runtime: fix conservative and precise GC freeing live objects - #5822
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The conservative and precise GCs can free objects that are still in use on bare-metal targets and native -scheduler=tasks builds. This is possibly exposed by commit c9aa88b ("runtime (gc_blocks.go): use a linked stack to scan marked objects"). A pointer held only in a callee-saved register when a collection runs is not marked, so the object it points to is freed. Seen on cortex-m-qemu at -opt=0 and -opt=z, riscv-qemu at -opt=0, and native -scheduler=tasks at -opt=0. This change scans the current goroutine stack while those registers are still saved on it, which fixes the missed roots. It adds a regression test, gc-register-root.go, and a runtime package test, and runs the runtime package tests in CI. gc-register-root.go is skipped on simavr, where runtime.GC does not return, and on riscv-qemu, where it can time out under load. Both are unrelated to this change.
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The conservative and precise GCs can free objects that are still in use on bare-metal targets and native -scheduler=tasks builds. This is possibly exposed by commit c9aa88b ("runtime (gc_blocks.go): use a linked stack to scan marked objects").
A pointer held only in a callee-saved register when a collection runs is not marked, so the object it points to is freed. Seen on cortex-m-qemu at -opt=0 and -opt=z, riscv-qemu at -opt=0, and native -scheduler=tasks at -opt=0.
This change scans the current goroutine stack while those registers are still saved on it, which fixes the missed roots.
It adds a regression test, gc-register-root.go, and a runtime package test, and runs the runtime package tests in CI. gc-register-root.go is skipped on simavr, where runtime.GC does not return, and on riscv-qemu, where it can time out under load. Both are unrelated to this change.