Add use of asprintf()

Add asprintf(), pg_asprintf(), and psprintf() to simplify string
allocation and composition.  Replacement implementations taken from
NetBSD.

Reviewed-by: Álvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Asif Naeem <anaeem.it@gmail.com>

src/backend/utils/mmgr/mcxt.c

@@ -852,3 +852,52 @@ pnstrdup(const char *in, Size len)
 	out[len] = '\0';
 	return out;
 }
+
+/*
+ * asprintf()-like functions around palloc, adapted from
+ * http://ftp.netbsd.org/pub/pkgsrc/current/pkgsrc/pkgtools/libnbcompat/files/asprintf.c
+ */
+
+char *
+psprintf(const char *format, ...)
+{
+	va_list ap;
+	char   *retval;
+
+	va_start(ap, format);
+	retval = pvsprintf(format, ap);
+	va_end(ap);
+
+	return retval;
+}
+
+char *
+pvsprintf(const char *format, va_list ap)
+{
+	char *buf, *new_buf;
+	size_t len;
+	int retval;
+	va_list ap2;
+
+	len = 128;
+	buf = palloc(len);
+
+	va_copy(ap2, ap);
+	retval = vsnprintf(buf, len, format, ap);
+	Assert(retval >= 0);
+
+	if (retval < len)
+	{
+		new_buf = repalloc(buf, retval + 1);
+		va_end(ap2);
+		return new_buf;
+	}
+
+	len = (size_t)retval + 1;
+	pfree(buf);
+	buf = palloc(len);
+	retval = vsnprintf(buf, len, format, ap2);
+	va_end(ap2);
+	Assert(retval == len - 1);
+	return buf;
+}