Using HTTP::payload with/without HTTP::collect/HTTP_REQUEST_DATA?

Citizen_elah suggested you could use 'HTTP::payload length' in HTTP_RESPONSE in this recent post (Click here). I didn't realize you could use HTTP::payload length outside of HTTP_REQUEST_DATA or HTTP_RESPONSE_DATA.

After some quick testing on 9.4.6, I'm a bit confused on whether/why you can use HTTP::payload in HTTP_REQUEST or HTTP_RESPONSE. I always thought these two events were triggered when only the HTTP headers had been parsed. But it seems like the payload is also available:

  
 when HTTP_REQUEST { 
  
  Log the original payload 
 log local0. "\[HTTP::payload\]: [HTTP::payload], \[HTTP::payload length\]:\ 
 [HTTP::payload length], \[HTTP::header Content-Length\]: [HTTP::header Content-Length]" 
  
  Replace the full payload with some text 
 HTTP::payload replace 0 [HTTP::payload length] "new data here!" 
  
  Log the updated payload 
 log local0. "500 \[HTTP::payload\]: [HTTP::payload], \[HTTP::payload length\]:\ 
 [HTTP::payload length], \[HTTP::header Content-Length\]: [HTTP::header Content-Length]" 
  
 } 
 when HTTP_REQUEST priority 501 { 
  
  Log the updated payload 
 log local0. "501 \[HTTP::payload\]: [HTTP::payload], \[HTTP::payload length\]:\ 
 [HTTP::payload length], \[HTTP::header Content-Length\]: [HTTP::header Content-Length]" 
 } 
 

For a test request this is the log output:

: [HTTP::payload]: param=value, [HTTP::payload length]: 11, [HTTP::header Content-Length]: 11

: 500 [HTTP::payload]: new data here!, [HTTP::payload length]: 14, [HTTP::header Content-Length]: 11

: 501 [HTTP::payload]: new data here!, [HTTP::payload length]: 14, [HTTP::header Content-Length]: 14

So it looks like the Content-Length header is even updated correctly (as shown in the non-cached value from HTTP_REQUEST priority 501.

Is this new? Is the full payload always available or are there size constraints or other issues with accessing/modifying the payload in HTTP_REQUEST and/or HTTP_RESPONSE?

Thanks for any info,

Aaron