Add support to trace defer function calls under trace follow option

[archanaravindar]

Is an extension of the basic support added to mention depth upto which functions and their descendants need to be traced #3594
The same option can now trace defer functions and their descendants

Fixes #3743

[aarzilli]

I don't really get what this PR is trying to achieve. Right now the trace with depth thing is determining statically from the call graph which functions to trace. This PR seems to change that so that deferred functions are added, at runtime, to the traced functions. Why are we doing this and why only deferred functions and not any other kind of dynamic dispatch?

[archanaravindar]

Hi @aarzilli Thank you for reviewing the patch in detail and also suggesting improvements to optimize the code
This patch adds support for functions called from within defer {..} that cant be determined statically unlike other functions so that their execution can be traced more accurately. Defers are handled in this patch specifically because they are an explicit feature of Go and one of the most common dynamic dispatch types used within a Go program

[aarzilli]

All new code should be gofmt'd.

[aarzilli]

Defers are handled in this patch specifically because they are an explicit feature of Go and one of the most common dynamic dispatch types used within a Go program

Defers are an explicit feature of Go just like function variables and interfaces, I'm not even sure that they really are the most common type of dynamic dispatch compared to the others (or by how much if they are), but even if they were I'm not sure it's enough to single them out. Or even how you would explain that to users, i.e. this changes the behavior of follow-calls without updating its documentation, but how would you update it "defer calls are followed regardless of what they call but other other types of dynamic calls aren't"?

[archanaravindar]

Defers are handled in this patch specifically because they are an explicit feature of Go and one of the most common dynamic dispatch types used within a Go program

Defers are an explicit feature of Go just like function variables and interfaces, I'm not even sure that they really are the most common type of dynamic dispatch compared to the others (or by how much if they are), but even if they were I'm not sure it's enough to single them out. Or even how you would explain that to users, i.e. this changes the behavior of follow-calls without updating its documentation, but how would you update it "defer calls are followed regardless of what they call but other other types of dynamic calls aren't"?

Thank you for the detailed comment @aarzilli, it appears that with minimal modifications the code can trace functions that are dynamically passed as procedure parameters or returned from functions. Made changes to documentation and added couple of test cases for the same, kindly let us know your thoughts on this, Thanks again for your time on this!