The Unix convention is for a pid file to contain the process id of the process which created the file. This is described in http://www.pathname.com/fhs/2.2/fhs-5.13.html which I found via https://stackoverflow.com/questions/688343/reference-for-proper-handling-of-pid-file-on-unix.

mongocryptd produces a file called mongocryptd.pid with the following contents:

{ "port" : 27020, "pid" : 14620 }                                                                             

This contents is contrary to the expected contents of the pid file.

For example, using the common Unix idiom of

kill `cat *pid`

produces:

kill: illegal pid: {
kill: illegal pid: "port"
kill: illegal pid: :
kill: illegal pid: 27020,
kill: illegal pid: "pid"
kill: illegal pid: :
kill: illegal pid: }

Tested on 4.2 server built from source recently.