Clarify test/Makefile and snippet testing documentation

Hopefully this makes it easier for future development work to augment
the snippet-based testing.

test/Makefile

@@ -31,16 +31,18 @@ functest:
 randomtest: repeat_random
 	./randomtest.sh
 
-## test variables
 
+# run snippet tests
+snippets: test_variable_data test_library_constructors
+
+## test snippet behavior across env var setting over time:
 test_variable_data: test_variable_data.sh $(foreach f,${EXPECTATIONS},run_${f})
 	./test_variable_data.sh ${EXPECTATIONS}
 
 run_%: _run_test.c snippets/%.c
 	sed s/SNIPPET_NAME/$*/g < _run_test.c | ${CC} -o $@ ${CFLAGS} -x c - 
 
-## testing when interception points get called in library constructors:
-
+## test snippets in other library constructors:
 test_library_constructors: $(foreach f,${TEST_SNIPPETS},test_lib_${f})
 test_lib_%: test_constructors.sh use_lib_% lib%.so
 	./test_constructors.sh $*
@@ -60,4 +62,4 @@ clean:
 distclean: clean
 	@echo
 
-.PHONY: all test clean distclean randomtest
+.PHONY: all test clean distclean randomtest snippets test_variable_data test_library_constructors

test/snippets/README

@@ -1,35 +1,72 @@
-Bulk testing of function interception
-=====================================
+Testing Interception with Snippets
+==================================
 
-Faketime intercepts some C library functions.  We want to make it
-easier to apply certain generic tests across many different functions.
+Faketime intercepts some C library functions and system calls.  We
+want to make it easier to apply certain generic tests across many
+different functions.  We do that with snippets of C, which each can be
+applied in multiple testing frameworks.
 
-Including a new function
-------------------------
+Most snippets are just a minimalist invocation of a single function or
+syscall that is likely to be intercepted by libfaketime, though it's
+possible to test more complex snippets too.
 
-To test a function FOO, supply a C snippet in this directory named
-<FOO.c>.  This should be a small bit of code that exercises the
-function FOO.  It should be self-contained, and it should produce some
-kind of description of what happened to stdout.  Take a look at
-getpid.c for a simple example.
+Including a New Snippet
+-----------------------
 
-The data sent to stdout should include the const char* "where" value
-(which is an indication of which test framework is using the snippet).
+To cover a new bit of intercepted functionality, supply a C snippet in
+this directory named `FOO.c` (the name FOO should conform to C
+function names -- letters, numbers, and underscores; if you're testing
+interception of a single function, the simplest thing is to name
+snippet file after the intercepted function name).
 
-If the output of the snippet is expected to be stable when the
-associated variable (e.g. FAKETIME, FAKETIME_FAKEPID, or
-FAKERANDOM_SEED) is set, and is likely to vary otherwise, especially
-across a delay of a second or more, add a single-line file
-FOO.variable that contains the name of the variable separated by a
-space and then the value of the variable.
+This file should contain a small bit of code that exercises the
+functionality in question.  It should be self-contained, and it should
+produce some kind of description of what happened to stdout.  The data
+sent to stdout should include the const char* "where" value (which is
+an indication of which test framework is using the snippet).  Take a
+look at getpid.c for a simple example.
 
-If the snippet needs to additional #include headers, please add them
-in include_headers.h. These #includes will be used by every snippet,
+If the snippet needs additional #include headers, please add them
+in `include_headers.h`. These #includes will be used by every snippet,
 so try to keep it minimal.
 
-Testing across functions
-------------------------
+Snippet Testing Frameworks
+--------------------------
 
-If you want to test something systemically, autogenerate code that
-uses each snippet.  See test_variable_data or
-test_library_constructors in ../Makefile for an example.
+We have the following frameworks that use the snippets:
+
+### Variable Data
+
+Most functionality intercepted by libfaketime will normally produce
+variant output when invoked multiple times across the span of a few
+seconds by different processes.  But if `FAKETIME` (or an analogous
+variable like `FAKERANDOM_SEED`) is set, the output data should remain
+constant.
+
+If this describes the functionality in a new snippet `FOO.c`, please
+also drop a single-line file `FOO.variable` in this directory, where
+the first word of the line is the variable that should hold the output
+constant, and the rest of the line is the value of that variable.
+
+See the `test_variable_data` target in `test/Makefile` for how this is
+implemented.
+
+### Library Constructors
+
+Library constructor routines are run by ld.so before the main process
+is launched.  If the LD_PRELOADed libfaketime is masking a symbol from
+libc, and another library has a constructor routine that invokes that
+symbol, it might get called before libfaketime has had a chance to
+initialize its followup pointers to the actual libc functionality.
+
+This framework is applied automatically to all snippets.
+
+See the `test_library_constructors` target in `test/Makefile` for how
+this is implemented.
+ 
+Adding a New Framework
+----------------------
+
+If you want to add a new framework that tests across all snippets,
+implement it in a few lines of `test/Makefile` and document it in the
+section above.