mvcc: check null before set FillPercent not to panic

[yudai]

Since CreateBucketIfNotExists() can return nil when it gets an error,
accessing FilePercent must be done after a nil check, not to cause
a panic.

We observed a panic during a defrag like below with etcd v3.2.10 under heavy load (larger IO wait). It seems it's due to a nil check failure. As the code is till there in the master, I'm sending a patch to fix the issue. Probably backporting to v3.2.x is required.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x8c76a7]

goroutine 905788 [running]:
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend.defragdb(0xc4401b81e0, 0xc44617a000, 0x2710, 0x0, 0x0)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend/backend.go:373 +0x297
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend.(*backend).defrag(0xc431512480, 0x0, 0x0)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend/backend.go:309 +0x206
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend.(*backend).Defrag(0xc431512480, 0x14b77c0, 0xc431512480)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/mvcc/backend/backend.go:275 +0x2b
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc.(*maintenanceServer).Defragment(0xc43bb680e0, 0x7f3918a11e98, 0xc457b4fcb0, 0x1520a10, 0x0, 0xc4bc21f810, 0x3)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc/maintenance.go:69 +0x8c
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc.(*authMaintenanceServer).Defragment(0xc426286120, 0x7f3918a11e98, 0xc457b4fcb0, 0x1520a10, 0xc426286120, 0xf8dc92, 0xa)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc/maintenance.go:169 +0x9e
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/etcdserverpb._Maintenance_Defragment_Handler.func1(0x7f3918a11e98, 0xc457b4fcb0, 0xeb0a40, 0x1520a10, 0xc469ed2c80, 0xc4bc21f801, 0xc4bc21f8b8, 0x4ec4e3)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/etcdserverpb/rpc.pb.go:3823 +0x86
github.com/coreos/etcd/cmd/vendor/github.com/grpc-ecosystem/go-grpc-prometheus.UnaryServerInterceptor(0x7f3918a11e98, 0xc457b4fcb0, 0xeb0a40, 0x1520a10, 0xc45f6d7820, 0xc45f6d7840, 0x40ff68, 0x20, 0xe4b5e0, 0x1)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/grpc-ecosystem/go-grpc-prometheus/server.go:29 +0xd2
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc.newUnaryInterceptor.func1(0x7f3918a11e98, 0xc457b4fcb0, 0xeb0a40, 0x1520a10, 0xc45f6d7820, 0xc45f6d7840, 0x0, 0xc4bc21f9d8, 0x40ff68, 0x50)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/api/v3rpc/interceptor.go:57 +0xc3
github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/etcdserverpb._Maintenance_Defragment_Handler(0xe77340, 0xc426286120, 0x7f3918a11e98, 0xc457b4fcb0, 0xc469ed2c30, 0xc42c106170, 0x0, 0x0, 0xc441efd7c8, 0xc430be0280)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/github.com/coreos/etcd/etcdserver/etcdserverpb/rpc.pb.go:3825 +0x177
github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc.(*Server).processUnaryRPC(0xc4437ee000, 0x14b73a0, 0xc442c85340, 0xc46b2ada00, 0xc452c089f0, 0x14f4750, 0x0, 0x0, 0x0)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc/server.go:843 +0xc41
github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc.(*Server).handleStream(0xc4437ee000, 0x14b73a0, 0xc442c85340, 0xc46b2ada00, 0x0)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc/server.go:1040 +0x15a6
github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc43d4ad440, 0xc4437ee000, 0x14b73a0, 0xc442c85340, 0xc46b2ada00)
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc/server.go:589 +0xa9
created by github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc.(*Server).serveStreams.func1
__/home/gyuho/etcd/release/etcd/gopath/src/github.com/coreos/etcd/cmd/vendor/google.golang.org/grpc/server.go:590 +0xa1

[gyuho]

lgtm. Thanks! /cc @xiang90

[xiang90]

lgtm