perf: Cache plain link validation for 30 minutes

Very often you're picking same documents again and again, there's no
need to validate them.

Also, document is JUST selected using search_link, so it's 99%
guaranteed to be valid.

The real purpose of this function is to provide "fetch from" feature,
not link validation like the name suggests.

It will get validated server side anyway.

frappe/client.py

@@ -427,7 +427,11 @@ def validate_link(doctype: str, docname: str, fields=None):
 	values.name = frappe.db.get_value(doctype, docname, cache=True)
 
 	fields = frappe.parse_json(fields)
-	if not values.name or not fields:
+	if not values.name:
+		return values
+
+	if not fields:
+		frappe.local.response_headers.set("Cache-Control", "private,max-age=1800,stale-while-revalidate=7200")
 		return values
 
 	try:

frappe/public/js/frappe/form/controls/link.js

@@ -685,11 +685,16 @@ frappe.ui.form.ControlLink = class ControlLink extends frappe.ui.form.ControlDat
 		// to avoid unnecessary request
 		if (value) {
 			return frappe
-				.xcall("frappe.client.validate_link", {
-					doctype: options,
-					docname: value,
-					fields: columns_to_fetch,
-				})
+				.xcall(
+					"frappe.client.validate_link",
+					{
+						doctype: options,
+						docname: value,
+						fields: columns_to_fetch,
+					},
+					"GET",
+					{ cache: !columns_to_fetch.length }
+				)
 				.then((response) => {
 					if (!this.docname || !columns_to_fetch.length) {
 						return response.name;