[RFC PATCH 15/15] config.mak.dev: add and use ASSERT_FOR_FANALYZER() macro

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add an ASSERT_FOR_FANALYZER() macro to quiet those -fanalyzer issues
that haven't been diagnosed yet to either change the code involved, or
to add an assert() or BUG() to it to placate GCC v12's -fanalyzer
mode.

As in the preceding commit we could also use -Wno-error=* here, which
was the initial implementation in config.mak.dev, i.e. something like:

	## -Wno-error=analyzer-null-dereference
	$(eval $(call fn_disable_analyzer, \
		-Wno-error=analyzer-null-dereference, \
		dir \
		gpg-interface \
		graph \
		range-diff \
		utf8 \
	))
	## -Wno-error=analyzer-null-dereference: pre-gcc12
	ifeq ($(filter gcc12,$(COMPILER_FEATURES)),)
	$(eval $(call fn_disable_analyzer, \
		-Wno-error=analyzer-null-dereference, \
		merge \
		builtin/name-rev \
	))
	endif

But any such approach will unfortunately quiet *all* in the relevant
files, including any new ones. Instead we want to quiet specific
issues involved with ASSERT_FOR_FANALYZER().

A drawback of this overall approach is that this only works under
e.g. CFLAGS=-O0, but not CFLAGS=-O3. The compiler takes the liberty to
re-arrange our code to get around these assert()s, and other new
issues will also crop up. All of this is expected (-fanalyzer is
subject to optimization passes, like most other options), but let's
focus on -O0 for now.

Commentary on specific cases:

 * builtin/name-rev.c: Since 45a14f578e1 (Revert "name-rev: release
   unused name strings", 2022-04-22) GCC v12's -fanalyzer has complained
   about this code, per René's analysis in [1] it's incorrect to do
   so. So let's add an ASSERT_FOR_FANALYZER() to placate it.

 * graph.c: In 0f0f389f120 (graph: tidy up display of left-skewed
   merges, 2019-10-15) a previous "assert" was removed, and another
   unconditional deference of the return value of
   first_interesting_parent() was added without checking it. Since it
   can return NULL let's add ASSERT_FOR_FANALYZER()'s here to note for
   -fanalyzer that we won't be dereferencing these in practice.

1. https://lore.kernel.org/git/dece627d-ccf8-2375-0c50-c59637e561d3@web.de/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/name-rev.c |  1 +
 config.mak.dev     |  4 ++++
 dir.c              |  1 +
 git-compat-util.h  | 16 ++++++++++++++++
 gpg-interface.c    |  2 ++
 graph.c            |  2 ++
 line-log.c         |  2 ++
 unpack-trees.c     |  1 +
 utf8.c             |  1 +
 9 files changed, 30 insertions(+)

diff --git a/builtin/name-rev.c b/builtin/name-rev.c
index 02ea9d16330..419159961b9 100644
--- a/builtin/name-rev.c
+++ b/builtin/name-rev.c
@@ -223,6 +223,7 @@ static void name_rev(struct commit *start_commit,
 			if (commit_is_before_cutoff(parent))
 				continue;
 
+			ASSERT_FOR_FANALYZER(name);
 			if (parent_number > 1) {
 				generation = 0;
 				distance = name->distance + MERGE_TRAVERSAL_WEIGHT;
diff --git a/config.mak.dev b/config.mak.dev
index d6f5be92297..1d47fc04163 100644
--- a/config.mak.dev
+++ b/config.mak.dev
@@ -83,6 +83,10 @@ endif
 
 DEVELOPER_CFLAGS += -fanalyzer
 
+ifeq ($(filter no-suppress-analyzer,$(DEVOPTS)),)
+DEVELOPER_CFLAGS += -DSUPPRESS_FSANITIZER
+endif
+
 ## -fanalyzer exists exists as of gcc10, but versions older than gcc12
 ## have a lot of false positives.
 ifeq ($(filter gcc12,$(COMPILER_FEATURES)),)
diff --git a/dir.c b/dir.c
index 3633c0852ee..c80b584b4a7 100644
--- a/dir.c
+++ b/dir.c
@@ -3781,6 +3781,7 @@ static void invalidate_one_directory(struct untracked_cache *uc,
 				     struct untracked_cache_dir *ucd)
 {
 	uc->dir_invalidated++;
+	ASSERT_FOR_FANALYZER(ucd);
 	ucd->valid = 0;
 	ucd->untracked_nr = 0;
 }
diff --git a/git-compat-util.h b/git-compat-util.h
index 96293b6c43a..a553884c048 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -984,6 +984,21 @@ static inline unsigned long cast_size_t_to_ulong(size_t a)
 	return (unsigned long)a;
 }
 
+/**
+ * Transitory helper macro to quiet currently known GCC -fsanitizer
+ * issues.
+ *
+ * We lie to it and say that we're confident that the given "expr" to
+ * ASSERT_FOR_FANALYZER() cannot be NULL (or 0). Those
+ * grandfathered-in issues should be fixed, but at least we're
+ * stemming the tide.
+ */
+#ifdef SUPPRESS_FSANITIZER
+#define ASSERT_FOR_FANALYZER(expr) assert((expr))
+#else
+#define ASSERT_FOR_FANALYZER(expr) do {} while (0)
+#endif
+
 #ifdef HAVE_ALLOCA_H
 # include <alloca.h>
 # define xalloca(size)      (alloca(size))
@@ -1037,6 +1052,7 @@ int xstrncmpz(const char *s, const char *t, size_t len);
 	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src))))
 static inline void copy_array(void *dst, const void *src, size_t n, size_t size)
 {
+	ASSERT_FOR_FANALYZER(dst);
 	if (n)
 		memcpy(dst, src, st_mult(size, n));
 }
diff --git a/gpg-interface.c b/gpg-interface.c
index 280f1fa1a58..9cba3b86e45 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -242,6 +242,7 @@ static void parse_gpg_output(struct signature_check *sigc)
 					next = strchrnul(line, ' ');
 					replace_cstring(&sigc->key, line, next);
 					/* Do we have signer information? */
+					ASSERT_FOR_FANALYZER(next);
 					if (*next && (sigcheck_gpg_status[i].flags & GPG_STATUS_UID)) {
 						line = next + 1;
 						next = strchrnul(line, '\n');
@@ -283,6 +284,7 @@ static void parse_gpg_output(struct signature_check *sigc)
 					 */
 					limit = strchrnul(line, '\n');
 					for (j = 9; j > 0; j--) {
+						ASSERT_FOR_FANALYZER(next);
 						if (!*next || limit <= next)
 							break;
 						line = next + 1;
diff --git a/graph.c b/graph.c
index 568b6e7cd41..39f7e95d4ab 100644
--- a/graph.c
+++ b/graph.c
@@ -1105,6 +1105,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			seen_this = 1;
 
 			for (j = 0; j < graph->num_parents; j++) {
+				ASSERT_FOR_FANALYZER(parents);
 				par_column = graph_find_new_column_by_commit(graph, parents->item);
 				assert(par_column >= 0);
 
@@ -1138,6 +1139,7 @@ static void graph_output_post_merge_line(struct git_graph *graph, struct graph_l
 			}
 		}
 
+		ASSERT_FOR_FANALYZER(first_parent);
 		if (col_commit == first_parent->item)
 			parent_col = col;
 	}
diff --git a/line-log.c b/line-log.c
index 51d93310a4d..1295f46deaf 100644
--- a/line-log.c
+++ b/line-log.c
@@ -154,6 +154,7 @@ static void range_set_union(struct range_set *out,
 	while (i < a->nr || j < b->nr) {
 		struct range *new_range;
 		if (i < a->nr && j < b->nr) {
+			ASSERT_FOR_FANALYZER(rb);
 			if (ra[i].start < rb[j].start)
 				new_range = &ra[i++];
 			else if (ra[i].start > rb[j].start)
@@ -166,6 +167,7 @@ static void range_set_union(struct range_set *out,
 			new_range = &ra[i++];
 		else                       /* a exhausted */
 			new_range = &rb[j++];
+		ASSERT_FOR_FANALYZER(new_range);
 		if (new_range->start == new_range->end)
 			; /* empty range */
 		else if (!out->nr || out->ranges[out->nr-1].end < new_range->start) {
diff --git a/unpack-trees.c b/unpack-trees.c
index a1d0ff3a4d3..b6e10b05a00 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -2195,6 +2195,7 @@ static int verify_clean_subdirectory(const struct cache_entry *ce,
 	char *pathbuf;
 	int cnt = 0;
 
+	ASSERT_FOR_FANALYZER(ce);
 	if (S_ISGITLINK(ce->ce_mode)) {
 		struct object_id oid;
 		int sub_head = resolve_gitlink_ref(ce->name, "HEAD", &oid);
diff --git a/utf8.c b/utf8.c
index de4ce5c0e68..3ffc0a97f3b 100644
--- a/utf8.c
+++ b/utf8.c
@@ -130,6 +130,7 @@ static ucs_char_t pick_one_utf8_char(const char **start, size_t *remainder_p)
 	 */
 	remainder = (remainder_p ? *remainder_p : 999);
 
+	ASSERT_FOR_FANALYZER(remainder < 1 || s);
 	if (remainder < 1) {
 		goto invalid;
 	} else if (*s < 0x80) {
-- 
2.36.1.1124.g577fa9c2ebd