[plasma] SIGSEGV in TestPlasmaRecovery seen in ci robot

--- PASS: TestPlasmaRecoverySimple (0.02s)=== RUN   TestPlasmaRecoveryunexpected fault address 0x894a4fatal error: fault[signal SIGSEGV: segmentation violation code=0x1 addr=0x894a4 pc=0x552d93] goroutine 780 [running]:runtime.throw(0x727e06, 0x5)        /home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:566 +0x95 fp=0xc42058f230 sp=0xc42058f210runtime.sigpanic()        /home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/sigpanic_unix.go:27 +0x288 fp=0xc42058f288 sp=0xc42058f230github.com/couchbase/nitro/skiplist.CompareInt(0x7f52983b4f18, 0x894a4, 0x894a4)        /opt/build/goproj/src/github.com/couchbase/nitro/skiplist/item.go:90 +0x43 fp=0xc42058f290 sp=0xc42058f288github.com/couchbase/plasma.(*pdMergeIterator).fetchMin(0xc420abf080)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:366 +0x2c4 fp=0xc42058f2d8 sp=0xc42058f290github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abf080)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:353 +0x59 fp=0xc42058f2f8 sp=0xc42058f2d8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abf080)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f318 sp=0xc42058f2f8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530cc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f338 sp=0xc42058f318github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abf020)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f358 sp=0xc42058f338github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abf020)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f378 sp=0xc42058f358github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530d40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f398 sp=0xc42058f378github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abefc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f3b8 sp=0xc42058f398github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abefc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f3d8 sp=0xc42058f3b8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530dc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f3f8 sp=0xc42058f3d8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abef60)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f418 sp=0xc42058f3f8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abef60)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f438 sp=0xc42058f418github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530e40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f458 sp=0xc42058f438github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abef00)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f478 sp=0xc42058f458github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abef00)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f498 sp=0xc42058f478github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530ec0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f4b8 sp=0xc42058f498github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abeea0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f4d8 sp=0xc42058f4b8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abeea0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f4f8 sp=0xc42058f4d8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530f40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f518 sp=0xc42058f4f8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abee40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f538 sp=0xc42058f518github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abee40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f558 sp=0xc42058f538github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420530fc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f578 sp=0xc42058f558github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abede0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f598 sp=0xc42058f578github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abede0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f5b8 sp=0xc42058f598github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531040)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f5d8 sp=0xc42058f5b8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abed80)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f5f8 sp=0xc42058f5d8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abed80)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f618 sp=0xc42058f5f8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205310c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f638 sp=0xc42058f618github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abed20)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f658 sp=0xc42058f638github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abed20)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f678 sp=0xc42058f658github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531140)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f698 sp=0xc42058f678github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abecc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f6b8 sp=0xc42058f698github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abecc0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f6d8 sp=0xc42058f6b8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205311c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f6f8 sp=0xc42058f6d8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abec60)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f718 sp=0xc42058f6f8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abec60)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f738 sp=0xc42058f718github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531240)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f758 sp=0xc42058f738github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abec00)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f778 sp=0xc42058f758github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abec00)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f798 sp=0xc42058f778github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205312c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f7b8 sp=0xc42058f798github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abeba0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f7d8 sp=0xc42058f7b8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abeba0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f7f8 sp=0xc42058f7d8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531340)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f818 sp=0xc42058f7f8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abeb40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f838 sp=0xc42058f818github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abeb40)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f858 sp=0xc42058f838github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205313c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f878 sp=0xc42058f858github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abeae0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f898 sp=0xc42058f878github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abeae0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f8b8 sp=0xc42058f898github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531440)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f8d8 sp=0xc42058f8b8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abea80)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f8f8 sp=0xc42058f8d8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abea80)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f918 sp=0xc42058f8f8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205314c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f938 sp=0xc42058f918github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abea20)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f958 sp=0xc42058f938github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abea20)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f978 sp=0xc42058f958github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531540)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f998 sp=0xc42058f978github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abe9c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058f9b8 sp=0xc42058f998github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abe9c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058f9d8 sp=0xc42058f9b8github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205315c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058f9f8 sp=0xc42058f9d8github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abe960)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058fa18 sp=0xc42058f9f8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abe960)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058fa38 sp=0xc42058fa18github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc420531640)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058fa58 sp=0xc42058fa38github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abe900)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058fa78 sp=0xc42058fa58github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abe900)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058fa98 sp=0xc42058fa78github.com/couchbase/plasma.(*pdJoinIterator).Next(0xc4205316c0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:310 +0x47 fp=0xc42058fab8 sp=0xc42058fa98github.com/couchbase/plasma.(*pdMergeIterator).next(0xc420abe8a0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:352 +0x4b fp=0xc42058fad8 sp=0xc42058fab8github.com/couchbase/plasma.(*pdMergeIterator).Next(0xc420abe8a0)        /opt/build/goproj/src/github.com/couchbase/plasma/iterator.go:346 +0x65 fp=0xc42058faf8 sp=0xc42058fad8github.com/couchbase/plasma.(*page).collectItems(0xc420530c00, 0x7f5274121b00, 0x0, 0x7f527d1d89f0, 0xc42561c0d0, 0x0, 0x1, 0xc42561c0d0, 0x0, 0x1, ...)        /opt/build/goproj/src/github.com/couchbase/plasma/page.go:773 +0x198 fp=0xc42058fba0 sp=0xc42058faf8github.com/couchbase/plasma.(*page).Compact(0xc420530c00, 0xc8)        /opt/build/goproj/src/github.com/couchbase/plasma/page.go:683 +0x7d fp=0xc42058fc68 sp=0xc42058fba0github.com/couchbase/plasma.(*Plasma).trySMOs2(0xc42033ec00, 0x7f52580716d0, 0xa7c800, 0xc420530c00, 0xc4200bcf00, 0xc42058fe01, 0x190, 0x19, 0xc8, 0x4, ...)        /opt/build/goproj/src/github.com/couchbase/plasma/plasma.go:1548 +0x9e fp=0xc42058fe48 sp=0xc42058fc68github.com/couchbase/plasma.(*Plasma).trySMOs(0xc42033ec00, 0x7f52580716d0, 0xa7c800, 0xc420530c00, 0xc4200bcf00, 0xc420530c01, 0x0)        /opt/build/goproj/src/github.com/couchbase/plasma/plasma.go:1539 +0x96 fp=0xc42058feb0 sp=0xc42058fe48github.com/couchbase/plasma.(*Writer).Delete(0xc4201046a0, 0xc420360708, 0x0, 0x0)        /opt/build/goproj/src/github.com/couchbase/plasma/plasma.go:1714 +0xf2 fp=0xc42058ff30 sp=0xc42058feb0github.com/couchbase/plasma.doDelete(0xc4201046a0, 0xc420568680, 0x1, 0x1b774)        /opt/build/goproj/src/github.com/couchbase/plasma/plasma_test.go:130 +0x9c fp=0xc42058ff70 sp=0xc42058ff30runtime.goexit()        /home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/asm_amd64.s:2086 +0x1 fp=0xc42058ff78 sp=0xc42058ff70created by github.com/couchbase/plasma.TestPlasmaRecovery        /opt/build/goproj/src/github.com/couchbase/plasma/plasma_test.go:282 +0x312