aya-obj: Mutate BTF in-place without clone

[dave-tucker]

The BTF we're working on is Cow anyway so modifying in-place is fine. All we need to do is store some information before we start our mutable iteration to avoid concurrently borrowing types both mutably and immutably.

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[tamird]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @dave-tucker)

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aya-obj/src/btf/btf.rs line 531 at r1 (raw file):

                    let mut members = vec![];
                    for member in d.entries.iter() {

so i think we can avoid the need for datasec_member_name_offset here if we realize that we're about to throw away d, so we can write:

let entries = std::mem::take(d.entries);
// now we've dropped the mutable borrow of types.types, i hope
//
// populate members as before
types.types[i] = ...

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aya-obj/src/btf/btf.rs line 584 at r1 (raw file):

                        for e in &mut d.entries.iter_mut() {
                            let (name_offset, linkage) =
                                var_types.get(&e.btf_type).ok_or(BtfError::InvalidDatasec)?;

i think we can do similar tricks here; hollow out d instead of cloning it, which drops the mutable reference, and then write a new one. that avoids var_types which is also allocates.

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aya-obj/src/btf/btf.rs line 1256 at r1 (raw file):

        )));

        let name_offset = btf.add_string("foo");

should we extract local for this "foo", or perhaps assert that name_offset1 is equal to this name_offset below?

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aya-obj/src/btf/types.rs line 868 at r1 (raw file):

#[repr(C)]
#[derive(Clone, Debug)]

do we want to remove these clone impls?

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aya-obj/src/btf/types.rs line 877 at r1 (raw file):

impl DataSec {
    pub fn name_offset(&self) -> u32 {

why are these getters needed? the fields are pub(crate).

[dave-tucker]

Reviewable status: 0 of 3 files reviewed, 5 unresolved discussions (waiting on @tamird)

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aya-obj/src/btf/btf.rs line 531 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

so i think we can avoid the need for datasec_member_name_offset here if we realize that we're about to throw away d, so we can write:

let entries = std::mem::take(d.entries);
// now we've dropped the mutable borrow of types.types, i hope
//
// populate members as before
types.types[i] = ...

Done.

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aya-obj/src/btf/btf.rs line 584 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

i think we can do similar tricks here; hollow out d instead of cloning it, which drops the mutable reference, and then write a new one. that avoids var_types which is also allocates.

Done.

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aya-obj/src/btf/btf.rs line 1256 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

should we extract local for this "foo", or perhaps assert that name_offset1 is equal to this name_offset below?

Done.

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aya-obj/src/btf/types.rs line 868 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

do we want to remove these clone impls?

We can't because Btf is Clone on write. So all everything contained within also needs to be clone since we derive the clone impl.

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aya-obj/src/btf/types.rs line 877 at r1 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

why are these getters needed? the fields are pub(crate).

Done.

[tamird]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @dave-tucker)

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aya-obj/src/btf/btf.rs line 511 at r2 (raw file):

                    let members = entries
                        .iter()
                        .map(|e| {

Can "e" be "member"? Reduces the diff.

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aya-obj/src/btf/btf.rs line 562 at r2 (raw file):

                        // we can query by name in symbol_offsets
                        let mut entries = mem::take(&mut d.entries);
                        let mut fixed_section = d.clone();

Is this needed because there's a private field?

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aya-obj/src/btf/btf.rs line 564 at r2 (raw file):

                        let mut fixed_section = d.clone();

                        for e in entries.iter_mut() {

s/e/d/ to match old name and reduce diff

[dave-tucker]

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @tamird)

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aya-obj/src/btf/btf.rs line 562 at r2 (raw file):

Previously, tamird (Tamir Duberstein) wrote…

Is this needed because there's a private field?

Yes. But at least we've pulled out the Vec now.