commit 7c4558292c671977645c344ac3b5760cfe11147d
author Nicolas Hillegeer <aktau@google.com> Sat Oct 12 14:59:46 2024 -0700
committer Gopher Robot <gobot@golang.org> Mon Oct 14 01:08:16 2024 +0000
tree f8fba1479dec5953f1dd61517e57b3c355201d7e
parent dfd7e92aa331f2d584bfefaab2d0432a1bb8c91d

internal/gocore: fix large types dereference (Go 1.22+)

viewcore supports Go 1.22+ allocation headers since CL 608475. The CL
description mentions:

> This gets the goroot test passing again, which is a low bar... but
> it's something.

This CL fixes an untested path, and modifies test.go to hit that path by
allocating a "large" object (>32760 bytes). When allocating a large
object, viewcore panics:

```
panic: asking for field of non-struct [recovered]
        panic: asking for field of non-struct

goroutine 170 [running]:
testing.tRunner.func1.2({0x72ce80, 0x7b86c0})
        /usr/lib/google-golang/src/testing/testing.go:1632 +0x230
testing.tRunner.func1()
        /usr/lib/google-golang/src/testing/testing.go:1635 +0x35e
panic({0x72ce80?, 0x7b86c0?})
        /usr/lib/google-golang/src/runtime/panic.go:785 +0x132
golang.org/x/debug/internal/gocore.(*Type).field(...)
        /usr/local/google/home/aktau/gob/go/debug/internal/gocore/type.go:85
golang.org/x/debug/internal/gocore.region.Field(...)
        /usr/local/google/home/aktau/gob/go/debug/internal/gocore/region.go:163
golang.org/x/debug/internal/gocore.(*Process).readSpans(0xc0001fe000, {0xc0001fe000?, 0xc000000f50?, 0xc000000500?}, {0xc000aa0480?, 0x1, 0xc0005d7260?})
        /usr/local/google/home/aktau/gob/go/debug/internal/gocore/process.go:596 +0x5125
golang.org/x/debug/internal/gocore.(*Process).readHeap(0xc0001fe000)
        /usr/local/google/home/aktau/gob/go/debug/internal/gocore/process.go:254 +0x569
golang.org/x/debug/internal/gocore.Core(0xc0001f4000)
```

Indeed, `largeType` is not a struct, but a pointer to a struct:

```
// src/runtime/mheap.go
type mspan struct {
	_    sys.NotInHeap
	// ...
	largeType             *_type        // malloc header for large objects.
}
```

Add a `Deref()` operation to fix this. Afterwards, the test does not panic
anymore. Though no attempt has been made to verify that the results are good.

Change-Id: I3569e5e1fba14a311507944f75e2ee2d27616a15
Reviewed-on: https://go-review.googlesource.com/c/debug/+/619875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Nicolas Hillegeer <aktau@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>