runtime: "unexpected signal during runtime execution" during bgscavenge on plan9

[bcmills]

#!watchflakes
post <- goos == "plan9" && log ~ `unexpected signal during runtime execution` && log ~ `runtime\.bgscavenge`

From plan9-386-0intro (https://build.golang.org/log/a94f5d80b1a5d38ecdf8b02bed7bfb305f569118), a cmd/vet test failure that appears to have nothing to do with cmd/vet:

--- FAIL: TestVet (1.94s)
    --- FAIL: TestVet/assign (0.43s)
        vet_test.go:162: error check failed: 
            assign.go:18: missing error "self-assignment of x to x"
            assign.go:20: missing error "self-assignment of s.x to s.x"
            assign.go:22: missing error "self-assignment of s.l.0. to s.l.0."
            Unmatched Errors:
            fatal error: unexpected signal during runtime execution
            [signal sys: trap: fault write code=0x0 addr=0x0 pc=0xa960]
            goroutine 4 [running]:
            runtime.throw(0x96394d, 0x2a)
            	/tmp/workdir-gnot/go/src/runtime/panic.go:1106 +0x63 fp=0x11429f44 sp=0x11429f30 pc=0x2e783
            runtime.sigpanic()
            	/tmp/workdir-gnot/go/src/runtime/os_plan9.go:79 +0x333 fp=0x11429f78 sp=0x11429f44 pc=0x2b033
            runtime.newobject(0x92d1a0, 0x0)
            	/tmp/workdir-gnot/go/src/runtime/malloc.go:1153 fp=0x11429f7c sp=0x11429f78 pc=0xa960
            runtime.bgscavenge(0x1143c000)
            	/tmp/workdir-gnot/go/src/runtime/mgcscavenge.go:211 +0x5b fp=0x11429fe8 sp=0x11429f7c pc=0x1d83b
            runtime.goexit()
            	/tmp/workdir-gnot/go/src/runtime/asm_386.s:1337 +0x1 fp=0x11429fec sp=0x11429fe8 pc=0x573f1
            created by runtime.gcenable
            	/tmp/workdir-gnot/go/src/runtime/mgc.go:215 +0x6a
            goroutine 1 [chan receive, locked to thread]:
            runtime.gopark(0x96ee48, 0x1143c030, 0x1143170e, 0x2)
            	/tmp/workdir-gnot/go/src/runtime/proc.go:304 +0xd8
            runtime.chanrecv(0x1143c000, 0x0, 0x11400001, 0x1515a)
            	/tmp/workdir-gnot/go/src/runtime/chan.go:563 +0x275
            runtime.chanrecv1(0x1143c000, 0x0)
            	/tmp/workdir-gnot/go/src/runtime/chan.go:433 +0x1c
            runtime.gcenable()
            	/tmp/workdir-gnot/go/src/runtime/mgc.go:216 +0x7e
            runtime.main()
            	/tmp/workdir-gnot/go/src/runtime/proc.go:166 +0x183
            runtime.goexit()
            	/tmp/workdir-gnot/go/src/runtime/asm_386.s:1337 +0x1
FAIL
FAIL	cmd/vet	34.760s

CC @aclements @mknyszek @0intro @fhs

[mknyszek]

This is very odd. It's failing very, very early in the runtime (right when the background scavenger is spinning up) trying to allocate a timer. And the problem seems to be that newobject has a nil type parameter? I'm not even sure how that can happen.

[ianlancetaylor]

The type parameter passed to newobject is 0x92d1a0. The 0x0 is the result parameter. The addr=0x0 printed at the start of the backtrace is meaningless, as this is not a fault signal, it is a trap signal. The trap signal does not set the address field (sigcode1) on Plan 9.

But that said I have no idea why the binary would get a trap signal. I don't know enough about Plan 9 to know what might cause that. If this happened only once I'm inclined to ignore it.

CC @0intro

[fhs]

Not sure if this is related, but cmd/vet crashed from a trap signal in a trybot run:

fatal error: sys: trap: general protection violation pc=0x00200393
[signal sys: trap: code=0x0 addr=0x0 pc=0x200393]

goroutine 1 [running]:
runtime.throw(0x10812000, 0x35)
	/tmp/workdir-gnot/go/src/runtime/panic.go:1106 +0x63 fp=0x108a1498 sp=0x108a1484 pc=0x2da03
runtime.sigpanic()
	/tmp/workdir-gnot/go/src/runtime/os_plan9.go:104 +0x289 fp=0x108a14cc sp=0x108a1498 pc=0x2a3a9
go/internal/gcimporter.(*importReader).declare(0x108a5b40, 0x338e40, 0x108d9140)
	/tmp/workdir-gnot/go/src/go/internal/gcimporter/iimport.go:310 +0x43 fp=0x108a14e4 sp=0x108a14cc pc=0x200393
go/internal/gcimporter.(*importReader).obj(0x108a5b40, 0x10821ff4, 0x4)
	/tmp/workdir-gnot/go/src/go/internal/gcimporter/iimport.go:270 +0x796 fp=0x108a1580 sp=0x108a14e4 pc=0x200136
go/internal/gcimporter.(*iimporter).doDecl(0x10862500, 0x108d9020, 0x10821ff4, 0x4)
	/tmp/workdir-gnot/go/src/go/internal/gcimporter/iimport.go:202 +0x160 fp=0x108a15c8 sp=0x108a1580 pc=0x1ff1f0
go/internal/gcimporter.iImportData(0x10851410, 0x108dfce0, 0x109e0001, 0x4ff96, 0x7fdff, 0x1085e820, 0x14, 0x0, 0x0, 0x0, ...)
	/tmp/workdir-gnot/go/src/go/internal/gcimporter/iimport.go:153 +0xb5f fp=0x108a1788 sp=0x108a15c8 pc=0x1fec0f
go/internal/gcimporter.Import(0x10851410, 0x108dfce0, 0x1085e820, 0x14, 0x0, 0x0, 0x108dabc0, 0x0, 0x0, 0x0)
	/tmp/workdir-gnot/go/src/go/internal/gcimporter/gcimporter.go:159 +0x421 fp=0x108a188c sp=0x108a1788 pc=0x1fda21
go/importer.(*gcimports).ImportFrom(0x108e17f0, 0x1085e820, 0x14, 0x0, 0x0, 0x0, 0x8c, 0x1085c630, 0x4e9324)
	/tmp/workdir-gnot/go/src/go/importer/importer.go:102 +0x58 fp=0x108a18b8 sp=0x108a188c pc=0x205008
go/importer.(*gcimports).Import(0x108e17f0, 0x1085e820, 0x14, 0x14, 0x1085c680, 0x291401)
	/tmp/workdir-gnot/go/src/go/importer/importer.go:95 +0x48 fp=0x108a18e0 sp=0x108a18b8 pc=0x204f88
cmd/vendor/golang.org/x/tools/go/analysis/unitchecker.run.func2(0x1085e941, 0x14, 0x290cc0, 0x52d201, 0x0)
	/tmp/workdir-gnot/go/src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go:221 +0x78 fp=0x108a1910 sp=0x108a18e0 pc=0x214748
cmd/vendor/golang.org/x/tools/go/analysis/unitchecker.importerFunc.Import(0x108e1800, 0x1085e941, 0x14, 0x0, 0x0, 0xb4f00)
	/tmp/workdir-gnot/go/src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go:396 +0x2b fp=0x108a1928 sp=0x108a1910 pc=0x21437b
go/types.(*Checker).importPackage(0x1085c7e0, 0x8b4, 0x1085e941, 0x14, 0x10860210, 0x20, 0x0)
	/tmp/workdir-gnot/go/src/go/types/resolver.go:158 +0x48e fp=0x108a1998 sp=0x108a1928 pc=0x1cf0ae
go/types.(*Checker).collectObjects(0x1085c7e0)
	/tmp/workdir-gnot/go/src/go/types/resolver.go:253 +0x383 fp=0x108a1cc4 sp=0x108a1998 pc=0x1cf573
go/types.(*Checker).checkFiles(0x1085c7e0, 0x108dabb8, 0x1, 0x2, 0x0, 0x0)
	/tmp/workdir-gnot/go/src/go/types/check.go:253 +0x89 fp=0x108a1ce8 sp=0x108a1cc4 pc=0x1b72d9
go/types.(*Checker).Files(...)
	/tmp/workdir-gnot/go/src/go/types/check.go:246
go/types.(*Config).Check(0x108dfd00, 0x10820bc2, 0x7, 0x10851410, 0x108dabb8, 0x1, 0x2, 0x108d8a20, 0x0, 0x1087cae0, ...)
	/tmp/workdir-gnot/go/src/go/types/api.go:348 +0xf4 fp=0x108a1d1c sp=0x108a1ce8 pc=0x1adab4
cmd/vendor/golang.org/x/tools/go/analysis/unitchecker.run(0x10851410, 0x1087ca80, 0x10880500, 0x15, 0x20, 0x108a1e40, 0x1082089c, 0x108a1de8, 0x108ac020, 0x2cb0bc)
	/tmp/workdir-gnot/go/src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go:235 +0x3e7 fp=0x108a1df0 sp=0x108a1d1c pc=0x213bb7
cmd/vendor/golang.org/x/tools/go/analysis/unitchecker.Run(0xdfffef94, 0x23, 0x10880500, 0x15, 0x20)
	/tmp/workdir-gnot/go/src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go:131 +0xd8 fp=0x108a1f54 sp=0x108a1df0 pc=0x213288
cmd/vendor/golang.org/x/tools/go/analysis/unitchecker.Main(0x10880500, 0x15, 0x20)
	/tmp/workdir-gnot/go/src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go:118 +0x1ed fp=0x108a1f98 sp=0x108a1f54 pc=0x21302d
main.main()
	/tmp/workdir-gnot/go/src/cmd/vet/main.go:35 +0x276 fp=0x108a1fb8 sp=0x108a1f98 pc=0x24f726
runtime.main()
	/tmp/workdir-gnot/go/src/runtime/proc.go:203 +0x261 fp=0x108a1ff0 sp=0x108a1fb8 pc=0x2fe71
runtime.goexit()
	/tmp/workdir-gnot/go/src/runtime/asm_386.s:1337 +0x1 fp=0x108a1ff4 sp=0x108a1ff0 pc=0x53601

[gopherbot]

Change https://golang.org/cl/233797 mentions this issue: dashboard: change "plan9" SlowBot alias to refer to plan9-arm