Fix example for form handling and file upload in Lua (example for #70)

examples/handle_form.lua

@@ -0,0 +1,74 @@
+mg.write("HTTP/1.0 200 OK\r\n")
+mg.write("Connection: close\r\n")
+mg.write("Content-Type: text/plain; charset=utf-8\r\n")
+mg.write("Cache-Control: max-age=0, must-revalidate\r\n")
+mg.write("\r\n")
+
+-- Read the entire body data (POST content) into "bdata" variable.
+-- Use a string builder pattern for performance reasons
+stringtab = {}
+bdata = ""
+repeat
+  local add_data = mg.read()
+  if add_data then
+    stringtab[#stringtab+1] = add_data
+  end
+until (add_data == nil);
+bdata = table.concat(stringtab)
+stringtab = nil
+
+-- Get the boundary string.
+bs = "--" .. ((mg.request_info.content_type):upper():match("BOUNDARY=(.*)"));
+
+-- The POST data has to start with the boundary string.
+-- Check this and remove the starting boundary.
+if bdata:sub(1, #bs) ~= bs then
+  error "invalid format of POST data"
+end
+bdata = bdata:sub(#bs)
+
+-- The boundary now starts with CR LF.
+bs = "\r\n" .. bs
+
+-- Now loop through all the parts
+while #bdata>4 do
+   -- Find the header of new part.
+   part_header_end = bdata:find("\r\n\r\n", 1, true)
+
+   -- Parse the header.
+   local form_field_name, file_name
+   h = bdata:sub(1, part_header_end+2)
+   for key,val in h:gmatch("([^%:\r\n]*)%s*%:%s*([^\r\n]*)\r\n") do
+      if key:upper() == "CONTENT-DISPOSITION" then
+          form_field_name = val:match('name=%"([^%"]*)%"')
+          file_name = val:match('filename=%"([^%"]*)%"')
+      end
+   end
+
+   -- Remove the header from "bdata".
+   bdata = bdata:sub(part_header_end+4)
+
+   -- Find the end of the body by locating the boundary string.
+   local part_body_end = bdata:find(bs, 1, true)
+
+   -- Isolate the content, and drop it from "bdata".
+   local form_field_value = bdata:sub(1,part_body_end-1)
+   bdata = bdata:sub(part_body_end+#bs)
+
+   -- Now the data (file content or field value) is isolated: We know form_field_name and form_field_value.
+   -- Here the script should do something useful with the data. This example just sends it back to the client.
+   if form_field_name then
+     mg.write("Field name: " .. form_field_name .. "\r\n")
+   end
+   if file_name then
+     mg.write("File name: " .. file_name .. "\r\n")
+   end
+   local len = #form_field_value
+   if len<50 then
+     mg.write("Field value: " .. form_field_value .. "\r\n")
+   else
+     mg.write("Field value: " .. form_field_value:sub(1, 40) .. " .. (" .. len .. " bytes)\r\n")
+   end
+   mg.write("\r\n")
+
+end