refactor skip_prefix to return a boolean

The skip_prefix() function returns a pointer to the content
past the prefix, or NULL if the prefix was not found. While
this is nice and simple, in practice it makes it hard to use
for two reasons:

  1. When you want to conditionally skip or keep the string
     as-is, you have to introduce a temporary variable.
     For example:

       tmp = skip_prefix(buf, "foo");
       if (tmp)
	       buf = tmp;

  2. It is verbose to check the outcome in a conditional, as
     you need extra parentheses to silence compiler
     warnings. For example:

       if ((cp = skip_prefix(buf, "foo"))
	       /* do something with cp */

Both of these make it harder to use for long if-chains, and
we tend to use starts_with() instead. However, the first line
of "do something" is often to then skip forward in buf past
the prefix, either using a magic constant or with an extra
strlen(3) (which is generally computed at compile time, but
means we are repeating ourselves).

This patch refactors skip_prefix() to return a simple boolean,
and to provide the pointer value as an out-parameter. If the
prefix is not found, the out-parameter is untouched. This
lets you write:

  if (skip_prefix(arg, "foo ", &arg))
	  do_foo(arg);
  else if (skip_prefix(arg, "bar ", &arg))
	  do_bar(arg);

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

18 files changed, 73 insertions, 58 deletions

diff --git a/advice.c b/advice.c
index c50ebdf5f..9b4203319 100644
--- a/advice.c
+++ b/advice.c
@@ -61,9 +61,12 @@ void advise(const char *advice, ...)
 
 int git_default_advice_config(const char *var, const char *value)
 {
-	const char *k = skip_prefix(var, "advice.");
+	const char *k;
 	int i;
 
+	if (!skip_prefix(var, "advice.", &k))
+		return 0;
+
 	for (i = 0; i < ARRAY_SIZE(advice_config); i++) {
 		if (strcmp(k, advice_config[i].name))
 			continue;
diff --git a/branch.c b/branch.c
index 660097bc2..46e8aa86d 100644
--- a/branch.c
+++ b/branch.c
@@ -50,11 +50,11 @@ static int should_setup_rebase(const char *origin)
 
 void install_branch_config(int flag, const char *local, const char *origin, const char *remote)
 {
-	const char *shortname = skip_prefix(remote, "refs/heads/");
+	const char *shortname = NULL;
 	struct strbuf key = STRBUF_INIT;
 	int rebasing = should_setup_rebase(origin);
 
-	if (shortname
+	if (skip_prefix(remote, "refs/heads/", &shortname)
 	    && !strcmp(local, shortname)
 	    && !origin) {
 		warning(_("Not setting branch %s as its own upstream."),
diff --git a/builtin/branch.c b/builtin/branch.c
index 652b1d2d1..0591b22a4 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -294,13 +294,13 @@ static char *resolve_symref(const char *src, const char *prefix)
 {
 	unsigned char sha1[20];
 	int flag;
-	const char *dst, *cp;
+	const char *dst;
 
 	dst = resolve_ref_unsafe(src, sha1, 0, &flag);
 	if (!(dst && (flag & REF_ISSYMREF)))
 		return NULL;
-	if (prefix && (cp = skip_prefix(dst, prefix)))
-		dst = cp;
+	if (prefix)
+		skip_prefix(dst, prefix, &dst);
 	return xstrdup(dst);
 }
 
diff --git a/builtin/clone.c b/builtin/clone.c
index b12989d1c..a5b2d9db3 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -584,11 +584,11 @@ static void update_remote_refs(const struct ref *refs,
 static void update_head(const struct ref *our, const struct ref *remote,
 			const char *msg)
 {
-	if (our && starts_with(our->name, "refs/heads/")) {
+	const char *head;
+	if (our && skip_prefix(our->name, "refs/heads/", &head)) {
 		/* Local default branch link */
 		create_symref("HEAD", our->name, NULL);
 		if (!option_bare) {
-			const char *head = skip_prefix(our->name, "refs/heads/");
 			update_ref(msg, "HEAD", our->old_sha1, NULL, 0,
 				   UPDATE_REFS_DIE_ON_ERR);
 			install_branch_config(0, head, option_origin, our->name);
@@ -703,9 +703,12 @@ static void write_refspec_config(const char* src_ref_prefix,
 					strbuf_addf(&value, "+%s:%s%s", our_head_points_at->name,
 						branch_top->buf, option_branch);
 			} else if (remote_head_points_at) {
+				const char *head = remote_head_points_at->name;
+				if (!skip_prefix(head, "refs/heads/", &head))
+					die("BUG: remote HEAD points at non-head?");
+
 				strbuf_addf(&value, "+%s:%s%s", remote_head_points_at->name,
-						branch_top->buf,
-						skip_prefix(remote_head_points_at->name, "refs/heads/"));
+						branch_top->buf, head);
 			}
 			/*
 			 * otherwise, the next "git fetch" will
diff --git a/builtin/commit.c b/builtin/commit.c
index 5e2221c8e..ec7534123 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1020,7 +1020,7 @@ static int message_is_empty(struct strbuf *sb)
 static int template_untouched(struct strbuf *sb)
 {
 	struct strbuf tmpl = STRBUF_INIT;
-	char *start;
+	const char *start;
 
 	if (cleanup_mode == CLEANUP_NONE && sb->len)
 		return 0;
@@ -1029,8 +1029,7 @@ static int template_untouched(struct strbuf *sb)
 		return 0;
 
 	stripspace(&tmpl, cleanup_mode == CLEANUP_ALL);
-	start = (char *)skip_prefix(sb->buf, tmpl.buf);
-	if (!start)
+	if (!skip_prefix(sb->buf, tmpl.buf, &start))
 		start = sb->buf;
 	strbuf_release(&tmpl);
 	return rest_is_empty(sb, start - sb->buf);
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 72378e681..3c1924155 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -297,7 +297,7 @@ static void credit_people(struct strbuf *out,
 	if (!them->nr ||
 	    (them->nr == 1 &&
 	     me &&
-	     (me = skip_prefix(me, them->items->string)) != NULL &&
+	     skip_prefix(me, them->items->string, &me) &&
 	     starts_with(me, " <")))
 		return;
 	strbuf_addf(out, "\n%c %s ", comment_line_char, label);
diff --git a/builtin/push.c b/builtin/push.c
index f8dfea41e..f50e3d5e7 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -127,11 +127,10 @@ static NORETURN int die_push_simple(struct branch *branch, struct remote *remote
 	 * them the big ugly fully qualified ref.
 	 */
 	const char *advice_maybe = "";
-	const char *short_upstream =
-		skip_prefix(branch->merge[0]->src, "refs/heads/");
+	const char *short_upstream = branch->merge[0]->src;
+
+	skip_prefix(short_upstream, "refs/heads/", &short_upstream);
 
-	if (!short_upstream)
-		short_upstream = branch->merge[0]->src;
 	/*
 	 * Don't show advice for people who explicitly set
 	 * push.default.
diff --git a/builtin/remote.c b/builtin/remote.c
index c9102e8fe..a8efe3da4 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -250,9 +250,7 @@ static struct string_list branch_list;
 
 static const char *abbrev_ref(const char *name, const char *prefix)
 {
-	const char *abbrev = skip_prefix(name, prefix);
-	if (abbrev)
-		return abbrev;
+	skip_prefix(name, prefix, &name);
 	return name;
 }
 #define abbrev_branch(name) abbrev_ref((name), "refs/heads/")
diff --git a/column.c b/column.c
index 1a468debb..ca878bcea 100644
--- a/column.c
+++ b/column.c
@@ -336,8 +336,9 @@ static int column_config(const char *var, const char *value,
 int git_column_config(const char *var, const char *value,
 		      const char *command, unsigned int *colopts)
 {
-	const char *it = skip_prefix(var, "column.");
-	if (!it)
+	const char *it;
+
+	if (!skip_prefix(var, "column.", &it))
 		return 0;
 
 	if (!strcmp(it, "ui"))
diff --git a/commit.c b/commit.c
index 881be3baa..dfc0eb024 100644
--- a/commit.c
+++ b/commit.c
@@ -556,8 +556,7 @@ static void record_author_date(struct author_date_slab *author_date,
 	     buf;
 	     buf = line_end + 1) {
 		line_end = strchrnul(buf, '\n');
-		ident_line = skip_prefix(buf, "author ");
-		if (!ident_line) {
+		if (!skip_prefix(buf, "author ", &ident_line)) {
 			if (!line_end[0] || line_end[1] == '\n')
 				return; /* end of header */
 			continue;
@@ -1183,8 +1182,7 @@ static void parse_gpg_output(struct signature_check *sigc)
 	for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
 		const char *found, *next;
 
-		found = skip_prefix(buf, sigcheck_gpg_status[i].check + 1);
-		if (!found) {
+		if (!skip_prefix(buf, sigcheck_gpg_status[i].check + 1, &found)) {
 			found = strstr(buf, sigcheck_gpg_status[i].check);
 			if (!found)
 				continue;
diff --git a/config.c b/config.c
index a1aef1cf3..ba882a1f5 100644
--- a/config.c
+++ b/config.c
@@ -138,8 +138,7 @@ int git_config_include(const char *var, const char *value, void *data)
 	if (ret < 0)
 		return ret;
 
-	type = skip_prefix(var, "include.");
-	if (!type)
+	if (!skip_prefix(var, "include.", &type))
 		return ret;
 
 	if (!strcmp(type, "path"))
diff --git a/credential-cache--daemon.c b/credential-cache--daemon.c
index 390f19425..3b370ca5e 100644
--- a/credential-cache--daemon.c
+++ b/credential-cache--daemon.c
@@ -109,14 +109,12 @@ static int read_request(FILE *fh, struct credential *c,
 	const char *p;
 
 	strbuf_getline(&item, fh, '\n');
-	p = skip_prefix(item.buf, "action=");
-	if (!p)
+	if (!skip_prefix(item.buf, "action=", &p))
 		return error("client sent bogus action line: %s", item.buf);
 	strbuf_addstr(action, p);
 
 	strbuf_getline(&item, fh, '\n');
-	p = skip_prefix(item.buf, "timeout=");
-	if (!p)
+	if (!skip_prefix(item.buf, "timeout=", &p))
 		return error("client sent bogus timeout line: %s", item.buf);
 	*timeout = atoi(p);
 
diff --git a/credential.c b/credential.c
index e54753c75..4d79d320f 100644
--- a/credential.c
+++ b/credential.c
@@ -40,8 +40,7 @@ static int credential_config_callback(const char *var, const char *value,
 	struct credential *c = data;
 	const char *key, *dot;
 
-	key = skip_prefix(var, "credential.");
-	if (!key)
+	if (!skip_prefix(var, "credential.", &key))
 		return 0;
 
 	if (!value)
diff --git a/fsck.c b/fsck.c
index abed62bac..bdbea2b99 100644
--- a/fsck.c
+++ b/fsck.c
@@ -278,20 +278,18 @@ static int fsck_ident(const char **ident, struct object *obj, fsck_error error_f
 
 static int fsck_commit(struct commit *commit, fsck_error error_func)
 {
-	const char *buffer = commit->buffer, *tmp;
+	const char *buffer = commit->buffer;
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
 	int parents = 0;
 	int err;
 
-	buffer = skip_prefix(buffer, "tree ");
-	if (!buffer)
+	if (!skip_prefix(buffer, "tree ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'tree' line");
 	if (get_sha1_hex(buffer, tree_sha1) || buffer[40] != '\n')
 		return error_func(&commit->object, FSCK_ERROR, "invalid 'tree' line format - bad sha1");
 	buffer += 41;
-	while ((tmp = skip_prefix(buffer, "parent "))) {
-		buffer = tmp;
+	while (skip_prefix(buffer, "parent ", &buffer)) {
 		if (get_sha1_hex(buffer, sha1) || buffer[40] != '\n')
 			return error_func(&commit->object, FSCK_ERROR, "invalid 'parent' line format - bad sha1");
 		buffer += 41;
@@ -318,14 +316,12 @@ static int fsck_commit(struct commit *commit, fsck_error error_func)
 		if (p || parents)
 			return error_func(&commit->object, FSCK_ERROR, "parent objects missing");
 	}
-	buffer = skip_prefix(buffer, "author ");
-	if (!buffer)
+	if (!skip_prefix(buffer, "author ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'author' line");
 	err = fsck_ident(&buffer, &commit->object, error_func);
 	if (err)
 		return err;
-	buffer = skip_prefix(buffer, "committer ");
-	if (!buffer)
+	if (!skip_prefix(buffer, "committer ", &buffer))
 		return error_func(&commit->object, FSCK_ERROR, "invalid format - expected 'committer' line");
 	err = fsck_ident(&buffer, &commit->object, error_func);
 	if (err)
diff --git a/git-compat-util.h b/git-compat-util.h
index b6f03b36d..d29e1dff0 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -349,13 +349,32 @@ extern void set_die_is_recursing_routine(int (*routine)(void));
 extern int starts_with(const char *str, const char *prefix);
 extern int ends_with(const char *str, const char *suffix);
 
-static inline const char *skip_prefix(const char *str, const char *prefix)
+/*
+ * If the string "str" begins with the string found in "prefix", return 1.
+ * The "out" parameter is set to "str + strlen(prefix)" (i.e., to the point in
+ * the string right after the prefix).
+ *
+ * Otherwise, return 0 and leave "out" untouched.
+ *
+ * Examples:
+ *
+ *   [extract branch name, fail if not a branch]
+ *   if (!skip_prefix(ref, "refs/heads/", &branch)
+ *	return -1;
+ *
+ *   [skip prefix if present, otherwise use whole string]
+ *   skip_prefix(name, "refs/heads/", &name);
+ */
+static inline int skip_prefix(const char *str, const char *prefix,
+			      const char **out)
 {
 	do {
-		if (!*prefix)
-			return str;
+		if (!*prefix) {
+			*out = str;
+			return 1;
+		}
 	} while (*str++ == *prefix++);
-	return NULL;
+	return 0;
 }
 
 #if defined(NO_MMAP) || defined(USE_WIN32_MMAP)
diff --git a/parse-options.c b/parse-options.c
index b536896f2..e7dafa80d 100644
--- a/parse-options.c
+++ b/parse-options.c
@@ -231,7 +231,8 @@ static int parse_long_opt(struct parse_opt_ctx_t *p, const char *arg,
 			continue;
 
 again:
-		rest = skip_prefix(arg, long_name);
+		if (!skip_prefix(arg, long_name, &rest))
+			rest = NULL;
 		if (options->type == OPTION_ARGUMENT) {
 			if (!rest)
 				continue;
@@ -280,12 +281,13 @@ is_abbreviated:
 				continue;
 			}
 			flags |= OPT_UNSET;
-			rest = skip_prefix(arg + 3, long_name);
-			/* abbreviated and negated? */
-			if (!rest && starts_with(long_name, arg + 3))
-				goto is_abbreviated;
-			if (!rest)
-				continue;
+			if (!skip_prefix(arg + 3, long_name, &rest)) {
+				/* abbreviated and negated? */
+				if (starts_with(long_name, arg + 3))
+					goto is_abbreviated;
+				else
+					continue;
+			}
 		}
 		if (*rest) {
 			if (*rest != '=')
diff --git a/transport.c b/transport.c
index 325f03e1e..59c9727d8 100644
--- a/transport.c
+++ b/transport.c
@@ -192,7 +192,9 @@ static void set_upstreams(struct transport *transport, struct ref *refs,
 
 static const char *rsync_url(const char *url)
 {
-	return !starts_with(url, "rsync://") ? skip_prefix(url, "rsync:") : url;
+	if (!starts_with(url, "rsync://"))
+		skip_prefix(url, "rsync:", &url);
+	return url;
 }
 
 static struct ref *get_refs_via_rsync(struct transport *transport, int for_push)
diff --git a/urlmatch.c b/urlmatch.c
index ec87cba75..3d4c54b5c 100644
--- a/urlmatch.c
+++ b/urlmatch.c
@@ -483,8 +483,7 @@ int urlmatch_config_entry(const char *var, const char *value, void *cb)
 	int user_matched = 0;
 	int retval;
 
-	key = skip_prefix(var, collect->section);
-	if (!key || *(key++) != '.') {
+	if (!skip_prefix(var, collect->section, &key) || *(key++) != '.') {
 		if (collect->cascade_fn)
 			return collect->cascade_fn(var, value, cb);
 		return 0; /* not interested */