Up until 19 century, the main foreign threats to China came from north.
There they have built the great wall to stop nomadic warriors tribes like xiongnu,huns,mongols,manchu from raiding China.
China in the north can be threaten by chaotic warrior tribes, or by a powerful empire like mongols,czars or Soviet Union.
Putin's Russia is neither of this, it has internal order but lacks power to conquer China.
If Russia breaks, the eastern provinces can come under American influence due to cultural ad physical proximity.
China doesn't want an American foothold near its border, that's why it supports North Korea and it will support the future North Korea: Russia.
In short Putin's Russia is a buffer from global western influence.
The real threat for China comes from south, those countries had clashes in the past with China, and China sequestrations of water from Tibet's rivers doesn't help.
They are densely populated countries with physical barriers such as Himalaya and jungle in SE Asia so a "special military operation" is not feasible if those countries get help from US or Australia.
A bellicose China might move then in West's side.
