HACKING: Add subtleties.

doc/HACKING.md

@@ -155,7 +155,7 @@ valgrind installed, and build with DEVELOPER=1 (currently the default).
   header order, and checks formatted quotes from BOLTs if BOLTDIR
   exists (currently disabled, since BOLTs are being re-edited).
 
-* unit tests - run by `make check`, these are run-*.c files in test/
+* unit tests - run by `make check`, these are `run-*.c` files in test/
   subdirectories which can test routines inside C source files.  You
   should insert `/* AUTOGENERATED MOCKS START */` and `/* AUTOGENERATED MOCKS END */`
   lines, and `make update-mocks` will automatically generate stub functions
@@ -169,6 +169,25 @@ valgrind installed, and build with DEVELOPER=1 (currently the default).
 
 Our Travis CI instance (see `.travis.yml`) runs all these for each pull request.
 
+Subtleties
+----------
+
+There are a few subtleties you should be aware of as you modify deeper parts
+of the code:
+
+* `structeq` will not work on some structures.
+  For example, it will not work with `struct short_channel_id` --- use
+  `short_channel_id_eq` for comparing those.
+* `command_success`, `command_fail`, and `command_fail_detailed` will free the
+  `cmd` you pass in.
+  This also means that if you `tal`-allocated anything from the `cmd`, they
+  will also get freed at those points and will no longer be accessible
+  afterwards.
+* When making a structure part of a list, you will instance a
+  `struct list_node`.
+  This has to be the *first* field of the structure, or else `dev-memleak`
+  command will think your structure has leaked.
+
 Further Information
 -------------------