#946 use a better way

Clear cache when `didOpen`. This avoids clearing the cache according to the situation in the diagnostic business.

script/provider/diagnostic.lua

@@ -131,9 +131,10 @@ local function mergeDiags(a, b, c)
     return t
 end
 
+-- enable `push`, disable `clear`
 function m.clear(uri)
     await.close('diag:' .. uri)
-    if not m.cache[uri] then
+    if m.cache[uri] == nil then
         return
     end
     m.cache[uri] = nil
@@ -144,6 +145,7 @@ function m.clear(uri)
     log.info('clearDiagnostics', uri)
 end
 
+-- enable `push` and `send`
 function m.clearCache(uri)
     m.cache[uri] = false
 end
@@ -256,11 +258,6 @@ function m.doDiagnostic(uri, isScopeDiag)
         end
     end
 
-    -- always re-sent diagnostics of current file
-    if not isScopeDiag then
-        m.cache[uri] = nil
-    end
-
     pushResult()
 
     local lastPushClock = time.time()
@@ -435,6 +432,7 @@ files.watch(function (ev, uri) ---@async
         m.refresh(uri)
     elseif ev == 'open' then
         if ws.isReady(uri) then
+            m.clearCache(uri)
             xpcall(m.doDiagnostic, log.error, uri)
         end
     elseif ev == 'close' then