refactor(lsp): avoid using coroutine when parsing frames

runtime/lua/vim/lsp/rpc.lua

@@ -25,16 +25,18 @@ end
 --- We ignore lines ending with `\n` that don't contain `content-length`, since some servers
 --- write log to standard output and there's no way to avoid it.
 --- See https://github.com/neovim/neovim/pull/35743#pullrequestreview-3379705828
---- @param header string The header to parse
+--- @param ptr vim._core.stringbuffer.ptr The ptr to buffer to parse
+--- @param start integer The starting index of the buffer to parse, 0-based
+--- @param len integer The length of the header to parse
 --- @return integer
-local function get_content_length(header)
+local function get_content_length(ptr, start, len)
   local state = 'name'
-  local i, len = 1, #header
+  local i, end_ = start, start + len
   local j, name = 1, 'content-length'
   local buf = strbuffer.new()
   local digit = true
-  while i <= len do
-    local c = header:byte(i)
+  while i < end_ do
+    local c = ptr[i]
     if state == 'name' then
       if c >= 65 and c <= 90 then -- lower case
         c = c + 32
@@ -54,7 +56,7 @@ local function get_content_length(header)
         i = i - 1
       end
     elseif state == 'value' then
-      if c == 13 and header:byte(i + 1) == 10 then -- must end with \r\n
+      if c == 13 and ptr[i + 1] == 10 then -- must end with \r\n
         local value = buf:get()
         if digit then
           return vim._assert_integer(value)
@@ -73,7 +75,11 @@ local function get_content_length(header)
     end
     i = i + 1
   end
-  error('Content-Length not found in header: ' .. header)
+  local header = strbuffer.new()
+  for k = start, end_ - 1 do
+    header:put(string.char(ptr[k]))
+  end
+  error('Content-Length not found in header: ' .. header:tostring())
 end
 
 local M = {}
@@ -190,33 +196,36 @@ local default_dispatchers = {
   end,
 }
 
---- @async
-local function message_decoder()
-  local strbuf = strbuffer.new()
-  while true do
-    local header_len ---@type integer?
-    local ptr, len = strbuf:ref()
-    for i = 0, len - 4 do
-      -- Find the header boundary "\r\n\r\n"
-      -- (compare bytes instead of string.find(), to avoid a string alloc).
-      if ptr[i] == 13 and ptr[i + 1] == 10 and ptr[i + 2] == 13 and ptr[i + 3] == 10 then
-        header_len = i + 2
-        break
-      end
-    end
-    if header_len then
-      local header = strbuf:get(header_len)
-      strbuf:skip(2) -- skip past header boundary
-      local content_length = get_content_length(header)
-      while strbuffer.len(strbuf) < content_length do
-        strbuf:put(coroutine.yield())
-      end
-      local body = strbuf:get(content_length)
-      strbuf:put(coroutine.yield(body))
-    else
-      strbuf:put(coroutine.yield())
+--- Parse one `Content-Length` framed message from `strbuf`.
+---
+--- Returns a body after consuming one full frame, returns nil if more bytes are needed.
+--- Raises an error if the buffered data is not a valid frame.
+---
+---@param strbuf vim._core.stringbuffer
+---@return string?
+local function message_decoder(strbuf)
+  local header_len ---@type integer?
+  local ptr, len = strbuf:ref()
+  for i = 0, len - 4 do
+    -- Find the header boundary "\r\n\r\n"
+    -- (compare bytes instead of string.find(), to avoid a string alloc).
+    if ptr[i] == 13 and ptr[i + 1] == 10 and ptr[i + 2] == 13 and ptr[i + 3] == 10 then
+      header_len = i + 2
+      break
     end
   end
+
+  if not header_len then
+    return nil
+  end
+
+  local content_length = get_content_length(ptr, 0, header_len)
+  if strbuffer.len(strbuf) < header_len + 2 + content_length then
+    return nil
+  end
+
+  strbuf:skip(header_len + 2) -- skip past header boundary
+  return strbuf:get(content_length)
 end
 
 --- @private