builtins: annotate always-empty prefix parameters

It's usually a bad idea for a builtin's cmd_foo() to ignore the "prefix"
argument it gets, as it needs to prepend that string when accessing any
paths given by the user.

But if a builtin does not ask for the git wrapper to run repository
setup (via the RUN_SETUP or RUN_SETUP_GENTLY flags), then we know the
prefix will always be NULL (it is adjusting for the chdir() done during
repo setup, but there cannot be one if we did not set up the repo). In
those cases it's OK to ignore "prefix", but it's worth annotating for a
few reasons:

  1. It serves as documentation to somebody reading the code about what
     we expect.

  2. If the flags in git.c ever change, the run-time assertion may help
     detect the problem (though only if the command is run from a
     subdirectory of the repository).

  3. It notes to the compiler that we are OK ignoring "prefix". In
     particular, this silences -Wunused-parameter. It _could_ also help
     the compiler generate better code (because it will know the prefix
     is NULL), but in practice this is quite unlikely to matter.

Note that I've only added this annotation to commands which triggered
-Wunused-parameter. It would be correct to add it to any builtin which
doesn't ask for RUN_SETUP, but most of the rest of them do the sensible
thing with "prefix" by passing it to parse_options(). So they're much
more likely to just work if they ever switched to RUN_SETUP, and aren't
worth annotating.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/builtin.h b/builtin.h
index 46cc789789..cb0db67681 100644
--- a/builtin.h
+++ b/builtin.h
@@ -107,6 +107,16 @@ void setup_auto_pager(const char *cmd, int def);
 
 int is_builtin(const char *s);
 
+/*
+ * Builtins which do not use RUN_SETUP should never see
+ * a prefix that is not empty; use this to protect downstream
+ * code which is not prepared to call prefix_filename(), etc.
+ */
+#define BUG_ON_NON_EMPTY_PREFIX(prefix) do { \
+	if ((prefix)) \
+		BUG("unexpected prefix in builtin: %s", (prefix)); \
+} while (0)
+
 int cmd_add(int argc, const char **argv, const char *prefix);
 int cmd_am(int argc, const char **argv, const char *prefix);
 int cmd_annotate(int argc, const char **argv, const char *prefix);