fix(lsp): fix unnecessary buffers being added on empty diagnostics (#18275)

Some language servers send empty `textDocument/publishDiagnostics`
messages after indexing the project with URIs corresponding to unopened buffers.
This commit guards against opening buffers corresponding to empty diagnostics.

runtime/lua/vim/lsp/diagnostic.lua

@@ -185,7 +185,12 @@ end
 function M.on_publish_diagnostics(_, result, ctx, config)
   local client_id = ctx.client_id
   local uri = result.uri
-  local bufnr = vim.uri_to_bufnr(uri)
+  local fname = vim.uri_to_fname(uri)
+  local diagnostics = result.diagnostics
+  if #diagnostics == 0 and vim.fn.bufexists(fname) == 0 then
+    return
+  end
+  local bufnr = vim.fn.bufadd(fname)
 
   if not bufnr then
     return
@@ -193,7 +198,6 @@ function M.on_publish_diagnostics(_, result, ctx, config)
 
   client_id = get_client_id(client_id)
   local namespace = M.get_namespace(client_id)
-  local diagnostics = result.diagnostics
 
   if config then
     for _, opt in pairs(config) do

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -3,6 +3,7 @@ local helpers = require('test.functional.helpers')(after_each)
 local clear = helpers.clear
 local exec_lua = helpers.exec_lua
 local eq = helpers.eq
+local neq = require('test.helpers').neq
 
 describe('vim.lsp.diagnostic', function()
   local fake_uri
@@ -227,5 +228,43 @@ describe('vim.lsp.diagnostic', function()
       eq(exec_lua([[return vim.str_byteindex(..., 7, true)]], line), result[1].col)
       eq(exec_lua([[return vim.str_byteindex(..., 8, true)]], line), result[1].end_col)
     end)
+
+    it('does not create buffer on empty diagnostics', function()
+      local bufnr
+
+      -- No buffer is created without diagnostics
+      bufnr = exec_lua [[
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = "file:///fake/uri2",
+          diagnostics = {},
+        }, {client_id=client_id})
+        return vim.fn.bufnr(vim.uri_to_fname("file:///fake/uri2"))
+      ]]
+      eq(bufnr, -1)
+
+      -- Create buffer on diagnostics
+      bufnr = exec_lua [[
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = "file:///fake/uri2",
+          diagnostics = {
+            make_error('Diagnostic', 0, 0, 0, 0),
+          },
+        }, {client_id=client_id})
+        return vim.fn.bufnr(vim.uri_to_fname("file:///fake/uri2"))
+      ]]
+      neq(bufnr, -1)
+      eq(exec_lua([[return #vim.diagnostic.get(...)]], bufnr), 1)
+
+      -- Clear diagnostics after buffer was created
+      bufnr = exec_lua [[
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = "file:///fake/uri2",
+          diagnostics = {},
+        }, {client_id=client_id})
+        return vim.fn.bufnr(vim.uri_to_fname("file:///fake/uri2"))
+      ]]
+      neq(bufnr, -1)
+      eq(exec_lua([[return #vim.diagnostic.get(...)]], bufnr), 0)
+    end)
   end)
 end)