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+
+Writing tests for Tor: an incomplete guide
+==========================================
+
+Tor uses a variety of testing frameworks and methodologies to try to
+keep from introducing bugs.  The major ones are:
+
+   1. Unit tests written in C and shipped with the Tor distribution.
+
+   2. Integration tests written in Python and shipped with the Tor
+      distribution.
+
+   3. Integration tests written in Python and shipped with the Stem
+      library.  Some of these use the Tor controller protocol.
+
+   4. System tests written in Python and SH, and shipped with the
+      Chutney package.  These work by running many instances of Tor
+      locally, and sending traffic through them.
+
+   5. The Shadow network simulator.
+
+How to run these tests
+----------------------
+
+=== The easy version
+
+To run all the tests that come bundled with Tor, run "make check"
+
+To run the Stem tests as well, fetch stem from the git repository,
+set STEM_SOURCE_DIR to the checkout, and run "make test-stem".
+
+To run the Chutney tests as well, fetch chutney from the git repository,
+set CHUTNEY_PATH to the checkout, and run "make test-network".
+
+To run all of the above, run "make test-full".
+
+To run all of the above, plus tests that require a working connection to the
+internet, run "make test-full-online".
+
+=== Running particular subtests
+
+The Tor unit tests are divided into separate programs and a couple of
+bundled unit test programs.
+
+Separate programs are easy.  For example, to run the memwipe tests in
+isolation, you just run ./src/test/test-memwipe .
+
+To run tests within the unit test programs, you can specify the name
+of the test.  The string ".." can be used as a wildcard at the end of the
+test name.  For example, to run all the cell format tests, enter
+"./src/test/test cellfmt/..".  To run
+
+Many tests that need to mess with global state run in forked subprocesses in
+order to keep from contaminating one another.  But when debugging a failing test,
+you might want to run it without forking a subprocess.  To do so, use the
+"--no-fork" option with a single test.  (If you specify it along with
+multiple tests, they might interfere.)
+
+You can turn on logging in the unit tests by passing one of "--debug",
+"--info", "--notice", or "--warn".  By default only errors are displayed.
+
+Unit tests are divided into "./src/test/test" and "./src/test/test-slow".
+The former are those that should finish in a few seconds; the latter tend to
+take more time, and may include CPU-intensive operations, deliberate delays,
+and stuff like that.
+
+=== Finding test coverage
+
+When you configure Tor with the --enable-coverage option, it should
+build with support for coverage in the unit tests, and in a special
+"tor-cov" binary.
+
+Then, run the tests you'd like to see coverage from.  If you have old
+coverage output, you may need to run "reset-gcov" first.
+
+Now you've got a bunch of files scattered around your build directories
+called "*.gcda".  In order to extract the coverage output from them, make a
+temporary directory for them and run "./scripts/test/coverage ${TMPDIR}",
+where ${TMPDIR} is the temporary directory you made.  This will create a
+".gcov" file for each source file under tests, containing that file's source
+annotated with the number of times the tests hit each line.  (You'll need to
+have gcov installed.)
+
+You can get a summary of the test coverage for each file by running
+"./scripts/test/cov-display ${TMPDIR}/*" .  Each line lists the file's name,
+the number of uncovered lines, the number of uncovered lines, and the
+coverage percentage.
+
+For a summary of the test coverage for each _function_, run
+"./scripts/test/cov-display -f ${TMPDIR}/*" .
+
+=== Comparing test coverage
+
+Sometimes it's useful to compare test coverage for a branch you're writing to
+coverage from another branch (such as git master, for example).  But you
+can't run "diff" on the two coverage outputs directly, since the actual
+number of times each line is executed aren't so important, and aren't wholly
+deterministic.
+
+Instead, follow the instructions above for each branch, creating a separate
+temporary directory for each.  Then, run "./scripts/test/cov-diff ${D1}
+${D2}", where D1 and D2 are the directories you want to compare.  This will
+produce a diff of the two directories, with all lines normalized to be either
+covered or uncovered.
+
+To count new or modified uncovered lines in D2, you can run:
+
+    "./scripts/test/cov-diff ${D1} ${D2}" | grep '^+ *\#' |wc -l
+
+
+What kinds of test should I write?
+----------------------------------
+
+Integration testing and unit testing are complementary: it's probably a
+good idea to make sure that your code is hit by both if you can.
+
+If your code is very-low level, and its behavior is easily described in
+terms of a relation between inputs and outputs, or a set of state
+transitions, then it's a natural fit for unit tests.  (If not, please
+consider refactoring it until most of it _is_ a good fit for unit
+tests!)
+
+If your code adds new externally visible functionality to Tor, it would
+be great to have a test for that functionality.  That's where
+integration tests more usually come in.
+
+Unit and regression tests: Does this function do what it's supposed to?
+-----------------------------------------------------------------------
+
+Most of Tor's unit tests are made using the "tinytest" testing framework.
+You can see a guide to using it in the tinytest manual at
+
+   https://github.com/nmathewson/tinytest/blob/master/tinytest-manual.md
+
+To add a new test of this kind, either edit an existing C file in src/test/,
+or create a new C file there.  Each test is a single function that must
+be indexed in the table at the end of the file.  We use the label "done:" as
+a cleanup point for all test functions.
+
+(Make sure you read tinytest-manual.md before proceeding.)
+
+I use the term "unit test" and "regression tests" very sloppily here.
+
+=== A simple example
+
+Here's an example of a test function for a simple function in util.c:
+
+    static void
+    test_util_writepid(void *arg)
+    {
+      (void) arg;
+
+      char *contents = NULL;
+      const char *fname = get_fname("tmp_pid");
+      unsigned long pid;
+      char c;
+
+      write_pidfile(fname);
+
+      contents = read_file_to_str(fname, 0, NULL);
+      tt_assert(contents);
+
+      int n = sscanf(contents, "%lu\n%c", &pid, &c);
+      tt_int_op(n, OP_EQ, 1);
+      tt_int_op(pid, OP_EQ, getpid());
+
+    done:
+      tor_free(contents);
+    }
+
+This should look pretty familiar to you if you've read the tinytest
+manual.  One thing to note here is that we use the testing-specific
+function "get_fname" to generate a file with respect to a temporary
+directory that the tests use.  You don't need to delete the file;
+it will get removed when the tests are done.
+
+Also note our use of OP_EQ instead of == in the tt_int_op() calls.
+We define OP_* macros to use instead of the binary comparison
+operators so that analysis tools can more easily parse our code.
+(Coccinelle really hates to see == used as a macro argument.)
+
+Finally, remember that by convention, all *_free() functions that
+Tor defines are defined to accept NULL harmlessly.  Thus, you don't
+need to say "if (contents)" in the cleanup block.
+
+=== Exposing static functions for testing
+
+Sometimes you need to test a function, but you don't want to expose
+it outside its usual module.
+
+To support this, Tor's build system compiles a testing version of
+each module, with extra identifiers exposed.  If you want to
+declare a function as static but available for testing, use the
+macro "STATIC" instead of "static".  Then, make sure there's a
+macro-protected declaration of the function in the module's header.
+
+For example, crypto_curve25519.h contains:
+
+#ifdef CRYPTO_CURVE25519_PRIVATE
+STATIC int curve25519_impl(uint8_t *output, const uint8_t *secret,
+                           const uint8_t *basepoint);
+#endif
+
+The crypto_curve25519.c file and the test_crypto.c file both define
+CRYPTO_CURVE25519_PRIVATE, so they can see this declaration.
+
+=== Mock functions for testing in isolation
+
+Often we want to test that a function works right, but the function to
+be tested depends on other functions whose behavior is hard to observe,
+or which require a working Tor network, or something like that.
+
+To write tests for this case, you can replace the underlying functions
+with testing stubs while your unit test is running.  You need to declare
+the underlying function as 'mockable', as follows:
+
+   MOCK_DECL(returntype, functionname, (argument list));
+
+and then later implement it as:
+
+    MOCK_IMPL(returntype, functionname, (argument list))
+    {
+       /* implementation here */
+    }
+
+For example, if you had a 'connect to remote server' function, you could
+declare it as:
+
+
+   MOCK_DECL(int, connect_to_remote, (const char *name, status_t *status));
+
+When you declare a function this way, it will be declared as normal in
+regular builds, but when the module is built for testing, it is declared
+as a function pointer initialized to the actual implementation.
+
+In your tests, if you want to override the function with a temporary
+replacement, you say:
+
+   MOCK(functionname, replacement_function_name);
+
+And later, you can restore the original function with:
+
+   UNMOCK(functionname);
+
+For more information, see the definitions of this mocking logic in
+testsupport.h.
+
+
+=== Advanced techniques: Namespaces
+
+XXXX write this.  danah boyd made us some really awesome stuff here.
+
+
+Integration tests: Calling Tor from the outside
+-----------------------------------------------
+
+XXXX WRITEME
+
+Writing integration tests with Stem
+-----------------------------------
+
+XXXX WRITEME
+
+System testing with Chutney
+---------------------------
+
+XXXX WRITEME
+
+Who knows what evil lurks in the timings of networks? The Shadow knows!
+-----------------------------------------------------------------------
+
+XXXX WRITEME
+