Implement a work-around for a mis-behaved vsnprintf on Windows.

On Windows, vsnprintf returns -1 if the buffer is too small instead of
the number of characters needed. This wrapper computes the needed buffer
size by trying various sizes with exponential growth. A large growth
factor is used so as only few trials are required if a really large
result needs to be stored.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.c

@@ -459,3 +459,29 @@ int mingw_rename(const char *pold, const char *pnew)
 	errno = EACCES;
 	return -1;
 }
+
+#undef vsnprintf
+/* this is out of line because it uses alloca() behind the scenes,
+ * which must not be called in a loop (alloca() reclaims the allocations
+ * only at function exit)
+ */
+static int try_vsnprintf(size_t size, const char *fmt, va_list args)
+{
+	char buf[size];	/* gcc-ism */
+	return vsnprintf(buf, size, fmt, args);
+}
+
+int mingw_vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
+{
+	int len = vsnprintf(buf, size, fmt, args);
+	if (len >= 0)
+		return len;
+	/* ouch, buffer too small; need to compute the size */
+	if (size < 250)
+		size = 250;
+	do {
+		size *= 4;
+		len = try_vsnprintf(size, fmt, args);
+	} while (len < 0);
+	return len;
+}

git-compat-util.h

@@ -503,6 +503,10 @@ int git_fstat(int fd, struct stat *buf);
 #define lstat(x,y) git_lstat(x,y)
 #define stat(x,y) git_lstat(x,y)
 #define fstat(x,y) git_fstat(x,y)
+
+int mingw_vsnprintf(char *buf, size_t size, const char *fmt, va_list args);
+#define vsnprintf mingw_vsnprintf
+
 #endif /* __MINGW32__ */
 
 #endif