Add git__fmt as an easier to use snprintf

Checking the return value of snprintf is a pain, as it must be
>= 0 and < sizeof(buffer).  git__fmt is a simple wrapper to
perform these checks.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/common.h

@@ -19,6 +19,7 @@
 #endif
 
 #include "cc-compat.h"
+#include "git/common.h"
 #include "util.h"
 #include "thread-utils.h"
 #include "errors.h"

src/util.c

 #define GIT__NO_HIDE_MALLOC
 #include "common.h"
+#include <stdarg.h>
+#include <stdio.h>
 
 void *git__malloc(size_t n)
 {
@@ -25,6 +27,19 @@ char *git__strdup(const char *s)
 	return r;
 }
 
+int git__fmt(char *buf, size_t buf_sz, const char *fmt, ...)
+{
+	va_list va;
+	int r;
+
+	va_start(va, fmt);
+	r = vsnprintf(buf, buf_sz, fmt, va);
+	va_end(va);
+	if (r < 0 || r >= buf_sz)
+		return GIT_ERROR;
+	return r;
+}
+
 int git__prefixcmp(const char *str, const char *prefix)
 {
 	for (;;) {

src/util.h

@@ -26,6 +26,8 @@ extern char *git__strdup(const char *);
 # define strdup(a)          GIT__FORBID_MALLOC
 #endif
 
+extern int git__fmt(char *, size_t, const char *, ...)
+	GIT_FORMAT_PRINTF(3, 4);
 extern int git__prefixcmp(const char *str, const char *prefix);
 extern int git__suffixcmp(const char *str, const char *suffix);