fix nil err panic in httpd WriteResponse

[phemmer]

Got the below panic from my client when it tried to do an invalid query. The problem and fix is obvious, so putting up a PR.

[panic:runtime error: invalid memory address or nil pointer dereference] goroutine 25614 [running]:
runtime/debug.Stack(0xc42068b420, 0xc420108600, 0xbe91793e8d120376)
	/usr/lib/go/src/runtime/debug/stack.go:24 +0xa7
github.com/influxdata/influxdb/services/httpd.(*Handler).recovery.func1.1(0xc42068b420, 0xc420108600, 0xbe91793e8d120376, 0x55582cbe9a, 0x12f00c0, 0xc4201f8a50, 0x12a57a0, 0xc421ab27e0)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1537 +0xcd
panic(0xccd700, 0x12dc3b0)
	/usr/lib/go/src/runtime/panic.go:491 +0x283
github.com/influxdata/influxdb/services/httpd.(*msgpackFormatter).WriteResponse(0xc421b214e0, 0x0, 0x0, 0x0, 0x12945a0, 0xc421b21580, 0x0, 0x0, 0x0)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/response_writer.go:211 +0x979
github.com/influxdata/influxdb/services/httpd.(*responseWriter).WriteResponse(0xc42068b460, 0x0, 0x0, 0x0, 0x12945a0, 0xc421b21580, 0x52, 0x52, 0x3d)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/response_writer.go:59 +0x5a
github.com/influxdata/influxdb/services/httpd.(*Handler).httpError(0xc4201f8a50, 0x7f94723b5b68, 0xc42068b460, 0xc420428ae0, 0x52, 0x190)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1265 +0x1bd
github.com/influxdata/influxdb/services/httpd.(*Handler).serveQuery(0xc4201f8a50, 0x7f94723b5b68, 0xc42068b460, 0xc420108600, 0x0, 0x0)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:387 +0x1bd4
github.com/influxdata/influxdb/services/httpd.(*Handler).(github.com/influxdata/influxdb/services/httpd.serveQuery)-fm(0x7f94723b5b68, 0xc42068b460, 0xc420108600, 0x0, 0x0)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:136 +0x5c
github.com/influxdata/influxdb/services/httpd.authenticate.func1(0x7f94723b5b68, 0xc42068b460, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1337 +0x7d2
net/http.HandlerFunc.ServeHTTP(0xc4202e7b40, 0x7f94723b5b68, 0xc42068b460, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.(*Handler).responseWriter.func1(0x12a2ca0, 0xc421c44e60, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1514 +0xab
net/http.HandlerFunc.ServeHTTP(0xc4202e7b60, 0x12a2ca0, 0xc421c44e60, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.gzipFilter.func1(0x12a2d20, 0xc42068b440, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/gzip.go:39 +0x215
net/http.HandlerFunc.ServeHTTP(0xc4202e7b80, 0x12a2d20, 0xc42068b440, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.cors.func1(0x12a2d20, 0xc42068b440, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1459 +0x102
net/http.HandlerFunc.ServeHTTP(0xc4202e7ba0, 0x12a2d20, 0xc42068b440, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.requestID.func1(0x12a2d20, 0xc42068b440, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1490 +0x179
net/http.HandlerFunc.ServeHTTP(0xc4202e7bc0, 0x12a2d20, 0xc42068b440, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.(*Handler).logging.func1(0x12a2d20, 0xc42068b420, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1498 +0xe1
net/http.HandlerFunc.ServeHTTP(0xc4202e7be0, 0x12a2d20, 0xc42068b420, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/influxdata/influxdb/services/httpd.(*Handler).recovery.func1(0x12a57a0, 0xc421ab27e0, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:1551 +0x14c
net/http.HandlerFunc.ServeHTTP(0xc4202e7c00, 0x12a57a0, 0xc421ab27e0, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:1918 +0x44
github.com/bmizerany/pat.(*PatternServeMux).ServeHTTP(0xc4202e7960, 0x12a57a0, 0xc421ab27e0, 0xc420108600)
	/tmp/.gdm_26020/src/github.com/bmizerany/pat/mux.go:117 +0x15b
github.com/influxdata/influxdb/services/httpd.(*Handler).ServeHTTP(0xc4201f8a50, 0x12a57a0, 0xc421ab27e0, 0xc420108600)
	/tmp/.gdm_32513/src/github.com/influxdata/influxdb/services/httpd/handler.go:288 +0x24d
net/http.serverHandler.ServeHTTP(0xc420c90270, 0x12a57a0, 0xc421ab27e0, 0xc420108600)
	/usr/lib/go/src/net/http/server.go:2619 +0xb4
net/http.(*conn).serve(0xc4225a4280, 0x12a6de0, 0xc421920740)
	/usr/lib/go/src/net/http/server.go:1801 +0x71d
created by net/http.(*Server).Serve
	/usr/lib/go/src/net/http/server.go:2720 +0x288

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated
• Sign CLA (if not already signed)

[jsternberg]

Can you update the changelog and give yourself credit?

I'll merge after that and I think this needs to be backported to 1.4.

[jsternberg]

Can you also add a test case for this? Thanks.

[phemmer]

@jsternberg updated

[jsternberg]

Looks good to me. I'll push the merge button the minute that CI completes successfully.

Thanks for the help.

[jsternberg]

This is backported now. I believe we're planning to release 1.4.3 very soon so you should see it within the next week or so assuming something unexpected doesn't happen.