Flaky "invalid memory address or nil pointer" error in rules package's tests

[simonpasquier]

Spotted in https://travis-ci.org/prometheus/prometheus/builds/413101424

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa684b0]
goroutine 111 [running]:
github.com/prometheus/prometheus/rules.(*RecordingRule).Eval(0xc420185880, 0xc17a20, 0xc4203e0000, 0x270c56ef, 0xed2fb8ea0, 0xee04e0, 0x0, 0x0, 0xee04e0, 0xc17a20, ...)
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/recording.go:73 +0xf0
github.com/prometheus/prometheus/rules.(*Group).Eval.func1(0xc179a0, 0xc4200a6050, 0x270c56ef, 0xed2fb8ea0, 0xee04e0, 0xc4200e2500, 0x0, 0xc1b4e0, 0xc420185880)
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:384 +0x2ec
github.com/prometheus/prometheus/rules.(*Group).Eval(0xc4200e2500, 0xc179a0, 0xc4200a6050, 0x270c56ef, 0xed2fb8ea0, 0xee04e0)
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:452 +0xd0
github.com/prometheus/prometheus/rules.(*Group).run.func1()
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:230 +0x105
github.com/prometheus/prometheus/rules.(*Group).run(0xc4200e2500, 0xc179a0, 0xc4200a6050)
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:243 +0x38f
github.com/prometheus/prometheus/rules.(*Manager).Update.func1.1(0xc4201c4140, 0xc4200e2500)
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:679 +0xa7
created by github.com/prometheus/prometheus/rules.(*Manager).Update.func1
	/home/travis/gopath/src/github.com/prometheus/prometheus/rules/manager.go:674 +0x64
FAIL	github.com/prometheus/prometheus/rules	0.430s

[jojohappy]

I can not reproduce this test failed and the link seems to be passed. I would love to fix this bug.

[simonpasquier]

@jojohappy right the failed build log has been overwritten. I don't know how to reproduce it unfortunately.

[gouthamve]

To reproduce, run it enough times and grab popcorn :)

➜  prometheus git:(master) for i in {1..5000}
do
   go test ./rules -count=1
done

ok  	github.com/prometheus/prometheus/rules	0.122s
ok  	github.com/prometheus/prometheus/rules	0.127s
ok  	github.com/prometheus/prometheus/rules	0.132s
ok  	github.com/prometheus/prometheus/rules	0.130s
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1426c3a]

goroutine 189 [running]:
github.com/prometheus/prometheus/rules.(*RecordingRule).Eval(0xc4201a64d0, 0x15ab2e0, 0xc420a667b0, 0x270c56ef, 0xed308ca2a, 0x1814b20, 0x0, 0x0, 0x1814b20, 0x15ab2e0, ...)
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/recording.go:73 +0xca
github.com/prometheus/prometheus/rules.(*Group).Eval.func1(0x15ab260, 0xc4200ac048, 0x270c56ef, 0xed308ca2a, 0x1814b20, 0xc4200eeb40, 0x0, 0x15aebe0, 0xc4201a64d0)
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:384 +0x26f
github.com/prometheus/prometheus/rules.(*Group).Eval(0xc4200eeb40, 0x15ab260, 0xc4200ac048, 0x270c56ef, 0xed308ca2a, 0x1814b20)
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:452 +0xad
github.com/prometheus/prometheus/rules.(*Group).run.func1()
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:230 +0xc0
github.com/prometheus/prometheus/rules.(*Group).run(0xc4200eeb40, 0x15ab260, 0xc4200ac048)
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:243 +0x288
github.com/prometheus/prometheus/rules.(*Manager).Update.func1.1(0xc4200baeb0, 0xc4200eeb40)
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:679 +0x60
created by github.com/prometheus/prometheus/rules.(*Manager).Update.func1
	/Users/goutham/go/src/github.com/prometheus/prometheus/rules/manager.go:674 +0x56
FAIL	github.com/prometheus/prometheus/rules	0.115s

[lock]

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