reflect: tests fail with -linkshared

[crawshaw]

On tip:

$ go install -buildmode=shared runtime sync/atomic
$ go test -short -linkshared reflect
runtime: typeOff 0x289e0 base 0xc4203bbdc0 not in ranges:
	types 0x7ff1e846ad00 etypes 0x7ff1e84b4b10
	types 0xaaa6a0 etypes 0xd841a0
fatal error: runtime: type offset base pointer out of range

goroutine 75 [running]:
runtime.throw(0x7ff1e82656dc, 0x2e)
	/usr/local/google/home/crawshaw/go/src/runtime/panic.go:596 +0x97 fp=0xc42009dd18 sp=0xc42009dcf8
runtime.resolveTypeOff(0xc4203bbdc0, 0x289e0, 0xc420128750)
	/usr/local/google/home/crawshaw/go/src/runtime/type.go:223 +0x356 fp=0xc42009dd78 sp=0xc42009dd18
runtime.(*_type).typeOff(0xc4203bbdc0, 0x289e0, 0xc42009dde0)
	/usr/local/google/home/crawshaw/go/src/runtime/type.go:239 +0x35 fp=0xc42009dda0 sp=0xc42009dd78
reflect.resolveTypeOff(0xc4203bbdc0, 0x289e0, 0x0)
	/usr/local/google/home/crawshaw/go/src/runtime/runtime1.go:515 +0x35 fp=0xc42009ddc8 sp=0xc42009dda0
reflect.(*rtype).typeOff(0xc4203bbdc0, 0xc4000289e0, 0xc42009de88)
	/usr/local/google/home/crawshaw/go/src/reflect/type.go:679 +0x35 fp=0xc42009ddf0 sp=0xc42009ddc8
reflect.(*rtype).ptrTo(0xc4203bbdc0, 0xc4203bbdc0)
	/usr/local/google/home/crawshaw/go/src/reflect/type.go:1430 +0x57b fp=0xc42009dec0 sp=0xc42009ddf0
reflect.PtrTo(0xe23120, 0xc4203bbdc0, 0xe23120, 0xc4203bbdc0)
	/usr/local/google/home/crawshaw/go/src/reflect/type.go:1425 +0x47 fp=0xc42009dee8 sp=0xc42009dec0
reflect_test.TestPtrTo(0xc42012de10)
	/usr/local/google/home/crawshaw/go/src/reflect/all_test.go:2485 +0xa1 fp=0xc42009dfa8 sp=0xc42009dee8
testing.tRunner(0xc42012de10, 0xd808f8)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:657 +0xa0 fp=0xc42009dfd0 sp=0xc42009dfa8
runtime.goexit()
	/usr/local/google/home/crawshaw/go/src/runtime/asm_amd64.s:2197 +0x1 fp=0xc42009dfd8 sp=0xc42009dfd0
created by testing.(*T).Run
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:697 +0x2e4

goroutine 1 [chan receive]:
testing.(*T).Run(0xc4200d3110, 0x63a653, 0x9, 0xd808f8, 0xc4200add01)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:698 +0x30e
testing.runTests.func1(0xc4200d3110)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:882 +0x69
testing.tRunner(0xc4200d3110, 0xc4200adde0)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:657 +0xa0
testing.runTests(0xc42000d540, 0xe1c940, 0x85, 0x85, 0xc420011970)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:888 +0x2ee
testing.(*M).Run(0xc4200adf20, 0xc4200adf20)
	/usr/local/google/home/crawshaw/go/src/testing/testing.go:822 +0x109
main.main()
	reflect/_test/_testmain.go:334 +0x118

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
	/usr/local/google/home/crawshaw/go/src/runtime/asm_amd64.s:2197 +0x1

goroutine 38 [sleep]:
time.Sleep(0x3b9aca00)
	/usr/local/google/home/crawshaw/go/src/runtime/time.go:59 +0x105
reflect_test.selectWatcher()
	/usr/local/google/home/crawshaw/go/src/reflect/all_test.go:1416 +0x3e
created by reflect_test.TestSelect.func1
	/usr/local/google/home/crawshaw/go/src/reflect/all_test.go:1146 +0x37
FAIL	reflect	0.089s

May just be a bogus test.

First seen on #16602.

[bradfitz]

/cc @mwhudson @ianlancetaylor

[mwhudson]

So we have a type that lives on the heap ("base 0xc4203bbdc0") but has a non-zero ptrToThis? How does that happen?

[mwhudson]

Oh because when PtrTo needs to create a new type it starts by copying *unsafe.Pointer, which has a non-zero ptrToThis. One line fix incoming.

[mwhudson]

Well no, because the obvious fix, to whit this:

(master *)mwhudson@aeglos:/opt/opensource/go/src$ git diff
diff --git a/src/reflect/type.go b/src/reflect/type.go
index fbfda3a..4085909 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -1463,6 +1463,7 @@ func (t *rtype) ptrTo() *rtype {
        pp := *prototype
 
        pp.str = resolveReflectName(newName(s, "", "", false))
+       pp.ptrToThis = 0
 
        // For the type structures linked into the binary, the
        // compiler provides a good hash of the string.

causes this failure:

(master *)mwhudson@aeglos:/opt/opensource/go/src$ go test -short -linkshared reflect 
--- FAIL: TestTypelinksSorted (0.00s)
	all_test.go:4807: typelinks not sorted: "utf8.acceptRange" [2667] > "***[179999]*runtime.bucket" [2668]
FAIL
FAIL	reflect	0.092s

for reasons I totally don't understand at this point.

[crawshaw]

I believe TestTypelinksSorted was not designed with multiple modules in mind, and needs some rethinking. I'm happy to do this if you like.

[mwhudson]

Oh right yeah, this test is just invalid in this case isn't it. So I think the fix is valid.

[mwhudson]

FWIW, this bug isn't directly related to dynamic linking, for example this fails in the same way:

package main

import (
	"reflect"
	"unsafe"
)

var x unsafe.Pointer
var y = &x
var z = &y

func main() {
	var v unsafe.Pointer
	println(z)
	t := reflect.TypeOf(v)
	for i := 0; i < 10; i++ {
		t = reflect.PtrTo(t)
	}
}

[gopherbot]

CL https://golang.org/cl/36731 mentions this issue.

[rsc]

Cherry-pick for Go 1.8 https://go-review.googlesource.com/36718

[gopherbot]

CL https://golang.org/cl/36718 mentions this issue.