relevant portion of log:

2016/03/22 14:28:41 Task failed, retrying after a sleep of 0: http request failed: Service Unavailable (HTTP 503)
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/store_inline
2016/03/22 14:28:41 PUT  /fake/cloudstorage
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/finalize_gs_upload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 Task failed, retrying after a sleep of 0: http request failed: Service Unavailable (HTTP 503)
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/store_inline
2016/03/22 14:28:41 PUT  /fake/cloudstorage
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/finalize_gs_upload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/store_inline
2016/03/22 14:28:41 Task failed, retrying after a sleep of 0: http request failed: Service Unavailable (HTTP 503)
2016/03/22 14:28:41 PUT  /fake/cloudstorage
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/finalize_gs_upload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/store_inline
2016/03/22 14:28:41 PUT  /fake/cloudstorage
2016/03/22 14:28:41 Task failed, retrying after a sleep of 0: http request failed: Service Unavailable (HTTP 503)
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/finalize_gs_upload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 POST /_ah/api/isolateservice/v1/preupload
2016/03/22 14:28:41 Task failed, retrying after a sleep of 0: Put http://127.0.0.1:49760/fake/cloudstorage?digest=639b8c971d725db4b2eed1f1794c6c8a75ba09fb: EOF
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x28 pc=0x6327e]

goroutine 80 [running]:
github.com/luci/luci-go/client/isolatedclient.(*compressed).Read(0xc2080be8c0, 0xc208350000, 0x8000, 0x8000, 0x366450, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:250 +0x6e
io/ioutil.(*nopCloser).Read(0xc208346570, 0xc208350000, 0x8000, 0x8000, 0xc208078b00, 0x0, 0x0)
	<autogenerated>:4 +0x89
io.(*multiReader).Read(0xc2083028a0, 0xc208350000, 0x8000, 0x8000, 0x4010, 0x0, 0x0)
	/usr/local/go/src/io/multi.go:13 +0xab
io.Copy(0x5c87a8, 0xc2082f8390, 0x5c8700, 0xc2083028a0, 0x4017, 0x0, 0x0)
	/usr/local/go/src/io/io.go:362 +0x1f6
net/http.(*transferWriter).WriteBody(0xc2080100e0, 0x5c85e8, 0xc208078b00, 0x0, 0x0)
	/usr/local/go/src/net/http/transfer.go:207 +0x226
net/http.(*Request).write(0xc2080ee410, 0x5c85e8, 0xc208078b00, 0x69300, 0xc2082dcae0, 0x0, 0x0)
	/usr/local/go/src/net/http/request.go:448 +0xada
net/http.(*persistConn).writeLoop(0xc2082c48f0)
	/usr/local/go/src/net/http/transport.go:951 +0x28d
created by net/http.(*Transport).dialConn
	/usr/local/go/src/net/http/transport.go:661 +0xcbc

goroutine 1 [chan receive]:
testing.RunTests(0x4039f0, 0x5056c0, 0x9, 0x9, 0x508d01)
	/usr/local/go/src/testing/testing.go:576 +0x58e
testing.(*M).Run(0xc20802e0f0, 0x510160)
	/usr/local/go/src/testing/testing.go:485 +0x6c
main.main()
	github.com/luci/luci-go/client/isolatedclient/_test/_testmain.go:68 +0x1d5

goroutine 47 [runnable]:
compress/flate.(*compressor).findMatch(0xc2080cd0e0, 0x53ec, 0x2f85, 0x2, 0xac14, 0x2, 0x0, 0x0)
	/usr/local/go/src/compress/flate/deflate.go:150
compress/flate.(*compressor).deflate(0xc2080cd0e0)
	/usr/local/go/src/compress/flate/deflate.go:286 +0x62e
compress/flate.(*compressor).write(0xc2080cd0e0, 0xc2080ae000, 0x1, 0x1, 0x10001, 0x0, 0x0)
	/usr/local/go/src/compress/flate/deflate.go:379 +0xc7
compress/flate.(*Writer).Write(0xc2080cd0e0, 0xc20809e000, 0x10001, 0x10001, 0x5c8c70, 0x0, 0x0)
	/usr/local/go/src/compress/flate/deflate.go:534 +0x5f
compress/zlib.(*Writer).Write(0xc2080c44e0, 0xc20809e000, 0x10001, 0x10001, 0x0, 0x0, 0x0)
	/usr/local/go/src/compress/zlib/writer.go:156 +0x119
bytes.(*Reader).WriteTo(0xc2082dc9c0, 0x5c8f78, 0xc2080c44e0, 0x5c8fa0, 0x0, 0x0)
	/usr/local/go/src/bytes/reader.go:131 +0xda
io.Copy(0x5c8f78, 0xc2080c44e0, 0x5c86d8, 0xc2082dc9c0, 0x0, 0x0, 0x0)
	/usr/local/go/src/io/io.go:354 +0xb2
github.com/luci/luci-go/client/isolatedclient.func·006()
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:262 +0x14f
created by github.com/luci/luci-go/client/isolatedclient.(*compressed).reset
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:267 +0x170

goroutine 77 [IO wait]:
net.(*pollDesc).Wait(0xc2080d2760, 0x72, 0x0, 0x0)
	/usr/local/go/src/net/fd_poll_runtime.go:84 +0x47
net.(*pollDesc).WaitRead(0xc2080d2760, 0x0, 0x0)
	/usr/local/go/src/net/fd_poll_runtime.go:89 +0x43
net.(*netFD).accept(0xc2080d2700, 0x0, 0x5c3e10, 0xc208346318)
	/usr/local/go/src/net/fd_unix.go:419 +0x40b
net.(*TCPListener).AcceptTCP(0xc20811e108, 0x5c8688, 0x0, 0x0)
	/usr/local/go/src/net/tcpsock_posix.go:234 +0x4e
net.(*TCPListener).Accept(0xc20811e108, 0x0, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/tcpsock_posix.go:244 +0x4c
net/http/httptest.(*historyListener).Accept(0xc20803ba10, 0x0, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/http/httptest/server.go:48 +0x7a
net/http.(*Server).Serve(0xc208030480, 0x5c8330, 0xc20803ba10, 0x0, 0x0)
	/usr/local/go/src/net/http/server.go:1728 +0x92
created by net/http/httptest.(*Server).Start
	/usr/local/go/src/net/http/httptest/server.go:109 +0x35e

goroutine 28 [semacquire]:
sync.(*WaitGroup).Wait(0xc2080be8d0)
	/usr/local/go/src/sync/waitgroup.go:132 +0x169
github.com/luci/luci-go/client/isolatedclient.(*compressed).Close(0xc2080be8c0, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:231 +0xae
github.com/luci/luci-go/client/isolatedclient.(*compressed).Seek(0xc2080be8c0, 0x0, 0x0, 0xc2080db910, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:240 +0x6f
github.com/luci/luci-go/client/internal/lhttp.(*retriable).Do(0xc20802e550, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/internal/lhttp/client.go:167 +0x8a
github.com/luci/luci-go/client/internal/retry.(*Config).Do(0x4ea260, 0x5c84e0, 0xc20802e550, 0x5c9940, 0xc2083f8060)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/internal/retry/retry.go:38 +0xdc
github.com/luci/luci-go/client/isolatedclient.(*isolateServer).doPushGCS(0xc208078900, 0xc2080c4300, 0x5c8f18, 0xc2082dc9c0, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:206 +0x285
github.com/luci/luci-go/client/isolatedclient.(*isolateServer).doPush(0xc208078900, 0xc2080c4300, 0x5c8f18, 0xc2082dc9c0, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:158 +0x361
github.com/luci/luci-go/client/isolatedclient.(*isolateServer).Push(0xc208078900, 0xc2080c4300, 0x5c8f18, 0xc2082dc9c0, 0x0, 0x0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient.go:125 +0xb9
github.com/luci/luci-go/client/isolatedclient.TestIsolateServerRetryGCSPartial(0xc2080c62d0)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient_test.go:81 +0x6f8
testing.tRunner(0xc2080c62d0, 0x505768)
	/usr/local/go/src/testing/testing.go:447 +0xbf
created by testing.RunTests
	/usr/local/go/src/testing/testing.go:555 +0xa8b

goroutine 29 [chan receive]:
testing.func·007()
	/usr/local/go/src/testing/testing.go:559 +0x5f
created by testing.RunTests
	/usr/local/go/src/testing/testing.go:560 +0xb9b

goroutine 30 [chan receive]:
testing.(*T).Parallel(0xc2080c6360)
	/usr/local/go/src/testing/testing.go:413 +0x7f
github.com/luci/luci-go/client/isolatedclient.TestIsolateServerBadURL(0xc2080c6360)
	/b/build/slave/luci-go-osx/build/infra/go/src/github.com/luci/luci-go/client/isolatedclient/isolatedclient_test.go:98 +0x33
testing.tRunner(0xc2080c6360, 0x505780)
	/usr/local/go/src/testing/testing.go:447 +0xbf
created by testing.RunTests
	/usr/local/go/src/testing/testing.go:555 +0xa8b

goroutine 31 [chan receive]:
testing.func·007()
	/usr/local/go/src/testing/testing.go:559 +0x5f
created by testing.RunTests
	/usr/local/go/src/testing/testing.go:560 +0xb9b
FAIL	github.com/luci/luci-go/client/isolatedclient	0.118s


Full log: https://luci-logdog.appspot.com/v/?s=bb%2Fchromium.infra%2Fluci-go-osx%2F521%2F%2B%2Frecipes%2Fsteps%2Fgo_test%2F0%2Fstdout

Build: https://build.chromium.org/p/chromium.infra/builders/luci-go-osx/builds/521