ruby: Raise instead of hanging if grpc is used before and after fork

[dylanahsmith]

Fixes #7951

Problem

If grpc core is initialized before forking, then used after forking then it can cause the process to hang or crash. Since grpc core doesn't support using grpc before and after forking, we should make this unsupported usage clear so that it is easy to debug this invalid usage.

Solution

Store the pid for the process that initializes grpc, then check it before creating a new channel or using it in a way that could hang and raise. If the pid changes, then these channel methods that would hang will raise a runtime exception.

These checks aren't needed on windows where it doesn't support forking, so they are stubbed out using conditional compilation.

Regression tests are also included that hang when run without the changes to the code under test.

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[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,950,903      Total (=)      1,950,903

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,661,275      Total (>)     10,661,268

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,950,903      Total (=)      1,950,903

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,661,266      Total (<)     10,661,267

 No significant differences in binary sizes

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,950,903      Total (=)      1,950,903

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,661,269      Total (<)     10,661,273

 No significant differences in binary sizes

[apolcyn]

Overall I think this looks good. One nit comment so far and I'll come back for another look.

One hesitation is that this is a sort of "best effort" debugging aid, and so it's possible that gRPC can still be "used" without running in to this fork guard - but I think the tradeoff there is ok.

[dylanahsmith]

Yes, it is possible that grpc can be used without running into the fork guard, but not every use is resulting in ruby hanging, so I focused on adding the guard to methods that would hang when used after forking with grpc initialized before forking.

I've added a couple more fork guards to handle server usage and making rpc calls using the return_op: true option.

Raising an exception instead of crashing is meant to be a debugging aid, but it wasn't meant to be "best effort" in preventing the issue. Is there any specific usage in mind that you think should be protected by a fork guard?

[apolcyn]

Looks good, can we please squash down these commits into one?

[apolcyn]

nit, can we use braces around the body of this if?

[dylanahsmith]

commits have been squashed

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,961,059      Total (=)      1,961,059

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,883,613      Total (>)     10,883,608

 No significant differences in binary sizes