[PATCH 2/3] strbuf_nested_expand(): allow expansion to interrupt in the middle

[Junio C Hamano <gitster@pobox.com>]

This itself does not do a "nested" expansion, but it paves a way for
supporting an extended syntax to express a function that works on an
expanded substring, e.g. %[function(param...)expanded-string%], by
allowing the callback function to tell where the argument to the function
ends.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 strbuf.c |   23 +++++++++++++++++++----
 strbuf.h |    3 ++-
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/strbuf.c b/strbuf.c
index a6153dc..2bbc49c 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -214,25 +214,40 @@ void strbuf_addf(struct strbuf *sb, const char *fmt, ...)
 	strbuf_setlen(sb, sb->len + len);
 }
 
-void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
-		   void *context)
+void strbuf_nested_expand(struct strbuf *sb, const char **format_p,
+			  expand_fn_t fn, void *context)
 {
+	const char *format = *format_p;
 	for (;;) {
 		const char *percent;
 		size_t consumed;
 
 		percent = strchrnul(format, '%');
 		strbuf_add(sb, format, percent - format);
+		format = percent;
 		if (!*percent)
 			break;
-		format = percent + 1;
+		format++;
 
 		consumed = fn(sb, format, context);
-		if (consumed)
+		if ((ssize_t) consumed < 0)
+			break;
+		else if (consumed)
 			format += consumed;
 		else
 			strbuf_addch(sb, '%');
 	}
+	*format_p = format;
+}
+
+void strbuf_expand(struct strbuf *sb, const char *o_format, expand_fn_t fn,
+		   void *context)
+{
+	const char *format = o_format;
+	strbuf_nested_expand(sb, &format, fn, context);
+	if (*format)
+		die("format error: negative return from expand function: %s",
+		    o_format);
 }
 
 size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
diff --git a/strbuf.h b/strbuf.h
index d05e056..e602899 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -109,8 +109,9 @@ static inline void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2) {
 }
 extern void strbuf_adddup(struct strbuf *sb, size_t pos, size_t len);
 
-typedef size_t (*expand_fn_t) (struct strbuf *sb, const char *placeholder, void *context);
+typedef size_t (*expand_fn_t)(struct strbuf *sb, const char *placeholder, void *context);
 extern void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn, void *context);
+extern void strbuf_nested_expand(struct strbuf *sb, const char **format_p, expand_fn_t fn, void *context);
 struct strbuf_expand_dict_entry {
 	const char *placeholder;
 	const char *value;
-- 
1.6.5.99.g9ed7e