Add rudimentary test mocking support.

This is not the most beautiful possible implementation (it requires
decorating mockable functions with ugly macros), but it actually
works, and is portable across multiple compilers and architectures.

changes/fancy_testing

@@ -14,3 +14,15 @@
       tests (and a new src/or/tor-cov target) to build with gcov test
       coverage support.
 
+  o Testing:
+
+    - We now have rudimentary function mocking support that our unit
+      tests can use to test functions in isolation. Function mocking
+      lets the tests temporarily replace a function's dependencies with
+      stub functions, so that the tests can check the function without
+      invoking the other functions it calls.
+
+
+
+
+

src/common/testsupport.h

@@ -10,5 +10,71 @@
 #define STATIC static
 #endif
 
+/** Quick and dirty macros to implement test mocking.
+ *
+ * To use them, suppose that you have a function you'd like to mock
+ * with the signature "void writebuf(size_t n, char *buf)".  You can then
+ * declare the function as:
+ *
+ *     MOCK_DECL(void, writebuf, (size_t n, char *buf));
+ *
+ * and implement it as:
+ *
+ *     MOCK_IMPL(void
+ *     writebuf,(size_t n, char *buf)
+ *     {
+ *          ...
+ *     }
+ *
+ * For the non-testing build, this will expand simply into:
+ *
+ *     void writebuf(size_t n, char *buf);
+ *     void
+ *     writebuf(size_t n, char *buf)
+ *     {
+ *         ...
+ *     }
+ *
+ * But for the testing case, it will expand into:
+ *
+ *     void writebuf__real(size_t n, char *buf);
+ *     extern void (*writebuf)(size_t n, char *buf);
+ *
+ *     void (*writebuf)(size_t n, char *buf) = writebuf__real;
+ *     void
+ *     writebuf__real(size_t n, char *buf)
+ *     {
+ *         ...
+ *     }
+ *
+ * This is not a great mocking system!  It is deliberately "the simplest
+ * thing that could work", and pays for its simplicity in its lack of
+ * features, and in its uglification of the Tor code.  Replacing it with
+ * something clever would be a fine thing.
+ *
+ * @{ */
+#ifdef TOR_UNIT_TESTS
+#define MOCK_DECL(rv, funcname, arglist)     \
+  rv funcname ##__real arglist;              \
+  extern rv(*funcname) arglist
+#define MOCK_IMPL(rv, funcname, arglist)     \
+  rv(*funcname) arglist = funcname ##__real; \
+  rv funcname ##__real arglist
+#define MOCK(func, replacement)                 \
+  do {                                          \
+    (func) = (replacement);                     \
+  } while (0)
+#define UNMOCK(func)                            \
+  do {                                          \
+    func = func ##__real;                       \
+  } while (0)
+#else
+#define MOCK_DECL(rv, funcname, arglist) \
+  rv funcname arglist
+#define MOCK_IMPL(rv, funcname, arglist) \
+  rv funcname arglist
+#endif
+/** @} */
+
 #endif