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How to contribute to CFEngine

Thanks for considering contributing to CFEngine! We take pull-requests on GitHub and we have a public bug-tracker. Discussion is taking place on the dev-cfengine and help-cfengine mailing lists. You'll find us chatting on Freenode's IRC channels #cfengine and #cfengine-dev.

Normally, bug fixes have a higher chance of getting accepted than new features, but we certainly welcome feature contributions. If you have an idea for a new feature, it might be a good idea to open up a feature ticket in our bug-tracker and send a message to dev-cfengine mailing list, before actually contributing the code, in order to get discussion going.

Merged features and larger changes will be released in the first minor release (i.e. x.y.0). Please note that such pull requests should be ready for merging (i.e. adjusted for any feedback) at least two months before the scheduled release date in order to make it to the first minor release.

Top reasons pull-requests are rejected or delayed

  • Code does not follow style guidlines. See Coding Style.

  • Pull request addresses several disparate issues. In general, smaller pull-requests are better because they are easier to review and stay mergeable longer.

  • Big feature is added, but it is not configurable in compile-time. We are striving to keep CFEngine lightweight and fast, so big new features should be possible to disable with ./configure --disable-feature and linking to new libraries should be optional with ./configure --without-libfoo.

  • Messy commit log. Tidy up the commit log by squashing commits.

  • Missing ChangeLog description in commit message, which is mandatory for new features or bugfixes to be accepted. See ChangeLog Entries for details.

  • Code is out-of-date, does not compile, or does not pass all tests. Again, focused and small pull-requests are better.

  • No attached test case. Normally, all new code needs test cases. This means a functional test runnable with make check.

Code Overview

The CFEngine codebase can be usefully thought of as a few separate components: utilities (libutils), parsing (libpromises), evaluation (libpromises), actuation (mostly in cf-agent), network (libcfnet).

Over the past year, the structure of the codebase has undergone some change. The goal of the restructuring is to isolate separate components with explicit dependencies, and provide better unit test coverage.

For a general introduction to the tools, please read the man pages.

libcompat

These are replacement functions in cases where autoconf cannot find a function it expected to find on the platform. CFEngine takes an approach of relying on the platform (typically POSIX) as much as possible, rather than creating its own system abstraction layer.

libutils

Contains generally useful datastructures or utilities. The key point about libutils is that it is free of dependencies (except libcompat), so it does not know about any CFEngine structures or global state found in libpromises.

  • sequence.h: Loosely based on glib GSequence, essentially an array list.
  • map.h: General purpose map (hash table).
  • set.h: General purpose set, a wrapper of Map.
  • writer.h: Stream writer abstraction over strings and FILEs.
  • xml_writer.h: Utility for writing XML using a Writer.
  • csv_writer.h: Utility for writing CSV using a Writer.
  • buffer.h: Dynamic byte-array buffer.
  • json.h: JSON document model, supports de/serialization.
  • string_lib.h: General purpose string utilities.
  • logging.h: Log functions, use Log() instead of printf.
  • man.h: Utility for generating the man pages.
  • list.h: General purpose linked list.
  • ip_address.h: IP address parsing.
  • hashes.h: Hashing functions.
  • file_lib.h: General purpose file utilities.
  • misc_lib.h: Really general utilities.

libcfnet

Contains the networking layer for CFEngine. (At the time of writing, a bit of a moving target).

libpromises

This is the remainder of the old src directory, that which has not been categorized. The roadmap for the project remains to leave libpromises as a component for evaluation.

  • cf3.defs.h: Contains structure definitions used widely.
  • eval_context.h: Header for EvalContext, keeper of evaluation state.
  • dbm_api.h: Local database for agents.
  • mod_.c: Syntax definitions for all promise types (actuation modules).
  • syntax.h: Syntax utilities and type checking.
  • *files_.h": File utilities we haven't been able to decouple from evaluation.
  • locks.h: Manages various persistent locks, kept in a local database.
  • rlist.h: List for Rvals (of attributes).
  • expand.c: Evaluates promises.
  • parser.h: Parse a policy file.
  • policy.h: Policy document object model, essentially the AST output of the parsing stage.
  • sysinfo.c: Detects hard classes from the environment.
  • evalfunction.c: Where all the built-in functions are implemented.
  • crypto.h: Crypto utilities for some reason still tied to evaluation state.
  • generic_agent.h: Common code for all agent binaries.

Things you should not use in libpromises

  • cf3.extern.h: Remaining global variables.
  • prototypes3.h: The original singular header file.
  • item_lib.h: Item is a special purpose list that has been abused for unintended purposes.
  • assoc.h: An lval-rval pair, deprecated in favor of EvalContext symbol table.
  • scope.h: Old symbol table, this will move into EvalContext.

cf-agent

The binary cf-agent contains most actuation logic in the verify_*.h files. Each file more or less maps to a promise type.

As an example, the file verify_packages.h contains VerifyPackagesPromise(EvalContext *ctx, Promise *pp).

Lifecycle of cf-agent

The following outlines the normal execution of a cf-agent run.

  1. Read options and gather these in GenericAgentConfig.
  2. Create an EvalContext and call GenericAgentConfigApply(ctx, config).
  3. Discover environment and set hard classes, apply to EvalContext.
  4. Parse input policy file, get a Policy object.
  5. Run static checks on Policy object.
  6. Evaluate each Bundle in bundlesequence.
  7. Write reports to disk.

Bootstrapping cf-agent

The following outlines the steps taken by agent during a successful bootstrap to a policy server.

  1. Remove all files in inputs directory
  2. Write built-in inputs/failsafe.cf
  3. Write policy server address or hostname, as was the argument to --bootstrap option, to policy_server.dat.
  4. If the host was bootstrapped to the machine's own IP address, then it is a policy server, and the file state/am_policy_hub is touched as marker.
  5. cf-agent runs using failsafe.cf as input file: 5a. Runs cf-key to generate localhost.{priv,pub} keys inside ppkeys directory. 5b. Fetches policy files from the policy server. 5c. Starts cf-execd 5d. Runs cf-agent -f update.cf
  6. Agent finishes.
  7. cf-execd continues to run cf-agent periodically with policy from inputs directory.

cf-monitord

Monitoring probes are contained in mon_*.c files. These all have a common header file mon.h.

Coding Style

  • Loosely based on Allman-4 and the Google C++ Style Guide.

  • Keep in mind that code should be readable by non C experts. If you are a Guru, try to restrain yourself, only do magic when absolutely necessary.

  • 4 spaces indentation level, no tabs.

  • Function names are CamelCase (with first letter capital), variables and parameters are under_scored.

    • If you introduce a new namespace, you can use underscore as namespace-identifier separator, for example StrList_BinarySearch().
    • Avoid introducing extra long identifiers, like GenericAgentConfigParseWarningOptions().

  • Try not to include assignments inside if/while expressions unless they avoid great repetition. On the average case, just put the assignment on the previous line. So try NOT to do the following: if ((ret = open(...)) == -1)

  • Explicit comparisons are better than implicit, i.e. prefer writing if (number == 0) or if (pointer == NULL) instead of if (!number) or if (!pointer). It only makes sense to test booleans directly, for example if (is_valid) is good. Furthermore have the literal last in the comparison, not first, i.e. prefer writing if (open(...) == -1) instead of if (-1 == open(...)).

  • Control statements need to have braces on separate line, no matter how simple they are.

    • Caution, do-while loops should have the closing brace at the same line with while, so that it can't be confused with empty while statement. 
      do
{
    /* ... */

} while (condition);

  • C99 is encouraged in the language, use it.

    As for using C99-specific libc functions, you can mostly use them, because we provide replacement functions in libcompat, since many old Unix platforms are missing those. If there is no replacement for a C99-specific function, then either stick to C89, or write the libcompat replacement.

    Current functions known to be missing from libcompat (so stick to C89):

    • [s]scanf()

  • Fold new code at 78 columns.

  • Do not break string literals. Prefer having strings on a single line In order to improve grep-ability. If they do not fit on a single line, try breaking on punctuation. In worst case scenario, you are allowed to surpass the 78 columns limit.

    Bad:

    Log(LOG_LEVEL_INFO, "Some error occurred while reading installed "
    "packages cache.");

    Good:

    Log(LOG_LEVEL_INFO,
    "Some error occurred while reading installed packages cache");

  • Always use typedefs, no "struct X", or "enum Y" are allowed. Types defined with typedef should be in camelcase and no trailing "_t", "_f" etc.

  • Constify what can be const. Minimize use of global variables. Never declare a global variable in a library (e.g. libpromises) and change it in the programs.

  • Don't use static variables that change, since they are not thread-safe.

  • Sizes of stack-allocated buffers should be deduced using sizeof(). Never hard-code the size (like CF_BUFSIZE).

  • Avoid using type casts, unless absolutely necessary. Usually a compiler warning is better satisfied with correct code rather than using a type cast.

    • Type casts should be separated with one space from the variable, for example (struct sockaddr *) &myaddr.

  • Avoid pointless initialisation of variables, because they silence important compiler warnings. Only initialise variables when there is a reason to do so.

  • Document using Doxygen (within reason), preferably in the .c files, not the header files.

  • Read Linux Kernel coding style and libabc coding style. They contain many good practices.

  • http://en.wikipedia.org/wiki/Golden_Rule

C Platform Macros

It's important to have portability in a consistent way. In general we use autoconf to test for features (like system headers, defines, specific functions). So try to use the autoconf macros HAVE_DECL_X, HAVE_STRUCT_Y, HAVE_MYFUNCTION etc. See the autoconf manual existing tests section.

It is preferable to write feature-specific ifdefs, instead of OS-specific, but it's not always easy. If necessary use these platform-specific macros in C code:

  • Any Windows system: Use _WIN32. Don't use NT.
  • mingw-based Win32 build: Use __MINGW32__. Don't use MINGW.
  • Cygwin-based Win32 build: Use __CYGWIN__. Don't use CFCYG.
  • OS X: Use __APPLE__. Don't use DARWIN.
  • FreeBSD: Use __FreeBSD__. Don't use FREEBSD.
  • NetBSD: Use __NetBSD__. Don't use NETBSD.
  • OpenBSD: Use __OpenBSD__. Don't use OPENBSD.
  • AIX: Use _AIX. Don't use AIX.
  • Solaris: Use __sun. Don't use SOLARIS.
  • Linux: Use __linux__. Don't use LINUX.
  • HP/UX: Use __hpux (two underscores!). Don't use hpux.

Finally, it's best to avoid polluting the code logic with many ifdefs. Try restricting ifdefs in the header files, or in the beginning of the C files.

Output Message, Logging Conventions

CFEngine outputs messages about what its doing using the Log() function. It takes a LogLevel enum mapping closely to syslog priorities. Please try to do the following when writing output messages.

  • Log levels

    • LOG_LEVEL_CRIT For critical errors, process exits immediately.
    • LOG_LEVEL_ERR: For cf-agent, promise failed. For cf-serverd, some system error occurred that is worth logging to syslog.
    • LOG_LEVEL_NOTICE: Important information (not errors) that must not be missed by the user. For example cf-agent uses it in files promises when change tracking is enabled and the file changes.
    • LOG_LEVEL_INFO: For cf-agent, changes that the agent performs to the system, for example when a promise has been repaired. For cf-serverd, access_rules denials for connected clients.
    • LOG_LEVEL_VERBOSE :: Log human readable progress info useful to users (i.e. sysadmins). Also errors that are unimportant or expected in certain cases.
    • LOG_LEVEL_DEBUG: Log anything else (for example various progress info). Try to avoid "Entering function Foo()", but rather use for "While copying, got reply '%s' from server".

  • Do not decorate with symbols or indentation in messages and do not terminate the message with punctuation. Let Log() enforce the common formatting rules.

  • When quoting strings, use single quotes, e.g. "Some stuff '%s' happened in '%s'.

  • Keep in mind context, e.g. write "While copying, insufficient permissions" rather than "Insufficient permissions".

  • Use output sparingly, and use levels appropriately. Verbose logging tends to get very verbose.

  • Use platform-independent GetErrorStr() for strerror(errno). Write for example Log(LOG_LEVEL_ERR, "Failed to open ... (fopen: %s)", GetErrorStr());

  • Normally, try to keep each message to one line of output, produced by one call to Log().

  • Normally, do not circumvent Log() by writing to stdout or stderr.

Testing

It is extremely important to have automated tests for all code, and normally all new code should be covered by tests, though sometimes it can be hard to mock up the environment.

There are two types of tests in CFEngine. Unit tests are generally preferable to acceptance tests because they are more targeted and take less time to run. Most tests can be run using make check. See Unsafe Tests below.

  • Unit tests. Unit tests are a great way of testing some new module (header file). Ideally, the new functionality is written so that the environment can be easily injected and results readily verified.

  • Acceptance tests. These are tests that run cf-agent on a policy file that contains test and check bundles, i.e. it uses CFEngine to both make a change and check it. See also script tests/acceptance/testall.

Tip: In order to trigger assert() calls in the code, build with --enable-debug (passed to either ./autogen.sh or ./configure). If you get very large binary sizes you can also pass CFLAGS='-g -O0' to reduce that.

Unsafe Tests

Note that some acceptance tests are considered to be unsafe because they modify the system they are running on. One example is the tests for the "users" promise type, which does real manipulation of the user database on the system. Due to their potential to do damage to the host system, these tests are not run unless explicitly asked for. Normally, this is something you would want to do in a VM, so you can restore the OS to a pristine state afterwards.

To run all tests, including the unsafe ones, you either need to be logged in as root or have "sudo" configured to not ask for a password. Then run the following:

$ UNSAFE_TESTS=1 GAINROOT=sudo make check

Again: DO NOT do this on your main computer! Always use a test machine, preferable in a VM.

Emacs users

There is Elisp snippet in contrib/cfengine-code-style.el which defines the project's coding style. Please use it when working with source code. The easiest way to do so is to add

(add-to-list 'load-path "<core checkout directory>/contrib")
(require 'cfengine-code-style)

and run

ln -s contrib/dir-locals.el .dir-locals.el

in the top directory of the source code checkout.

ChangeLog Entries

When a new feature or a bugfix is being merged, it is often necessary to be accompanied by a proper entry in the ChangeLog file. Besides manually editing the file, we have an automatic way of generating them before the release, by properly formatting commit messages (see git-commit-template). Keep in mind that changelog entries should be written in a way that is understandable by non- programmers. This means that references to implementation details are not appropriate, leave this for the non-changelog part of the commit message. It is the behavior change which is important. This implies that refactorings that have no visible effect on behavior don't need a changelog entry.

If a changelog entry is needed, your pull request should have at least one commit with a "Changelog:" line in it, after the title. This may be one of the following:

  • To write arbitrary message in the ChangeLog: Changelog: <message>
  • To use the commit title line in the ChangeLog: Changelog: Title
  • To use the entire commit message in the ChangeLog: Changelog: Commit

It's worth noting that we strive to have bugtracker tickets for most changes, and they should be mentioned in the ChangeLog entries. In fact if anywhere in the commit message the string CFE-1234 is found (referring to a ticket from https://tracker.mender.io ), it will be automatically added to the ChangeLog.