to complement yesterday's diff about -Wmissing-prototypes, here's an
attempt to build got with -Wwrite-strings.  The purpose is to enforce
some `const's here and there, in the hope to make some parts clearer.

It correctly picked up a few places where we were using "char *" (or
"void *") but passing "const" pointers, and I'd also argue that in the
case of got_diff_objects_as_trees it's actually better to have the
pointers correctly marked as const.

There are also two downsides however:

 - i need to add a cast in got_dial_ssh.  I don't know how to trackle
   argv without extra casts with -Wwrite-strings.  The compiler
   complained that assigning to argv[x] a const char * was loosing
   const, but execv wants a "char *const *", and I can't have argv
   declared as `char *const argv[11]' either.  (why execv has such
   signature?  meh)

 - got_path_find_prog becomes more complex.  it mixes const and
   non-const char pointers, but it's carefully designed to not modify
   the const strings.  The compiler isn't smart enough to notice it (nor
   that it should.)  A possible solution in the long-term could be to
   maybe drop this function: it's just used to find the editor to exec;
   we could just try to exec $VISUAL, $EDITOR and /bin/ed in cascade and
   use the first that works.  not for today tho :)

so i'm not sure if we want this in or to just backport some changes and
call it a day.

P.S.: tracey: I'm probably making your life in the gotwebd branch harder
by adding these warnings (that turns into errors thanks to -Werror in
non release builds.)  I'll try to see if gotwebd breaks with these
warnings enabled and cook a diff soon(tm) ;)

-----------------------------------------------
commit c02440c56013c8818bf510752cfed899a4c8abd6 (wwritestr)
from: Omar Polo <op@omarpolo.com>
date: Sun Jun 26 09:01:05 2022 UTC
 
 build with -Wwrite-strings
 
diff 3d589bee0bbbe812bb91f3b0284fbf2596304132 c02440c56013c8818bf510752cfed899a4c8abd6
commit - 3d589bee0bbbe812bb91f3b0284fbf2596304132
commit + c02440c56013c8818bf510752cfed899a4c8abd6
blob - ef99224d337170364b2e5458bf6e2a3206c3e05a
blob + 2b72df1a3c9e149cff31460ae7090698a3416a61
--- Makefile.inc
+++ Makefile.inc
@@ -11,7 +11,7 @@ LIBEXECDIR ?= ${PREFIX}/libexec
 MANDIR ?= ${PREFIX}/man/man
 .else
 CFLAGS += -Werror -Wall -Wstrict-prototypes -Wmissing-prototypes
-CFLAGS += -Wunused-variable
+CFLAGS += -Wwrite-strings -Wunused-variable
 PREFIX ?= ${HOME}
 BINDIR ?= ${PREFIX}/bin
 LIBEXECDIR ?= ${BINDIR}
blob - e1fc7e795e917a7be22aff253610e8c66eef455c
blob + a6bd0096cd2d6e29e21d56f19ed44cd2365eb1ba
--- gotweb/gotweb.c
+++ gotweb/gotweb.c
@@ -237,7 +237,7 @@ struct gw_query_action {
 	unsigned int		 func_id;
 	const char		*func_name;
 	const struct got_error	*(*func_main)(struct gw_trans *);
-	char			*template;
+	const char		*template;
 };
 
 enum gw_query_actions {
@@ -2716,11 +2716,11 @@ gw_get_time_str(char **repo_age, time_t committer_time
 {
 	struct tm tm;
 	time_t diff_time;
-	char *years = "years ago", *months = "months ago";
-	char *weeks = "weeks ago", *days = "days ago", *hours = "hours ago";
-	char *minutes = "minutes ago", *seconds = "seconds ago";
-	char *now = "right now";
-	char *s;
+	const char *years = "years ago", *months = "months ago";
+	const char *weeks = "weeks ago", *days = "days ago", *hours = "hours ago";
+	const char *minutes = "minutes ago", *seconds = "seconds ago";
+	const char *now = "right now";
+	const char *s;
 	char datebuf[29];
 
 	*repo_age = NULL;
@@ -4763,7 +4763,7 @@ static const struct got_error *
 gw_colordiff_line(struct gw_trans *gw_trans, char *buf)
 {
 	const struct got_error *error = NULL;
-	char *color = NULL;
+	const char *color = NULL;
 	enum kcgi_err kerr = KCGI_OK;
 
 	if (strncmp(buf, "-", 1) == 0)
blob - 5dbe18b856a53a58bee722d773249a93e91993a8
blob + 4f537113e196997bfae7b568a9d90ab8fe9e77b0
--- include/got_diff.h
+++ include/got_diff.h
@@ -166,7 +166,7 @@ const struct got_error *got_diff_objects_as_blobs(off_
  */
 const struct got_error *got_diff_objects_as_trees(off_t **, size_t *,
     FILE *, FILE *, struct got_object_id *, struct got_object_id *,
-    struct got_pathlist_head *, char *, char *, int, int, int,
+    struct got_pathlist_head *, const char *, const char *, int, int, int,
     struct got_repository *, FILE *);
 
 /*
blob - 4f688a96c44c72c60f3157e3d1293fcca25111c6
blob + b7ce464748cabe3f0823af08f8961d87d13e0c7a
--- lib/dial.c
+++ lib/dial.c
@@ -67,7 +67,7 @@ got_dial_apply_unveil(const char *proto)
 }
 
 static int
-hassuffix(char *base, char *suf)
+hassuffix(const char *base, const char *suf)
 {
 	int nb, ns;
 
@@ -207,7 +207,7 @@ got_dial_ssh(pid_t *newpid, int *newfd, const char *ho
 	const struct got_error *error = NULL;
 	int pid, pfd[2];
 	char cmd[64];
-	char *argv[11];
+	const char *argv[11];
 	int i = 0, j;
 
 	*newpid = -1;
@@ -252,7 +252,7 @@ got_dial_ssh(pid_t *newpid, int *newfd, const char *ho
 		n = snprintf(cmd, sizeof(cmd), "git-%s-pack", direction);
 		if (n < 0 || n >= ssizeof(cmd))
 			err(1, "snprintf");
-		if (execv(GOT_DIAL_PATH_SSH, argv) == -1)
+		if (execv(GOT_DIAL_PATH_SSH, (char *const *)argv) == -1)
 			err(1, "execv");
 		abort(); /* not reached */
 	} else {
blob - 8337d25f3e5608a25af5b7e4a9940d9bcb2ceba5
blob + 773169c32d3cb119ac330201afe43d53b115085a
--- lib/diff.c
+++ lib/diff.c
@@ -62,7 +62,7 @@ diff_blobs(off_t **line_offsets, size_t *nlines,
 	const struct got_error *err = NULL, *free_err;
 	char hex1[SHA1_DIGEST_STRING_LENGTH];
 	char hex2[SHA1_DIGEST_STRING_LENGTH];
-	char *idstr1 = NULL, *idstr2 = NULL;
+	const char *idstr1 = NULL, *idstr2 = NULL;
 	off_t size1, size2;
 	struct got_diffreg_result *result;
 	off_t outoff = 0;
@@ -220,7 +220,7 @@ diff_blob_file(struct got_diffreg_result **resultp,
 {
 	const struct got_error *err = NULL, *free_err;
 	char hex1[SHA1_DIGEST_STRING_LENGTH];
-	char *idstr1 = NULL;
+	const char *idstr1 = NULL;
 	struct got_diffreg_result *result = NULL;
 
 	if (resultp)
@@ -904,9 +904,9 @@ show_object_id(off_t **line_offsets, size_t *nlines, c
 static const struct got_error *
 diff_objects_as_trees(off_t **line_offsets, size_t *nlines,
     FILE *f1, FILE *f2, struct got_object_id *id1, struct got_object_id *id2,
-    struct got_pathlist_head *paths,
-    char *label1, char *label2, int diff_context, int ignore_whitespace,
-    int force_text_diff, struct got_repository *repo, FILE *outfile)
+    struct got_pathlist_head *paths, const char *label1, const char *label2,
+    int diff_context, int ignore_whitespace, int force_text_diff,
+    struct got_repository *repo, FILE *outfile)
 {
 	const struct got_error *err;
 	struct got_tree_object *tree1 = NULL, *tree2 = NULL;
@@ -960,9 +960,9 @@ done:
 const struct got_error *
 got_diff_objects_as_trees(off_t **line_offsets, size_t *nlines,
     FILE *f1, FILE *f2, struct got_object_id *id1, struct got_object_id *id2,
-    struct got_pathlist_head *paths,
-    char *label1, char *label2, int diff_context, int ignore_whitespace,
-    int force_text_diff, struct got_repository *repo, FILE *outfile)
+    struct got_pathlist_head *paths, const char *label1, const char *label2,
+    int diff_context, int ignore_whitespace, int force_text_diff,
+    struct got_repository *repo, FILE *outfile)
 {
 	const struct got_error *err;
 	char *idstr = NULL;
blob - 70383527274d9e8c501d2480b0479657ced378b7
blob + e44ac0a4e3f7fe9caf5b350a89c7e5c5beba00be
--- lib/diff_output.c
+++ lib/diff_output.c
@@ -262,7 +262,7 @@ diff_output_match_function_prototype(char *prototype, 
 	struct diff_atom *start_atom, *atom;
 	const struct diff_data *data;
 	unsigned char buf[DIFF_FUNCTION_CONTEXT_SIZE];
-	char *state = NULL;
+	const char *state = NULL;
 	int rc, i, ch;
 
 	if (result->left->atoms.len > 0 && cc->left.start > 0) {
blob - 16d1a758fc03fec245de789888abc9cb072f408d
blob + 384b6d4beb4ecb887c5ba512e0dd1f882bb02d4b
--- lib/pack_create.c
+++ lib/pack_create.c
@@ -1829,7 +1829,7 @@ done:
 }
 
 static const struct got_error *
-hwrite(FILE *f, void *buf, off_t len, SHA1_CTX *ctx)
+hwrite(FILE *f, const void *buf, off_t len, SHA1_CTX *ctx)
 {
 	size_t n;
 
blob - 19dc9c983445c68dbea181cf6bfabcfd1ee9d949
blob + e3d7799acfdd15ffc55d19126b7c7e27a696bee6
--- lib/path.c
+++ lib/path.c
@@ -458,10 +458,11 @@ const struct got_error *
 got_path_find_prog(char **filename, const char *prog)
 {
 	const struct got_error *err = NULL;
+	const char *path;
 	char *p;
 	int len;
 	struct stat sbuf;
-	char *path, *pathcpy;
+	char *pathcpy, *dup = NULL;
 
 	*filename = NULL;
 
@@ -480,19 +481,22 @@ got_path_find_prog(char **filename, const char *prog)
 		return NULL;
 	}
 
-	if ((path = strdup(path)) == NULL)
+	if ((dup = strdup(path)) == NULL)
 		return got_error_from_errno("strdup");
-	pathcpy = path;
+	pathcpy = dup;
 
 	while ((p = strsep(&pathcpy, ":")) != NULL) {
-		if (*p == '\0')
-			p = ".";
+		const char *d;
 
 		len = strlen(p);
 		while (len > 0 && p[len-1] == '/')
 			p[--len] = '\0';	/* strip trailing '/' */
 
-		if (asprintf(filename, "%s/%s", p, prog) == -1) {
+		d = p;
+		if (*d == '\0')
+			d = ".";
+
+		if (asprintf(filename, "%s/%s", d, prog) == -1) {
 			err = got_error_from_errno("asprintf");
 			break;
 		}
@@ -503,7 +507,7 @@ got_path_find_prog(char **filename, const char *prog)
 		*filename = NULL;
 		continue;
 	}
-	free(path);
+	free(dup);
 	return err;
 }
 
blob - 8616b4e5f3930bdff53402d97880b3ae74a6fa5f
blob + de71b5845e334bff8e56366f910514f149cf3519
--- lib/reference.c
+++ lib/reference.c
@@ -998,8 +998,8 @@ got_ref_list(struct got_reflist_head *refs, struct got
 {
 	const struct got_error *err;
 	char *packed_refs_path, *path_refs = NULL;
-	char *abs_namespace = NULL;
-	char *buf = NULL, *ondisk_ref_namespace = NULL;
+	char *abs_namespace = NULL, *buf = NULL;
+	const char *ondisk_ref_namespace = NULL;
 	char *line = NULL;
 	FILE *f = NULL;
 	struct got_reference *ref;
blob - 9d84105f23ebcedee81341088dc0fd38cb0c4261
blob + c0e71877fd2d42e62b12140d90a427fecf8d0cee
--- regress/fetch/fetch_test.c
+++ regress/fetch/fetch_test.c
@@ -46,7 +46,7 @@ static int verbose;
 static int quiet;
 
 static void
-test_printf(char *fmt, ...)
+test_printf(const char *fmt, ...)
 {
 	va_list ap;
 
blob - e8001a520fd4f5763bc38dbfb1c94be3fdd0e531
blob + bbf71a79eb4fa68027df6f06b7a1d9926000a518
--- regress/path/path_test.c
+++ regress/path/path_test.c
@@ -34,7 +34,7 @@ static int verbose;
 static int quiet;
 
 static void
-test_printf(char *fmt, ...)
+test_printf(const char *fmt, ...)
 {
 	va_list ap;