Add an initial CodeStructure.md file to doc/HACKING.

Not complete or pretty, but better than nothing.

doc/HACKING/CodeStructure.md

+
+TODO: revise this to talk about how things are, rather than how things
+have changed.
+
+TODO: Make this into good markdown.
+
+
+
+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.

doc/include.am

@@ -36,6 +36,7 @@ EXTRA_DIST+= doc/asciidoc-helper.sh			\
 	     doc/HACKING/README.1st.md				\
 	     doc/HACKING/CodingStandards.md 			\
 	     doc/HACKING/CodingStandardsRust.md			\
+	     doc/HACKING/CodeStructure.md			\
 	     doc/HACKING/Fuzzing.md				\
 	     doc/HACKING/GettingStarted.md 			\
 	     doc/HACKING/GettingStartedRust.md 			\