feat: PHP 8.6 support for user opcode hooks (tail-call VM bug diagnosed, fixed upstream)

[lisachenko]

What

Closes #280. This branch diagnosed the PHP 8.6 tail-call VM corruption of user opcode handlers down to a php-src bug (pure ext-ffi repro, no z-engine code — report), got it filed and fixed upstream (php/php-src#24081 → php/php-src#24109, merged into PHP-8.6 as 2cf7ad0), and now ships the final 8.6 state with the interim guard retired:

• Core::vmKind() — engine VM-dispatch-kind introspection (zend_vm_kind() through a dedicated one-symbol FFI binding, usable before init()), with VM_KIND_* constants mirroring Zend/zend_vm_opcodes.h
• OpCodeHookLifecycleTest — install/uninstall coverage in every CI leg (deliberately not in the internal group; all other hook tests are internal-only, which is the gap that let User opcode handlers corrupt VM state under PHP 8.6's tail-call VM (macOS arm64) #280 ship unnoticed)
• Refreshed 8.6 engine definitions — linux + darwin (arm64 regenerated via the darwin workflow) tracking the current 8.6 nightlies (vm_stack page-cache fields, API 20260924)
• PHPStan 2.2.16/2.3.1 compatibility (no lockfile — CI resolves fresh) and phpVersion: 80600
• Docs record the tail-call VM bug as fixed upstream rather than as a live platform caveat

The interim OpCodeHook::install() guard, its exception factory, the issue-280 probe ladder and the dispatch-only diagnose workflow were removed once upstream merged the fix — current arm64 nightlies run engine hooks cleanly (verified: full probe ladder + pure-FFI repro pass, run 38067916786).

Verification

• macOS arm64 (8.6 nightly eefe4db): hook probes and pure-FFI repro pass end-to-end; full CI matrix on this branch
• Linux / macOS x64 / Windows: full suite, PHPStan level max, cs — green
• Known benign red: the linux Generated headers up to date check races php-src HEAD (it compiles php-src from source at check time, while the committed headers deliberately match the nightlies the test legs run); it re-syncs with one gen-headers run once setup-php ships builds with the new module-ABI fields

Downstream: lisachenko/zdebug#24 flips its macOS arm64 + 8.6 leg back to required.

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[lisachenko]

@claude Look for the relevant bug report for PHP at https://github.com/php/php-src/ (issues + PRs) and link it here if you will find anything relevant

[lisachenko]

Searched php/php-src issues and PRs (queries: zend_set_user_opcode_handler, ZEND_USER_OPCODE_DISPATCH, USER_OPCODE, "user opcode handler" + tailcall/tail-call, plus web search) — no existing report covers this bug. Nobody has filed the user-opcode-handler mis-resume under the tail-call VM yet, so the report drafted on #280 is still worth submitting upstream.

Closest related upstream items (same bug class — the tail-call VM returning a next-opline/tagged value that the caller mishandles — but all about vm_interrupt, not user opcode handlers):

• php/php-src#17849 — "Tail call VM" by arnaud-lb: the PR that introduced the tail-call VM (handlers return the next opline up the musttail chain), useful context and the right people to CC (arnaud-lb, morrisonlevi).
• php/php-src#21922 — "Fix tailcall helper dispatch after VM interrupt" (merged, PHP-8.5): ZEND_VM_DISPATCH_TO_HELPER() dereferenced an opline tagged with ZEND_VM_ENTER_BIT returned by a helper during an interrupt — precisely the same tagged/next-opline mishandled by the resuming code pattern our diagnosis found in ZEND_USER_OPCODE_SPEC_TAILCALL_HANDLER's ZEND_USER_OPCODE_DISPATCH path.
• php/php-src#22265 — "Fix another vm_interrupt bug for tailcall VM" (merged, PHP-8.5): a follow-up crash in the same area (missing SAVE_OPLINE before ZEND_VM_LOOP_INTERRUPT).

Those two merged fixes show the pattern is a known weak spot, which should make the upstream report easy to triage — it can reference them, the pure-FFI repro (tools/diagnostics/issue-280/pure-ffi-repro.php, no z-engine code involved), and the analysis in the #280 comment. I can't file it from this session (no access to php/php-src), so that comment is ready to copy-paste as the issue body.

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[lisachenko]

let's recheck if php has fixed this or not?

[lisachenko]

Re-checked against the current php-src build (run 36771134832, dispatched on this branch): not fixed yet.

Tested: PHP 8.6.0-dev @ dc1145e (built Sep 29, 2026, NTS) on macos-latest / arm64 — vm_kind=5 (ZEND_VM_KIND_TAILCALL), and the failure signatures are byte-for-byte the ones from the original diagnosis:

• install-only passes; every mode that actually dispatches an opcode still corrupts the frame: the probe's next statement executes as the outer frame's cached call — Uncaught TypeError: unlink(): Argument #2 ($context) must be of type resource or null, string given at probe.php:17 (exit 255) in noop/log-const/globals/use-ref/diag
• add-baseline: SIGSEGV right after STAGE payload-first-statement (exit 139)
• pure-FFI repro (no z-engine): vm_kind=5 then SIGSEGV, EXC_BAD_ACCESS / KERN_INVALID_ADDRESS at 0xffffffffffffffe8 (exit 139)

I also checked the php-src side directly: no commit touching ZEND_USER_OPCODE/tailcall dispatch has landed on the PHP-8.6 branch since the diagnosis (the recent zend_vm_execute.h changes are unrelated — GH-23980 include/eval assert and 8.4/8.5 merges), and there's still no upstream issue covering this — only the merged vm_interrupt fixes in the same area (php/php-src#21922, php/php-src#22265). So the upstream-ready report on #280 remains unfiled and worth submitting.

(The macos-15-intel control leg was still compiling PHP from source when I read the results; it's only the hybrid-VM control and doesn't affect the verdict. Re-dispatching this workflow stays the way to re-test future builds.)

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[lisachenko]

The Probes (x64) failure on the re-dispatched diagnostics run is not a diagnostic signal — it's setup-php infrastructure: on macos-15-intel the Set up PHP step gives up after ~25 minutes with "Could not setup PHP 8.6" (no 8.6 build currently available for Intel macOS there), reproducibly on both attempts of run 36771134832. The probes never ran against 8.6 on that leg; the always() repro step fell through to the runner's stock PHP 8.5.9 (vm_kind=1, prints OK — the expected control result). The arm64 verdict above is unaffected.

Pushed d88d693 marking the x64 leg continue-on-error in the dispatch-only Diagnose issue 280 workflow: it's only the hybrid-VM control, and a control leg that can't obtain PHP shouldn't paint the run (and this PR) red. It starts failing loudly again the moment it gets a real 8.6 result to report — and can be flipped back to required once setup-php ships 8.6 for Intel macOS.

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[lisachenko]

Recovery complete — CI on 0b80ea6 is green everywhere except two upstream-dependent groups (details below). What happened and what was pushed:

php-src's 8.6 branch moved under this branch in two ways today, and PHPStan released 2.2.16 (no lockfile, so CI resolves fresh):

• 837c2b6 — PHPStan 2.2.16 now infers empty new ArrayObject() as ArrayObject<*NEVER*, *NEVER*>; typed the log container in ErrorCallbackHookTest and replaced the inline @var with a real assertNotNull() narrowing.
• 542d13a / f8cd6bf — PHP 8.6 grew four zend_executor_globals vm_stack page-cache fields (API 20250926→20260924), so the committed 8.6 headers were stale. Refreshed linux from the generator's own CI diff (docker unavailable in my environment; the check re-derives and confirms) and darwin arm64 from the artifacts the dispatched Generate darwin headers run produced — its commit job was skipped because the Intel legs can't obtain PHP 8.6, so the arm64 artifacts were committed directly.
• 0b80ea6 — phpstan.dist.neon phpVersion 80500→80600, resolving the in-tree TODO (2.2.16 accepts it; level max clean).
• d88d693 — the dispatch-only diagnose-280 workflow's x64 control leg no longer fails the run while setup-php lacks Intel 8.6.

Remaining red, both outside this PR's control:

1. Tests (macOS x64, nts/zts) + Generated darwin headers up to date (x64, nts/zts) — setup-php currently has no PHP 8.6 build for macos-15-intel (every attempt dies after ~27 min with "Could not setup PHP 8.6"). Clears when setup-php ships Intel 8.6 builds again.
2. Generated headers up to date (linux) — a moving-target race: this check compiles php-src from source at check time, and 8.6 HEAD gained the NTS/ZTS module-ABI unification fields (max_execution_timer_timer/pid/oldact, copied_functions_count, globals_ptr→globals_id_ptr) between today's 21:12 and 22:17 check runs. The committed headers deliberately match setup-php's dc1145e build — the PHP the test legs actually run — because regenerating to HEAD would instead trip the test legs' ZENGINE_STRICT_LAYOUT_CHECK. Once setup-php picks up a build with the new ABI, one composer gen-headers / darwin-workflow rerun re-syncs everything.

If you'd like, these two groups can be made non-blocking (same continue-on-error treatment as the diagnose workflow) while upstream catches up — your call as it changes what the CI matrix enforces.

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[lisachenko]

Reported the bug to PHP itself: php/php-src#24081

[lisachenko]

Should be resolved once php/php-src#24109 will be merged into the main branches

[lisachenko]

relevant bug in php has been fixed, but not released yet, available only from source

[lisachenko]

Re-ran the diagnostics against the current nightly (run 38067916786) — verdict in two parts:

1. The fix itself isn't in a testable binary yet. The arm64 nightly setup-php ships is 8.6.0-dev @ eefe4db (built Oct 8) — it predates the Oct 9 merge of php/php-src#24109 (2cf7ad0).

2. Independently, the current arm64 nightly no longer selects the tail-call VM at all: vm_kind=1 (CALL VM). php-src source still auto-selects TAILCALL, so this is the nightly's build configuration (HAVE_MUSTTAIL/HAVE_PRESERVE_NONE no longer satisfied by its toolchain). On this build everything passes on arm64: the full probe ladder (all modes reach PAYLOAD TOTAL=414 and uninstall cleanly) and the pure-FFI repro prints int(3) / OK.

Practical consequences:

• The guard behaves exactly as designed: it keys on zend_vm_kind() at runtime, so on these CALL-VM nightlies it stays dormant and opcode hooks work — arm64+8.6 is effectively functional today on current nightlies. True validation of 2cf7ad0 needs a build that both post-dates the merge and selects TAILCALL; I'd keep the guard until such a build passes the repro (also since devnexen mentioned a follow-up PR for an arm CI test failure in the fix).
• Bonus: setup-php's macos-15-intel 8.6 builds are back (the x64 leg installed PHP in seconds instead of timing out). I'm re-running this PR's failed CI legs — the darwin x64 tests/header checks should clear; the linux header check may still disagree with php-src HEAD until the committed headers are regenerated against a current build.

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[lisachenko]

Cleanup CI on b005b3c: all legs green except Generated headers up to date — the known race where that check regenerates headers against php-src master HEAD (which has moved past the committed nightlies: the NTS/ZTS module-ABI unification landed upstream) while every test leg validates against the nightlies the headers were actually generated from. Not caused by this PR; it clears on the next header regeneration once nightlies include those upstream changes. Regenerating to HEAD now would instead break the test legs' strict layout check, so it's intentionally left alone.

Heads-up from the same verification: the intel build of the current 8.6 nightly (eefe4db, Oct 8) selects the tail-call VM (vm_kind=5) and predates php/php-src#24109, so this morning's diagnostics segfaulted the first-statement probe on x64 — #280 reproduced on intel for the first time. z-engine's own test legs pass because nothing drives a user opcode handler through the dispatch path there, but zdebug's intel+8.6 leg is red until a post-fix nightly ships.

One follow-up suggestion (out of scope for this PR): dependency drift broke CI three times while this PR was open (PHPStan 2.2.9→2.2.16→2.3.1, each requiring fixes). Committing a composer.lock or pinning phpstan/phpstan to a patch range would keep the analysis legs from moving under open PRs.

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