fix(lsp): blank line before Content-Length breaks header parsing #41524

Problem:
A blank line before the Content-Length header makes header parsing
fail with "Content-Length not found in header". An LF in the 'name'
state falls through to the 'invalid' state, which only return to
'name' at the *next* LF. That LF terminates the next line, so the
line with Content-Length is swallowed.
The same issue happens when a junk line is a partial match of the
header name (e.g. "Cont\nContent-Length: ...").

Solution:
When seeing an LF in the 'name' state, reset the cursor and stay in
'name' rather than entering 'invalid'.

runtime/lua/vim/lsp/rpc.lua

@@ -41,7 +41,9 @@ local function get_content_length(ptr, start, len)
       if c >= 65 and c <= 90 then -- lower case
         c = c + 32
       end
-      if (c == 32 or c == 9) and j == 1 then -- luacheck: ignore 542
+      if c == 10 then -- a blank line or a line with a prefix of header
+        j = 1
+      elseif (c == 32 or c == 9) and j == 1 then -- luacheck: ignore 542
         -- skip OWS for compatibility only
       elseif c == name:byte(j) then
         j = j + 1

test/functional/plugin/lsp_spec.lua

@@ -2052,6 +2052,28 @@ describe('LSP', function()
       end)
     end)
 
+    it('ignores blank lines before content-length', function()
+      exec_lua(function()
+        _G._send_msg_to_server(
+          'Info: Parsed 35 declarations\n\nContent-Length: ' .. #body .. ' \r\n\r\n' .. body
+        )
+      end)
+      verify_single_notification(function(method, args) ---@param args [string]
+        eq('body', method)
+        eq(body, args[1])
+      end)
+    end)
+
+    it('skips partial match of "content-length"', function()
+      exec_lua(function()
+        _G._send_msg_to_server('Cont\nContent-Length: ' .. #body .. ' \r\n\r\n' .. body)
+      end)
+      verify_single_notification(function(method, args) ---@param args [string]
+        eq('body', method)
+        eq(body, args[1])
+      end)
+    end)
+
     it('should not trim vim.NIL from the end of a list', function()
       local expected_handlers = {
         { NIL, {}, { method = 'shutdown', client_id = 1 } },