Add cflags support

[javierhonduco]

This PR adds support for passing a vector of cflags that will be passed to the bcc backend and processed by clang's preprocessor

Template

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

The BCC Python and C++ bindings allow passing CFLAGS to the BPF program. We are using this feature in a project with a Python BCC driver and are currently experimenting with Rust so we would love to be able to use it :smile:

• what changes does this pull request make?

Moves bpf_module_create_c_from_string to its own function, leaving new with the same signature, and adds new_with_cflags, to which you can pass CFLAGS has a second argument

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

It should not break backwards compatibility. I am a not experienced with Rust (first Rust PR!), so I am unsure if there is a better way to do this!

Test plan

Added a simple smoke test

➜  rust-bcc git:(cflags-support) ✗ cargo build --examples
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
➜  rust-bcc git:(cflags-support) ✗ sudo target/debug/examples/smoketest 
smoketest: empty table
smoketest: sized histogram
smoketest: hash insert and delete
smoketest: array
smoketest: cflags
smoketest passed

Thanks!

[javierhonduco]

cc @rdelfin In case you have any feedback!

[brayniac]

Thanks for opening this! Seems like a good feature to add.

I'd rather see this implemented using the builder pattern instead of adding a dedicated function to create w/ CFLAGs. This will give us more flexibility going forward and addresses the concern you had around adding more args in the future.

I'll see if I can take this on.

[brayniac]

@javierhonduco - can you see if the version in master since #128 landed meets your current needs?

[javierhonduco]

Sorry for the delay, just came from holidays :)

I'd rather see this implemented using the builder pattern instead of adding a dedicated function to create w/ CFLAGs. This will give us more flexibility going forward and addresses the concern you had around adding more args in the future.

Totally, this is way better! Just left some comments in #128! :)