ci: add build checking for side-effects in assert() calls

It is a big no-no to have side-effects in an assertion, because if the
assert() is compiled out, you don't get that side-effect, leading to the
code behaving differently.  That can be a large headache to debug.

We have roughly 566 assert() calls in our codebase (my grep might have
picked up things that aren't actually assert() calls, but most appeared
to be).  All but 9 of them can be determined by gcc to be free of side
effects with a clever redefine of assert() provided by Bruno De Fraine
(from
https://stackoverflow.com/questions/10593492/catching-assert-with-side-effects),
who upon request has graciously placed his two-liner into the public
domain without warranty of any kind.  The current 9 assert() calls
flagged by this clever redefinition of assert() appear to me to be free
of side effects as well, but are too complicated for a compiler/linker
to figure that since each assertion involves some kind of function call.
Add a CI job which will find and report these possibly problematic
assertions, and have the job suggest to the user that they replace these
with ASSERT() calls.

Example output from running:

```
ERROR: The compiler could not verify the following assert()
       calls are free of side-effects.  Please replace with
       ASSERT() calls.
/home/newren/floss/git/diffcore-rename.c:1409
	assert(!dir_rename_count || strmap_empty(dir_rename_count));
/home/newren/floss/git/merge-ort.c:1645
			assert(renames->deferred[side].trivial_merges_okay &&
			       !strset_contains(&renames->deferred[side].target_dirs,
						path));
/home/newren/floss/git/merge-ort.c:794
	assert(omittable_hint ==
	       (!starts_with(type_short_descriptions[type], "CONFLICT") &&
		!starts_with(type_short_descriptions[type], "ERROR")) ||
	       type == CONFLICT_DIR_RENAME_SUGGESTED);
/home/newren/floss/git/merge-recursive.c:1200
	assert(!merge_remote_util(commit));
/home/newren/floss/git/object-file.c:2709
	assert(would_convert_to_git_filter_fd(istate, path));
/home/newren/floss/git/parallel-checkout.c:280
	assert(is_eligible_for_parallel_checkout(pc_item->ce, &pc_item->ca));
/home/newren/floss/git/scalar.c:244
	assert(have_fsmonitor_support());
/home/newren/floss/git/scalar.c:254
	assert(have_fsmonitor_support());
/home/newren/floss/git/sequencer.c:4968
		assert(!(opts->signoff || opts->no_commit ||
			 opts->record_origin || should_edit(opts) ||
			 opts->committer_date_is_author_date ||
			 opts->ignore_date));
```

Note that if there are possibly problematic assertions, not necessarily
all of them will be shown in a single run, because the compiler errors
may include something like "ld: ... more undefined references to
`not_supposed_to_survive' follow" instead of listing each individually.
But in such cases, once you clean up a few that are shown in your first
run, subsequent runs will show (some of) the ones that remain, allowing
you to iteratively remove them all.

Helped-by: Bruno De Fraine <[email protected]>
Signed-off-by: Elijah Newren <[email protected]>
Acked-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

Makefile

@@ -2261,6 +2261,10 @@ ifdef WITH_BREAKING_CHANGES
 	BASIC_CFLAGS += -DWITH_BREAKING_CHANGES
 endif
 
+ifdef CHECK_ASSERTION_SIDE_EFFECTS
+	BASIC_CFLAGS += -DCHECK_ASSERTION_SIDE_EFFECTS
+endif
+
 ifdef INCLUDE_LIBGIT_RS
 	# Enable symbol hiding in contrib/libgit-sys/libgitpub.a without making
 	# us rebuild the whole tree every time we run a Rust build.

ci/check-unsafe-assertions.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+make CHECK_ASSERTION_SIDE_EFFECTS=1 >compiler_output 2>compiler_error
+if test $? != 0
+then
+	echo >&2 "ERROR: The compiler could not verify the following assert()"
+	echo >&2 "       calls are free of side-effects.  Please replace with"
+	echo >&2 "       ASSERT() calls."
+	grep undefined.reference.to..not_supposed_to_survive compiler_error |
+		sed -e s/:[^:]*$// | sort | uniq | tr ':' ' ' |
+		while read f l
+		do
+			printf "${f}:${l}\n  "
+			awk -v start="$l" 'NR >= start { print; if (/\);/) exit }' $f
+		done
+	exit 1
+fi
+rm compiler_output compiler_error

ci/run-static-analysis.sh

@@ -31,4 +31,6 @@ exit 1
 
 make check-pot
 
+${0%/*}/check-unsafe-assertions.sh
+
 save_good_tree

git-compat-util.h

@@ -1585,4 +1585,10 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset)
 	((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
 #endif /* !__GNUC__ */
 
+#ifdef CHECK_ASSERTION_SIDE_EFFECTS
+#undef assert
+extern int not_supposed_to_survive;
+#define assert(expr) ((void)(not_supposed_to_survive || (expr)))
+#endif /* CHECK_ASSERTION_SIDE_EFFECTS */
+
 #endif