net: TestLookupCNAME failures on plan9-arm with "open /net/dns: '/net/dns' cs: file does not exist"

[bcmills]

--- FAIL: TestLookupCNAME (36.02s)
    lookup_test.go:371: backoff 1s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:371: backoff 5s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:371: backoff 30s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:377: open /net/dns: '/net/dns' cs: file does not exist
--- FAIL: TestLookupDotsWithRemoteSource (0.00s)
    lookup_test.go:661: LookupAddr(8.8.8.8): open /net/dns: '/net/dns' cs: file does not exist (mode=go)
    lookup_test.go:674: LookupCNAME(www.mit.edu, mode=go): open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:682: LookupMX(google.com): open /net/dns: '/net/dns' cs: file does not exist (mode=go)
    lookup_test.go:695: LookupNS(google.com): open /net/dns: '/net/dns' cs: file does not exist (mode=go)
    lookup_test.go:708: LookupSRV(xmpp-server, tcp, google.com): open /net/dns: '/net/dns' cs: file does not exist (mode=go)
--- FAIL: TestLookupGoogleSRV (36.03s)
    lookup_test.go:94: backoff 1s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:94: backoff 5s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:94: backoff 30s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:100: open /net/dns: '/net/dns' cs: file does not exist
--- FAIL: TestLookupGmailNS (36.03s)
    lookup_test.go:189: backoff 1s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:189: backoff 5s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:189: backoff 30s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:195: open /net/dns: '/net/dns' cs: file does not exist
--- FAIL: TestLookupGmailMX (36.03s)
    lookup_test.go:143: backoff 1s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:143: backoff 5s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:143: backoff 30s after failure open /net/dns: '/net/dns' cs: file does not exist
    lookup_test.go:149: open /net/dns: '/net/dns' cs: file does not exist
FAIL
FAIL	net	106.874s

greplogs --dashboard -md -l -e '(?ms)\Aplan9-.*FAIL: TestLookupCNAME'

2021-05-12T16:18:01-07ff596/plan9-arm
2021-05-10T13:16:56-2870259/plan9-arm
2021-05-07T16:08:24-9bfa6f7/plan9-arm
2021-04-29T23:31:23-1df309e/plan9-arm
2021-04-29T17:47:48-6d95e5a/plan9-arm
2021-04-27T08:39:25-e0815d0/plan9-arm
2021-04-20T18:34:56-77860ad/plan9-arm
2021-04-19T14:42:58-a9c244a/plan9-arm
2021-04-14T04:02:01-6d8ba77/plan9-arm
2021-04-08T06:08:49-2123dfb/plan9-arm
2021-03-30T19:44:38-1318fb4/plan9-arm
2021-03-25T19:24:25-569c86d/plan9-arm
2020-12-25T23:54:24-2018b68/plan9-arm
2020-11-30T03:18:36-e5da18d/plan9-arm
2020-10-21T00:22:37-cfb087c/plan9-arm
2020-10-05T15:24:30-6f2c92e/plan9-arm
2020-09-28T08:16:57-05b626e/plan9-arm
2020-09-21T21:03:02-5853b4e/plan9-arm
2020-09-19T09:43:15-a386802/plan9-arm
2020-09-16T17:24:33-4f91591/plan9-arm
2020-09-16T17:13:14-1632851/plan9-arm
2020-09-15T12:46:25-03875bd/plan9-arm
2020-09-11T13:53:03-b22af9b/plan9-arm
2020-09-06T05:12:37-b60ec4c/plan9-arm
2020-08-24T16:53:02-fb5c3ea/plan9-arm
2020-07-13T17:48:55-e88ea87/plan9-arm
2020-07-07T03:43:31-6b34417/plan9-arm
2019-12-05T22:08:26-e751af1/plan9-arm
2019-12-05T20:14:44-d72dce8/plan9-arm
2019-11-27T15:06:55-4e3d580/plan9-arm
2019-11-27T04:05:12-22688f7/plan9-arm
2019-11-26T12:03:08-20bf6a4/plan9-arm
2019-11-15T20:56:35-9af8794/plan9-arm
2019-11-15T16:02:32-3bea90d/plan9-arm
2019-11-14T17:43:48-d3c2b1f/plan9-arm
2019-11-13T01:15:54-43ec1b1/plan9-arm
2019-11-12T05:24:00-d22b573/plan9-arm
2019-11-12T01:09:40-c2edcf4/plan9-arm
2019-11-08T18:00:54-a2cd2bd/plan9-arm
2019-11-06T16:17:30-a5936a4/plan9-arm
2019-11-03T01:44:46-d2c039f/plan9-arm
2019-11-02T21:51:12-2d031dc/plan9-arm
2019-11-01T14:19:56-8405cd3/plan9-arm
2019-10-31T15:14:45-02a5502/plan9-arm
2019-10-31T09:49:57-1085560/plan9-arm
2019-10-30T16:37:41-cf6e6ab/plan9-arm
2019-10-26T08:29:23-5d000a8/plan9-arm
2019-10-26T05:40:14-c3e8a20/plan9-arm
2019-10-26T02:52:28-8714e39/plan9-arm
2019-10-26T02:52:25-334291d/plan9-arm
2019-10-26T02:36:26-3706cd8/plan9-arm
2019-10-26T00:05:37-8031532/plan9-arm
2019-10-25T23:25:33-36a432f/plan9-arm
2019-10-22T15:15:06-95544cc/plan9-arm

[bcmills]

CC @millerresearch @0intro @fhs

(See previously #21335, #18391.)

[millerresearch]

Possibly a race condition on the builder, starting the reverse buildlet before the dns service is mounted into its namespace. I'll try adding a synchronisation in the buildlet script to make sure this doesn't happen.

[ianlancetaylor]

Hasn't happened since 2021-12-07. Calling this fixed.