tor/doc/HACKING/CodeStructure.md

TODO: revise this to talk about how things are, rather than how things
have changed.

For quite a while now, the program *tor* has been built from source
code in just two directories: **src/common** and **src/or**.

This has become more-or-less untenable, for a few reasons -- most
notably of which is that it has led our code to become more
spaghetti-ish than I can endorse with a clean conscience.

So to fix that, we've gone and done a huge code movement in our git
master branch, which will land in a release once Tor `0.3.5.1-alpha` is
out.

Here's what we did:

  * **src/common** has been turned into a set of static libraries.  These
all live in the **src/lib/*** directories.  The dependencies between
these libraries should have no cycles.  The libraries are:

    - **arch** -- Headers to handle architectural differences
    - **cc** -- headers to handle differences among compilers
    - **compress** -- wraps zlib, zstd, lzma
    - **container** -- high-level container types
    - **crypt_ops** -- Cryptographic operations. Planning to split this into
a higher and lower level library
    - **ctime** -- Operations that need to run in constant-time. (Properly,
data-invariant time)
    - **defs** -- miscelaneous definitions needed throughout Tor.
    - **encoding** -- transforming one data type into another, and various
data types into strings.
    - **err** -- lowest-level error handling, in cases where we can't use
the logs because something that the logging system needs has broken.
    - **evloop** -- Generic event-loop handling logic
    - **fdio** -- Low-level IO wrapper functions for file descriptors.
    - **fs** -- Operations on the filesystem
    - **intmath** -- low-level integer math and misc bit-twiddling hacks
    - **lock** -- low-level locking code
    - **log** -- Tor's logging module.  This library sits roughly halfway up
the library dependency diagram, since everything it depends on has to
be carefully crafted to *not* log.
    - **malloc** -- Low-level wrappers for the platform memory allocation functions.
    - **math** -- Higher-level mathematical functions, and floating-point math
    - **memarea** -- An arena allocator
    - **meminfo** -- Functions for querying the current process's memory
status and resources
    - **net** -- Networking compatibility and convenience code
    - **osinfo** -- Querying information about the operating system
    - **process** -- Launching and querying the status of other processes
    - **sandbox** -- Backend for the linux seccomp2 sandbox
    - **smartlist_core** -- The lowest-level of the smartlist_t data type.
Separated from the rest of the containers library because the logging
subsystem depends on it.
    - **string** -- Compatibility and convenience functions for manipulating
C strings.
    - **term** -- Terminal-related functions (currently limited to a getpass
function).
    - **testsupport** -- Macros for mocking, unit tests, etc.
    - **thread** -- Higher-level thread compatibility code
    - **time** -- Higher-level time management code, including format
conversions and monotonic time
    - **tls** -- Our wrapper around our TLS library
    - **trace** -- Formerly src/trace -- a generic event tracing API
    - **wallclock** -- Low-level time code, used by the log module.

  * To ensure that the dependency graph in **src/common** remains under
control, there is a tool that you can run called `make
check-includes`.  It verifies that each module in Tor only includes
the headers that it is permitted to include, using a per-directory
*.may_include* file.

  * The **src/or/or.h** header has been split into numerous smaller
headers.  Notably, many important structures are now declared in a
header called *foo_st.h*, where "foo" is the name of the structure.

  * The **src/or** directory, which had most of Tor's code, had been split
up into several directories.  This is still a work in progress:  This
code has not itself been refactored, and its dependency graph is still
a tangled web.  I hope we'll be working on that over the coming
releases, but it will take a while to do.

    - The new top-level source directories are:
        - **src/core** -- Code necessary to actually perform or use onion routing.
        - **src/feature** -- Code used only by some onion routing
configurations, or only for a special purpose.
        - **src/app** -- Top-level code to run, invoke, and configure the
lower-level code

    - The new second-level source directories are:
        - **src/core/crypto** -- High-level cryptographic protocols used in Tor
        - **src/core/mainloop** -- Tor's event loop, connection-handling, and
traffic-routing code.
        - **src/core/or** -- Parts related to handling onion routing itself
        - **src/core/proto** -- support for encoding and decoding different
wire protocols
        - **src/feature/api** -- Support for making Tor embeddable
        - **src/feature/client** -- Functionality which only Tor clients need
        - **src/feature/control** -- Controller implementation
        - **src/feature/dirauth** -- Directory authority
        - **src/feature/dircache** -- Directory cache
        - **src/feature/dirclient** -- Directory client
        - **src/feature/dircommon** -- Shared code between the other directory modules
        - **src/feature/hibernate** -- Hibernating when Tor is out of bandwidth
or shutting down
        - **src/feature/hs** -- v3 onion service implementation
        - **src/feature/hs_common** -- shared code between both onion service
implementations
        - **src/feature/nodelist** -- storing and accessing the list of relays on
the network.
        - **src/feature/relay** -- code that only relay servers and exit servers need.
        - **src/feature/rend** -- v2 onion service implementation
        - **src/feature/stats** -- statistics and history
        - **src/app/config** -- configuration and state for Tor
        - **src/app/main** -- Top-level functions to invoke the rest or Tor.

  * The `tor` executable is now built in **src/app/tor** rather than **src/or/tor**.

  * There are more static libraries than before that you need to build
into your application if you want to embed Tor.  Rather than
maintaining this list yourself, I recommend that you run `make
show-libs` to have Tor emit a list of what you need to link.