add env-var to record shell init time

[nerdvegas]

I have a case where I need to know the time when a tool was launched, which is done via ResolvedContext.execute_shell (eg, when you run rez-env blah -- mycommand). I would like to write this to an env-var.

The existing REZ_USED_TIMESTAMP (https://github.com/nerdvegas/rez/wiki/Environment-Variables#context-environment-variables) is similar to what I need, but not quite right. REZ_USED_TIMESTAMP is set to --timestamp if present, rather than the true time of resolve. Furthermore, it's time the context was resolved, not the time that a command was run, which can occur later (a context can be resolved once and used many times after - eg, suites).

I propose adding a new REZ_SHELL_INIT_TIMESTAMP env-var, which is set to epoch time every time a shell is created (via execute_shell).

Related - it would also be useful to set a var that indicates whether a command was run or not (ie command arg in execute_shell). The reason - I need to determine tool startup time, and if command is None, this generally means that an interactive shell was started, and I need to filter out those cases.

[instinct-vfx]

Seems like a useful addition to me! I am a bit unsure about setting it to --timestamp if present. That seems a bit odd to return a different value depending on the parameters. As a user i would rather expect that SHELL_INIT_TIMESTAMP always reflects the shell init time and not an arbitrarily set value.

When working with the value you could check if REZ_USED_REQUESTED_TIMESTAMP is non-zero and use that or else use REZ_SHELL_INIT_TIMESTAMP rather than have REZ_SHELL_INIT_TIMESTAMP reflect two different meanings.

Thoughts?

[nerdvegas]

Ah, the ticket reads a little ambiguously. I meant that REZ_USED_TIMESTAMP respects --timestamp, not that REZ_SHELL_INIT_TIMESTAMP should.

[instinct-vfx]

Ah, sorry, i misread that. That makes sense and i take back everything i said ;)