server/netget.c, docs/net-protocol.txt: set_var(): report IMMUTABLE f…

…lag on values in DEBUG builds [networkupstools#266]

Signed-off-by: Jim Klimov <[email protected]>

docs/net-protocol.txt

@@ -156,6 +156,11 @@ exponents, and comma for thousands separator are forbidden.
 For example: "1200.20" is valid, while "1,200.20" and "1200,20" and "1.2e4"
 are invalid.
 
+Also note that internally NUT programs can flag variables as 'IMMUTABLE':
+a value of such variable can not be changed after it was initially set
+(this is used e.g. for "override" settings where the device is known to
+provide bogus readings).  The flag is not currently exposed with `GET`
+commands, but can be a reason for a `SET` to fail.
 
 This replaces the old "VARTYPE" command.
 

server/netget.c

@@ -138,6 +138,19 @@ static void get_type(nut_ctype_t *client, const char *upsname, const char *var)
 
 	snprintf(buf, sizeof(buf), "TYPE %s %s", upsname, var);
 
+	if (node->flags & ST_FLAG_IMMUTABLE) {
+#if DEBUG
+		/* Properly exposing this needs also an update to
+		 * docs/net-protocol.txt (promote the paragraph
+		 * provided as a note currently) and to the NUT RFC
+		 * https://www.rfc-editor.org/info/rfc9271
+		 */
+		snprintfcat(buf, sizeof(buf), " IMMUTABLE");
+#endif
+		upsdebugx(3, "%s: UPS[%s] variable %s is IMMUTABLE",
+			__func__, upsname, var);
+	}
+
 	if (node->flags & ST_FLAG_RW)
 		snprintfcat(buf, sizeof(buf), " RW");