doc: Add Module.md explaining Tor modules

Initial document that contains guidelines to write a new module in Tor.

Closes #25991

Signed-off-by: David Goulet <dgoulet@torproject.org>

doc/HACKING/Module.md

@@ -0,0 +1,111 @@
+# Modules in Tor #
+
+This document describes the build system and coding standards when writing a
+module in Tor.
+
+## What is a module? ##
+
+In the context of the tor code base, a module is a subsystem that we can
+selectively enable or disable, at `configure` time.
+
+Currently, there is only one module:
+
+  - Directory Authority subsystem (dirauth)
+
+It is located in its own directory in `src/or/dirauth/`. To disable it, one
+need to pass `--disable-module-dirauth` at configure time. All modules are
+currently enabled by default.
+
+## Build System ##
+
+The changes to the build system are pretty straightforward.
+
+1. Locate in the `configure.ac` file this define: `m4_define(MODULES`. It
+   contains a list (white-space separated) of the module in tor. Add yours to
+   the list.
+
+2. Use the `AC_ARG_ENABLE([module-dirauth]` template for your new module. We
+   use the "disable module" approach instead of enabling them one by one. So,
+   by default, tor will build all the modules.
+
+   This will define the `HAVE_MODULE_<name>` statement which can be used in
+   the C code to conditionally compile things for your module. And the
+   `BUILD_MODULE_<name>` is also defined for automake files (e.g: include.am).
+
+3. In the `src/or/include.am` file, locate the `MODULE_DIRAUTH_SOURCES` value.
+   You need to create your own `_SOURCES` variable for your module and then
+   conditionally add the it to `LIBTOR_A_SOURCES` if you should build the
+   module.
+
+   It is then **very** important to add your SOURCES variable to
+   `src_or_libtor_testing_a_SOURCES` so the tests can build it.
+
+4. Do the same for header files, locate `ORHEADERS +=` which always add all
+   headers of all modules so the symbol can be found for the module entry
+   points.
+
+Finally, your module will automatically be included in the
+`TOR_MODULES_ALL_ENABLED` variable which is used to build the unit tests. They
+always build everything in order to tests everything.
+
+## Coding ##
+
+As mentioned above, a module must be isolated in its own directory (name of
+the module) in `src/or/`.
+
+There are couples of "rules" you want to follow:
+
+* Minimize as much as you can the number of entry points into your module.
+  Less is always better but of course that doesn't work out for every use
+  case. However, it is a good thing to always keep that in mind.
+
+* Do **not** use the `HAVE_MODULE_<name>` define outside of the module code
+  base. Every entry point should have a second definition if the module is
+  disabled. For instance:
+
+  ```
+  #ifdef HAVE_MODULE_DIRAUTH
+
+  int sr_init(int save_to_disk);
+
+  #else /* HAVE_MODULE_DIRAUTH */
+
+  static inline int
+  sr_init(int save_to_disk)
+  {
+    (void) save_to_disk;
+    return 0;
+  }
+
+  #endif /* HAVE_MODULE_DIRAUTH */
+
+  ```
+
+  The main reason for this approach is to avoid having conditional code
+  everywhere in the code base. It should be centralized as much as possible
+  which helps maintainability but also avoids conditional spaghetti code
+  making the code much more difficult to follow/understand.
+
+* It is possible that you end up with code that needs to be used by the rest
+  of the code base but is still part of your module. As a good example, if you
+  look at `src/or/shared_random_client.c`: it contains code needed by the hidden
+  service subsystem but mainly related to the shared random subsystem very
+  specific to the dirauth module.
+
+  This is fine but try to keep it as lean as possible and never use the same
+  filename as the one in the module. For example, this is a bad idea and
+  should never be done:
+
+    - `src/or/shared_random.c`
+    - `src/or/dirauth/shared_random.c`
+
+* When you include headers from the module, **always** use the full module
+  path in your statement. Example:
+
+  `#include "dirauth/dirvote.h"`
+
+  The main reason is that we do **not** add the module include path by default
+  so it needs to be specified. But also, it helps our human brain understand
+  which part comes from a module or not.
+
+  Even **in** the module itself, use the full include path like above.