Does your code look alien to the other code? Is the style the same? You can run this command to make sure it is the same:

      $ clang-format -style=file -i $(find lib src -iname '*.[ch]')

- Do all tests run? Go to the test documentation for more info. Also after opening the pull request, your code will be checked via TravisCI.

- Is your commit message descriptive? whatthecommit.com has some good examples how they should not look like.

- Is `rmlint` running okay inside of `valgrind` (i.e. no leaks and no memory violations)?

For language-translations/updates it is also okay to send the `.po` files via mail at sahib@online.de, since not every translator is necessarily a software developer.

## Testsuite

`rmlint` has a not yet complete but quite powerful testsuite. It is not complete yet (and probably never will), but it’s already a valuable boost of confidence in `rmlint's` correctness.

The tests are based on `pytest` and are written in `python>=3.0`. Every testcase just runs the (previously built) `rmlint` binary a and parses its json output. So they are technically blackbox-tests.

On every commit, those tests are additionally run on TravisCI.

### Control Variables

The behaviour of the testsuite can be controlled by certain environment variables which are:

- `RM_TS_DIR`: Testdir to create files in. Can be very large with some tests, sometimes `tmpfs` might therefore slow down your computer. By default `/tmp` will be used.

- `RM_TS_USE_VALGRIND`: Run each test inside of valgrind’s memcheck. (slow)

- `RM_TS_CHECK_LEAKS`: Fail test if valgrind indicates (definite) memory leak.

- `RM_TS_USE_GDB`: Run tests inside of `gdb`. Fatal signals will trigger a backtrace.

- `RM_TS_PEDANTIC`: Run each test several times with different optimization options and check for errors between the runs. (slow).

- `RM_TS_SLEEP`: Waits a long time before executing a command. Useful for starting the testcase and manually running rmlint on the priorly generated testdir.

- `RM_TS_PRINT_CMD`: Print the command that is currently run.

- `RM_TS_KEEP_TESTDIR`: If a test failed, keep the test files.

Additionally slow tests can be omitted with by appending `-k 'not slow'` to the commandline. More information on this syntax can be found on the pytest documentation.

Before each release we call the testsuite (at least) like this:

    $ sudo RM_TS_USE_VALGRIND=1 RM_TS_PRINT_CMD=1 RM_TS_PEDANTIC=1 pytest -s -a 'not slow and not known_issue'

The `sudo` here is there for executing some tests that need root access (like the creating of bad user and group ids). Most tests will work without.

### Coverage

To see which functions need more testcases we use `gcov` to detect which lines were executed (and how often) by the testsuite. Here’s a short quickstart using `lcov`:

    $ CFLAGS="-fprofile-arcs -ftest-coverage" LDFLAGS="-fprofile-arcs -ftest-coverage" scons -j4 DEBUG=1
    $ sudo RM_TS_USE_VALGRIND=1 RM_TS_PRINT_CMD=1 RM_TS_PEDANTIC=1 pytest -s -a 'slow and not known_issue'
    $ lcov --capture --directory . --output-file coverage.info
    $ genhtml coverage.info --output-directory out

The coverage results are updated from time to time here:

> http://sahib.github.io/rmlint/gcov/index.html

### Structure

    tests
    ├── test_formatters   # Tests for output formatters (like sh or json)
    ├── test_options      # Tests for normal options like --merge-directories etc.
    ├── test_types        # Tests for all lint types rmlint can find
    └── utils.py          # Common utilities shared among tests.

### Templates

A template for a testcase looks like this:

    from tests.utils import *

    def test_basic(usual_setup_usual_teardown):
        create_file('xxx', 'a')
        create_file('xxx', 'b')

        head, *data, footer = run_rmlint('-a city -S a')

        assert footer['duplicate_sets'] == 1
        assert footer['total_lint_size'] == 3
        assert footer['total_files'] == 2
        assert footer['duplicates'] == 1

### Rules

- Test should be able to run as normal user.

- If that’s not possible, check at the beginning of the testcase with this:

      if not runs_as_root():
          return

- Regressions in `rmlint` should get their own testcase so they do not appear again.

- Slow tests can be marked with a slow attribute:

      import pytest

      @pytest.mark.slow
      def test_debian_support(usual_setup_usual_teardown):
          assert random.choice([True, False]):

- Unresolved issues can be marked with known_issue attribute to avoid failing automated travis testing

## Buildsystem Helpers

### Environment Variables

CFLAGS:

  Extra flags passed to the compiler.

LDFLAGS:

  Extra flags passed to the linker.

CC:

  Which compiler to use?

    # Use clang and enable profiling, verbose build and enable debugging
    CC=clang CFLAGS='-pg' LDFLAGS='-pg' scons VERBOSE=1 DEBUG=1

### Variables

O=<level>:

  Set the optimization level.

  Valid levels are currently those that may be passed with the GCC/Clang option `-O`; these include `0`, `1`, `2`, `3`, `s`, `fast`, `g`, etc., depending on the compiler version.

  In addition, the level may be `debug` or `release`, which indicates that the optimization level should be whatever the build system currently defines to be the default for the associated build mode.

DEBUG=1:

  Enable a debugging build.

  This turns on extra tests; in particular, it turns on run-time assertions. By default, a debug build excludes optimizations that may hinder debugging, but this may be overridden with the `O` variable, as usual.

  Note that setting `DEBUG=1` does not enable the production of debugger symbols; to enable those, use `SYMBOLS=1` or `GDB=1`.

  This should always be enabled during development.

SYMBOLS=1:

  Enable debugger symbols.

  This option instructs the compiler to collect information that will help tools such as `gdb` present human-readable identifiers for a program’s functions and variables, etc. Note, though, that this information becomes obscured by optimizations, so make sure to set the optimization level appropriately.

GDB=1:

  Enable options that help a debugger (such as `gdb`).

  This option is equivalent to `DEBUG=1 SYMBOLS=1`.

VERBOSE=1:

  Print the exact compiler and linker commands. Useful for troubleshooting build errors.

CCFLAGS=<command line options>:

  Set the last compiler options.

  Internally, the build system maintains in `CCFLAGS` the list of options that are supplied to the compiler; this list is composed by combining the relevant environment variables (such as `CFLAGS`) along with the choices made by other build-time configurations.

  This command-line variable makes it possible to override an option in this list by supplying customized command-line options to be appended. For example: `GDB=1 CCFLAGS=-g1`.

  The string that is supplied as the value for this variable is parsed as per a POSIX shell command line, and so it may include shell quoting if necessary.

### Arguments

–prefix:

  Change the installation prefix. By default this is `/usr`, but some users might prefer `/usr/local` or `/opt`.

–actual-prefix:

  This is mainly useful for packagers. The `rmlint` binary knows where it is installed (which is needed to set e.g. the path to the gettext files). When installing a package, most of the time the build is installed to a local test environment first before being packed to `/usr`. In this case the `--prefix` would be set to the path of the temporary build env, while `--actual-prefix` would be set to `/usr`.

–libdir:

  Some distributions like Fedora use separate libdirectories for 64/32 bit. If this happens, you should set the correct one for 64 bit with `--libdir=lib64`.

–without-libelf:

  Do not link with `libelf`, which is needed for nonstripped binary detection.

–without-blkid:

  Do not link with `libblkid`, which is needed to differentiate between normal rotational harddisks and non-rotational disks.

–without-json-glib:

  Do not link with `libjson-glib`, which is needed to load json-cache files. Without this library a warning is printed when using `--replay`.

–without-fiemap:

  Do not attempt to use the `FIEMAP ioctl(2)`.

–without-gettext:

  Do not link with `libintl` and do not compile any message catalogs.

All `--without-*` options come with a `--with-*` option that inverses its effect. By default `rmlint` is built with all features available on the system, so you do not need to specify any `--with-*` option normally.

### Notable targets

install:

  Install all program parts system-wide.

config:

  Print a summary of all features that will be compiled and what the environment looks like.

man:

  Build the manpage.

docs:

  Build the online html docs (which you are reading now).

test:

  Build the tests (requires `python` and `pytest` installed). Optionally `valgrind` can be installed to run the tests through valgrind:

      $ USE_VALGRIND=1 pytest

xgettext:

  Extract a gettext `.pot` template from the source.

dist:

  Build a tarball suitable for release. Save it under `rmlint-$major-$minor-$patch.tar.gz`.

release:

  Same as `dist`, but reads the `.version` file and replaces the current version in the files that are not built by scons.

## Sourcecode layout

- All C-source lives in `lib`, the file names should be self explanatory.

- As an exception, the main lives in `src/rmlint.c`.

- All documentation is inside `docs`.

- All translation stuff should go to `po`.

- All packaging should be done in `pkg/<distribution>`.

- Tests are written in Python and live in `tests`.

## Hashfunctions

Here is a short comparison of the existing hashfunctions in `rmlint` (linear scale). For reference: Those plots were rendered with these sources - which are very ugly, sorry.

If you want to add new hashfunctions, you should have some arguments why it is valuable and possibly even benchmark it with the above scripts to see if it’s really that much faster.

Also keep in mind that most of the time the hashfunction is not the bottleneck.

## Optimizations

For sake of overview, here is a short list of optimizations implemented in `rmlint`:

### Obvious ones

- Do not compare each file with each other by content, use a hashfunction to reduce comparison overhead drastically (introduces possibility of collisions though).

- Only compare files of same size with each other.

- Use incremental hashing, i.e. hash block-wise each size group and stop as soon a difference occurs or the file is read fully.

- Create one reading thread for each physical disk. This gives a big speedup if files are roughly evenly spread over multiple physical disks [note: currently using 2 reading threads per disk as a workaround for a speed regression but hoping to fix this for rmlint 2.5].

- Disk traversal is similarly multi-threaded, one thread per disk.

- Create separate hashing threads (one for each file) so that the reader threads don’t have to wait for hashing to catch up.

### Subtle ones

- Check only executable files to be non-stripped binaries.

- Use `preadv(2)` based reading for small speeedups.

- Every thread in rmlint is shared, so only few calls to `pthread_create` are made.

### Insane ones

- Use `fiemap ioctl(2)` to analyze the harddisk layout of each file, so each block can read it in perfect order on a rotational device.

- Check the device ID of each file to see if it on a rotational (normal hard disks) or on a non-rotational device (like an SSD). On the latter the fiemap optimisation is bypassed.

- Use a common buffer pool for IO buffers and recycle used buffers to reduce memory allocation overheads.

- Use only one hashsum per group of same-sized files.

- Implement paranoia check using the same algorithm as the incremental hash. The difference is that large chunks of the file are read and kept in memory instead of just keeping the hash in memory. This avoids the need for a two-pass algorithm (find matches using hashes then confirm via bytewise comparison). Each file is read once only. This achieves bytewise comparison in O(N) time, even if there are large clusters of same-size files. The downside is that it is somewhat memory-intensive (can be configured by `--limit-mem` option).