bpf2go: add flag for alternative filename stem

[MarcusWichelmann]

In our project, the eBPF source files are named with underscores, like ext_vx_decap.c. We want to have the .go and .o files generated by bpf2go to follow the same naming scheme (e.g. ext_vx_decap_bpfel.go). But currently, when setting the <ident> parameter to ext_vx_decap, the result will be some broken Go type names like type ext_vx_decapPrograms

So instead, this PR adds a new -filename-stem flag, which allows specifying an alternative filename prefix while one can keep the ident as it is supposed to.
The -filename-stem flag is optional and if it's not set, the ident will be used instead. So this is not a breaking change.

Sample usage:

bpf2go [...] -filename-stem ext_vx_decap ExtVxDecap src/ext_vx_decap.c

[lmb]

To clarify, you want your Go types to be ExtVxDecap... but the output to be called ext_vx_decap_bpfel.go?

[MarcusWichelmann]

Exactly.

[lmb]

I think the flag is probably fine. I had another idea though: massage ident so that uppercase letters are turned into underscore followed by the lowercase letter. That would transform ExtFooBar into ext_foo_bar. What do you think?

[lmb]

Should we really lowercase an explicitly given stem?

[lmb]

Nit: Please call this -output-stem and call the new field outputStem, so it matches outputDir.

[lmb]

There should be a check to make sure b2g.outputStem doesn't contain directory separators. Said another way -output-stem ../foo should be rejected. A test for this would also be nice.

Something like filepath.Clean(outputStem) != outputStem or similar.

[MarcusWichelmann]

Thanks for the review!
An automatic transformation of the ident would also be fine, but will be hard to get right in cases like uppercase abbreviations as often seen (per convention) in Go type names. e.g. ExtIPV4Firewall -> ext_ipv4_firwall.
Of course, this is also algorithmically solvable, but there might still be cases where the algorithm will get it wrong and then the developer has no way to fix it manually. e.g. SFlowSomething -> sflow_something (not s_flow_something)
So a flag does probably provide better control.

But I'll work on your suggestions and update this PR.

[lmb]

I had a look around, and there are at least a couple open source cases that would break: https://sourcegraph.com/search?q=context:global+go:generate+.%2B%3F+bpf2go&patternType=regexp Let's stick to the flag.

[MarcusWichelmann]

Have done the requested changes now. I have squashed my changes, because the new commit changes everything from the previous commit.

I'm not sure, how you intended the usage of filepath.Clean, because that won't prevent one from using -output-stem "dir/some_stem". Instead I rely on the absence of path separation chars (/).

[MarcusWichelmann]

I rebased this branch onto current master.

How do you think about this PR, can this be merged?

[lmb]

Sorry for this, the PR fell through the cracks. I missed your last message somehow. Thank you for your contribution!