How to implement guard logic for buffered proxying and body existence in serverless plugins

When working with request and response bodies, buffered proxying needs to be enabled and the body needs to exist to perform operations.

Without guard logic in place, empty request / response bodies and incorrect buffering settings can cause the serverless plugin to throw errors in the Kong logs when the body is nil or buffering is disabled.

Guard logic can be put in place to protect any operations that expect a body to be present.

This is an example of how to configure a pre-function plugin to check for whether buffering is enabled, and if so serialize the request and response bodies to a log plugin output.

1. Create access-phase.lua and add the following contents:

   kong.log.warn("Access Start")
   kong.log.warn("Checking buffering")
   
   if ngx.ctx.buffered_proxying then
       kong.log.warn("Buffering already enabled.")
   else
       kong.log.warn("Enabling buffering...")
       kong.service.request.enable_buffering()
   end
   
   if not ngx.ctx.buffered_proxying then
       kong.log.warn("Buffering was not enabled.")
   else
       kong.log.warn("Buffering check passed, retrieving request body")
       local raw_body = kong.request.get_raw_body()
   
       if raw_body then
           kong.log.warn("Valid request body found, serialising to log")
           kong.log.set_serialize_value("request.body", raw_body)
       else
           kong.log.warn("No body to serialise")
       end
   end
   
   kong.log.warn("Access End")

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2. Create log-phase.lua and add the following contents:

   kong.log.warn("Log Start")
   
   kong.log.warn("Checking buffering")
   
   if not ngx.ctx.buffered_proxying then
       kong.log.warn("Buffering is disabled, cannot log the response body.")
   else
       kong.log.warn("Buffering check passed, retrieving response body")
       local raw_response_body = kong.service.response.get_raw_body()
   
       if raw_response_body then
           kong.log.warn("Valid response body found, serialising to log")
           kong.log.set_serialize_value("response.body", raw_response_body)
       else
           kong.log.warn("No body to serialise")
       end
   end

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3. Add the pre-function plugin to a route/service using the files as inputs:

   curl -X POST http://localhost:8001/routes/<route_name>/plugins \
   -F "name=pre-function"  \
   -F "config.access=@./access-phase.lua" \
   -F "config.log=@./log-phase.lua" \
   -H "Kong-Admin-Token:<RBAC Token>"

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The Kong error log will show the various log messages as they are executed.