fix(wasm): do not call attach() on re-entrancy

[flrgh]

Summary

kong.runloop.wasm.attach() may be called more than once when filters utilize the dispatch_http_call proxy-wasm SDK function. Previously this was assumed to be false, which can yield errors like this one:

2024-01-22 16:39:16 2024/01/22 22:39:16 [crit] 2464#0: *340 [lua] wasm.lua:790: attach(): failed attaching service(6bc44ccb-30c9-5f3d-acfa-017fa1b2ef82) filter chain to request: previous plan already attached, client: 172.17.0.1, server: kong, request: "GET /anything HTTP/1.1", host: "localhost:8000", request_id: "112ec236dcf487391a579a6f6f63fc0b"

This adds a ngx.ctx check and avoids calling proxy_wasm.attach() in this case.

Unfortunately it is incredibly non-trivial to add a test case for this at the moment, but the behavior change/correctness has been validated by hand.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR

Issue reference

KAG-3603

[hishamhm]

I'm still a bit surprised that calling proxy_wasm.start a second time works (i.e. that the last bit doesn't need to be inside the if block as well), but the changes done here definitely don't hurt, so if you managed to test this manually and get an improvement in behavior with this branch as-is, it's an LGTM from me!

[team-gateway-bot]

Backport failed for release/3.4.x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release/3.4.x
git worktree add -d .worktree/backport-12402-to-release/3.4.x origin/release/3.4.x
cd .worktree/backport-12402-to-release/3.4.x
git switch --create backport-12402-to-release/3.4.x
git cherry-pick -x 4d03ca45edfa54c76e5d2d4271892f37b7524ddd

[team-gateway-bot]

Backport failed for release/3.5.x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release/3.5.x
git worktree add -d .worktree/backport-12402-to-release/3.5.x origin/release/3.5.x
cd .worktree/backport-12402-to-release/3.5.x
git switch --create backport-12402-to-release/3.5.x
git cherry-pick -x 4d03ca45edfa54c76e5d2d4271892f37b7524ddd

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/8009

[team-gateway-bot]

Successfully created backport PR for release/3.6.x:

• [backport -> release/3.6.x] fix(wasm): do not call attach() on re-entrancy #12452

[thibaultcha]

Calling proxy_wasm.start() several times probably work since the underlying implementation uses ngx_wasm_ops and the filter chain is phase-aware; when invoked a second time, the filter chain loop will be hit but not entered, so ngx_http_wasm_ffi_start sort of becomes a NOP. This is the expected behavior since ngx_wasm_ops_resume is re-entrant. At least this would be my guess as to why it works.