The patch looks good to me. Thanks for improving comments around that code. I like the change to set waiting_for_ping_response in WalSndKeepalive. Thanks.
Ashutosh Bapat noticed that WalSndWaitForWal() is setting waiting_for_ping_response after sending a keepalive that does *not* request a reply. The bad consequence is that other callers that do require a reply end up in not sending a keepalive, because they think it was already sent previously. So the whole thing gets stuck.
He found that commit 41d5f8ad734 failed to remove the setting of waiting_for_ping_response after changing the "request" parameter WalSndKeepalive from true to false; that seems to have been an omission and it breaks the algorithm. Thread at [1].
The simplest fix is just to remove the line that sets waiting_for_ping_response, but I think it is less error-prone to have WalSndKeepalive set the flag itself, instead of expecting its callers to do it (and know when to). Patch attached. Also rewords some related commentary.