perf(lsp): use string.buffer for rpc loop

Avoids some table allocations.
In a quick test over 50000 iterations it reduces the time from 130ms to
74 ms

For the test setup details see:

https://github.com/mfussenegger/nvim-dap/pull/1394#issue-2725352391

runtime/lua/vim/lsp/rpc.lua

@@ -25,8 +25,8 @@ local function get_content_length(header)
     if line == '' then
       break
     end
-    local key, value = line:match('^%s*(%S+)%s*:%s*(.+)%s*$')
-    if key:lower() == 'content-length' then
+    local key, value = line:match('^%s*(%S+)%s*:%s*(%d+)%s*$')
+    if key and key:lower() == 'content-length' then
       return tonumber(value)
     end
   end
@@ -39,66 +39,96 @@ local header_start_pattern = ('content'):gsub('%w', function(c)
   return '[' .. c .. c:upper() .. ']'
 end)
 
+local has_strbuffer, strbuffer = pcall(require, 'string.buffer')
+
 --- The actual workhorse.
-local function request_parser_loop()
-  local buffer = '' -- only for header part
-  while true do
-    -- A message can only be complete if it has a double CRLF and also the full
-    -- payload, so first let's check for the CRLFs
-    local header_end, body_start = buffer:find('\r\n\r\n', 1, true)
-    -- Start parsing the headers
-    if header_end then
-      -- This is a workaround for servers sending initial garbage before
-      -- sending headers, such as if a bash script sends stdout. It assumes
-      -- that we know all of the headers ahead of time. At this moment, the
-      -- only valid headers start with "Content-*", so that's the thing we will
-      -- be searching for.
-      -- TODO(ashkan) I'd like to remove this, but it seems permanent :(
-      local buffer_start = buffer:find(header_start_pattern)
-      if not buffer_start then
-        error(
-          string.format(
-            "Headers were expected, a different response was received. The server response was '%s'.",
-            buffer
-          )
-        )
-      end
-      local header = buffer:sub(buffer_start, header_end + 1)
-      local content_length = get_content_length(header)
-      -- Use table instead of just string to buffer the message. It prevents
-      -- a ton of strings allocating.
-      -- ref. http://www.lua.org/pil/11.6.html
-      ---@type string[]
-      local body_chunks = { buffer:sub(body_start + 1) }
-      local body_length = #body_chunks[1]
-      -- Keep waiting for data until we have enough.
-      while body_length < content_length do
-        ---@type string
+---@type function
+local request_parser_loop
+
+if has_strbuffer then
+  request_parser_loop = function()
+    local buf = strbuffer.new()
+    while true do
+      local msg = buf:tostring()
+      local header_end = msg:find('\r\n\r\n', 1, true)
+      if header_end then
+        local header = buf:get(header_end + 1)
+        buf:skip(2) -- skip past header boundary
+        local content_length = get_content_length(header)
+        while #buf < content_length do
+          local chunk = coroutine.yield()
+          buf:put(chunk)
+        end
+        local body = buf:get(content_length)
+        local chunk = coroutine.yield(body)
+        buf:put(chunk)
+      else
         local chunk = coroutine.yield()
-          or error('Expected more data for the body. The server may have died.') -- TODO hmm.
-        table.insert(body_chunks, chunk)
-        body_length = body_length + #chunk
+        buf:put(chunk)
       end
-      local last_chunk = body_chunks[#body_chunks]
+    end
+  end
+else
+  request_parser_loop = function()
+    local buffer = '' -- only for header part
+    while true do
+      -- A message can only be complete if it has a double CRLF and also the full
+      -- payload, so first let's check for the CRLFs
+      local header_end, body_start = buffer:find('\r\n\r\n', 1, true)
+      -- Start parsing the headers
+      if header_end then
+        -- This is a workaround for servers sending initial garbage before
+        -- sending headers, such as if a bash script sends stdout. It assumes
+        -- that we know all of the headers ahead of time. At this moment, the
+        -- only valid headers start with "Content-*", so that's the thing we will
+        -- be searching for.
+        -- TODO(ashkan) I'd like to remove this, but it seems permanent :(
+        local buffer_start = buffer:find(header_start_pattern)
+        if not buffer_start then
+          error(
+            string.format(
+              "Headers were expected, a different response was received. The server response was '%s'.",
+              buffer
+            )
+          )
+        end
+        local header = buffer:sub(buffer_start, header_end + 1)
+        local content_length = get_content_length(header)
+        -- Use table instead of just string to buffer the message. It prevents
+        -- a ton of strings allocating.
+        -- ref. http://www.lua.org/pil/11.6.html
+        ---@type string[]
+        local body_chunks = { buffer:sub(body_start + 1) }
+        local body_length = #body_chunks[1]
+        -- Keep waiting for data until we have enough.
+        while body_length < content_length do
+          ---@type string
+          local chunk = coroutine.yield()
+            or error('Expected more data for the body. The server may have died.') -- TODO hmm.
+          table.insert(body_chunks, chunk)
+          body_length = body_length + #chunk
+        end
+        local last_chunk = body_chunks[#body_chunks]
 
-      body_chunks[#body_chunks] = last_chunk:sub(1, content_length - body_length - 1)
-      local rest = ''
-      if body_length > content_length then
-        rest = last_chunk:sub(content_length - body_length)
+        body_chunks[#body_chunks] = last_chunk:sub(1, content_length - body_length - 1)
+        local rest = ''
+        if body_length > content_length then
+          rest = last_chunk:sub(content_length - body_length)
+        end
+        local body = table.concat(body_chunks)
+        -- Yield our data.
+
+        --- @type string
+        local data = coroutine.yield(body)
+          or error('Expected more data for the body. The server may have died.')
+        buffer = rest .. data
+      else
+        -- Get more data since we don't have enough.
+        --- @type string
+        local data = coroutine.yield()
+          or error('Expected more data for the header. The server may have died.')
+        buffer = buffer .. data
       end
-      local body = table.concat(body_chunks)
-      -- Yield our data.
-
-      --- @type string
-      local data = coroutine.yield(body)
-        or error('Expected more data for the body. The server may have died.')
-      buffer = rest .. data
-    else
-      -- Get more data since we don't have enough.
-      --- @type string
-      local data = coroutine.yield()
-        or error('Expected more data for the header. The server may have died.')
-      buffer = buffer .. data
     end
   end
 end