global: convert intentionally-leaking config strings to consts

There are multiple cases where we intentionally leak config strings:

  - `struct gpg_format` is used to track programs that can be used for
    signing commits, either via gpg(1), gpgsm(1) or ssh-keygen(1). The
    user can override the commands via several config variables. As the
    array is populated once, only, and the struct memers are never
    written to or free'd.

  - `struct ll_merge_driver` is used to track merge drivers. Same as
    with the GPG format, these drivers are populated once and then
    reused. Its data is never written to or free'd, either.

  - `struct userdiff_funcname` and `struct userdiff_driver` can be
    configured via `diff.<driver>.*` to add additional drivers. Again,
    these have a global lifetime and are never written to or free'd.

All of these are intentionally kept alive and are never written to.
Furthermore, all of these are being assigned both string constants in
some places, and allocated strings in other places. This will cause
warnings once we enable `-Wwrite-strings`, so let's mark the respective
fields as `const char *` and cast away the constness when assigning
those values.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -34,7 +34,7 @@ static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;
 
 struct gpg_format {
 	const char *name;
-	char *program;
+	const char *program;
 	const char **verify_args;
 	const char **sigs;
 	int (*verify_signed_buffer)(struct signature_check *sigc,
@@ -783,7 +783,7 @@ static int git_gpg_config(const char *var, const char *value,
 
 	if (fmtname) {
 		fmt = get_format_by_name(fmtname);
-		return git_config_string(&fmt->program, var, value);
+		return git_config_string((char **) &fmt->program, var, value);
 	}
 
 	return 0;

merge-ll.c

@@ -27,7 +27,7 @@ typedef enum ll_merge_result (*ll_merge_fn)(const struct ll_merge_driver *,
 
 struct ll_merge_driver {
 	const char *name;
-	char *description;
+	const char *description;
 	ll_merge_fn fn;
 	char *recursive;
 	struct ll_merge_driver *next;
@@ -304,8 +304,13 @@ static int read_merge_config(const char *var, const char *value,
 		ll_user_merge_tail = &(fn->next);
 	}
 
-	if (!strcmp("name", key))
+	if (!strcmp("name", key)) {
-		return git_config_string(&fn->description, var, value);
+		/*
+		 * The description is leaking, but that's okay as we want to
+		 * keep around the merge drivers anyway.
+		 */
+		return git_config_string((char **) &fn->description, var, value);
+	}
 
 	if (!strcmp("driver", key)) {
 		if (!value)

userdiff.c

@@ -399,7 +399,7 @@ static struct userdiff_driver *userdiff_find_by_namelen(const char *name, size_t
 static int parse_funcname(struct userdiff_funcname *f, const char *k,
 		const char *v, int cflags)
 {
-	if (git_config_string(&f->pattern, k, v) < 0)
+	if (git_config_string((char **) &f->pattern, k, v) < 0)
 		return -1;
 	f->cflags = cflags;
 	return 0;
@@ -445,15 +445,15 @@ int userdiff_config(const char *k, const char *v)
 	if (!strcmp(type, "binary"))
 		return parse_tristate(&drv->binary, k, v);
 	if (!strcmp(type, "command"))
-		return git_config_string(&drv->external, k, v);
+		return git_config_string((char **) &drv->external, k, v);
 	if (!strcmp(type, "textconv"))
-		return git_config_string(&drv->textconv, k, v);
+		return git_config_string((char **) &drv->textconv, k, v);
 	if (!strcmp(type, "cachetextconv"))
 		return parse_bool(&drv->textconv_want_cache, k, v);
 	if (!strcmp(type, "wordregex"))
-		return git_config_string(&drv->word_regex, k, v);
+		return git_config_string((char **) &drv->word_regex, k, v);
 	if (!strcmp(type, "algorithm"))
-		return git_config_string(&drv->algorithm, k, v);
+		return git_config_string((char **) &drv->algorithm, k, v);
 
 	return 0;
 }

userdiff.h

@@ -7,19 +7,19 @@ struct index_state;
 struct repository;
 
 struct userdiff_funcname {
-	char *pattern;
+	const char *pattern;
 	int cflags;
 };
 
 struct userdiff_driver {
 	const char *name;
-	char *external;
+	const char *external;
-	char *algorithm;
+	const char *algorithm;
 	int binary;
 	struct userdiff_funcname funcname;
-	char *word_regex;
+	const char *word_regex;
-	char *word_regex_multi_byte;
+	const char *word_regex_multi_byte;
-	char *textconv;
+	const char *textconv;
 	struct notes_cache *textconv_cache;
 	int textconv_want_cache;
 };