buf: add buf_add_fmt()

include/common/buf.h

@@ -43,6 +43,13 @@ void buf_expand_tilde(struct buf *s);
  */
 void buf_expand_shell_variables(struct buf *s);
 
+/**
+ * buf_add_fmt - add format string to C string buffer
+ * @s: buffer
+ * @fmt: format string to be added
+ */
+void buf_add_fmt(struct buf *s, const char *fmt, ...);
+
 /**
  * buf_add - add data to C string buffer
  * @s: buffer

src/common/buf.c

@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #include <assert.h>
 #include <ctype.h>
+#include <stdarg.h>
 #include <stdbool.h>
+#include <stdio.h>
 #include <string.h>
 #include "common/buf.h"
 #include "common/macros.h"
@@ -95,6 +97,38 @@ buf_expand(struct buf *s, int new_alloc)
 	s->alloc = new_alloc;
 }
 
+void
+buf_add_fmt(struct buf *s, const char *fmt, ...)
+{
+	if (string_null_or_empty(fmt)) {
+		return;
+	}
+	size_t size = 0;
+	va_list ap;
+
+	va_start(ap, fmt);
+	int n = vsnprintf(NULL, size, fmt, ap);
+	va_end(ap);
+
+	if (n < 0) {
+		return;
+	}
+
+	size = (size_t)n + 1;
+	buf_expand(s, s->len + size);
+
+	va_start(ap, fmt);
+	n = vsnprintf(s->data + s->len, size, fmt, ap);
+	va_end(ap);
+
+	if (n < 0) {
+		return;
+	}
+
+	s->len += n;
+	s->data[s->len] = 0;
+}
+
 void
 buf_add(struct buf *s, const char *data)
 {

t/buf-simple.c

@@ -55,10 +55,25 @@ test_expand_title(void **state)
 	free(s.data);
 }
 
+static void
+test_buf_add_fmt(void **state)
+{
+	(void)state;
+
+	struct buf s = BUF_INIT;
+
+	buf_add(&s, "foo");
+	buf_add_fmt(&s, " %s baz %d", "bar", 10);
+	assert_string_equal(s.data, "foo bar baz 10");
+
+	buf_reset(&s);
+}
+
 int main(int argc, char **argv)
 {
 	const struct CMUnitTest tests[] = {
 		cmocka_unit_test(test_expand_title),
+		cmocka_unit_test(test_buf_add_fmt),
 	};
 
 	return cmocka_run_group_tests(tests, NULL, NULL);