cmd/go/internal/lockedfile: TestSpuriousEDEADLK flaky on plan9-arm since Oct. 7

[bcmills]

I'm not sure whether this is due to a change in the builder or a change in the runtime or standard library, but lockedfile.TestSpuriousEDEADLK has been failing sporadically on the plan9-arm builder, apparently due to the failure of a *lockedfile.File to provide the expected mutual-exclusion when open for writing.

The first such failure occurred at CL 260137, but it's not obvious to me how that could be related. (The plan9-386-0intro builder started working again around then — do the -386-0intro and -arm builders share an underlying filesystem?)

CC @millerresearch @fhs @0intro

--- FAIL: TestSpuriousEDEADLK (0.04s)
    lockedfile_test.go:255: Edit B unexpectedly did not block

2020-10-12T18:31:36-8994607/plan9-arm
2020-10-12T18:31:29-112c4d5/plan9-arm
2020-10-12T18:31:10-7b77ff4/plan9-arm
2020-10-12T18:31:01-5acec48/plan9-arm
2020-10-12T16:12:25-349a287/plan9-arm
2020-10-10T00:55:54-d317ba5/plan9-arm
2020-10-09T02:49:19-2be7788/plan9-arm
2020-10-09T02:14:32-8f26b57/plan9-arm
2020-10-09T01:09:06-f8df205/plan9-arm
2020-10-07T20:18:43-3f7b4d1/plan9-arm
2020-10-07T18:56:58-5c1567c/plan9-arm
2020-10-07T14:57:54-67edc0e/plan9-arm

[0intro]

While the plan9/386 builder is currently running a local ramfs (an "in memory" file system), I think the plan9/arm builders import their file system from a remote file server, so it may behave differently.

[millerresearch]

In this test the file being locked is in /tmp, which the plan9_arm builders also mount on ramfs.

[bcmills]

greplogs --dashboard -md -l -e 'FAIL: TestSpuriousEDEADLK' --since=2021-01-01

2021-10-04T22:00:19-739328c/plan9-arm
2021-09-23T21:59:30-4e308d7/plan9-arm
2021-09-17T18:51:48-7f36ef0/plan9-arm
2021-09-15T00:04:48-cb4e1de/plan9-arm
2021-09-07T22:23:36-a9a01a3/plan9-arm
2021-09-07T03:56:13-6226020/plan9-arm
2021-09-02T16:15:00-d01388b/plan9-arm
2021-06-21T16:58:36-16e82be/plan9-arm
2021-06-21T14:57:47-85a2e24/plan9-arm
2021-06-16T14:23:50-a6a853f/plan9-arm
2021-06-15T20:59:42-d77f4c0/plan9-arm
2021-06-07T21:18:03-dc8b558/plan9-arm
2021-06-04T17:33:24-831f937/plan9-arm
2021-05-21T23:46:07-f87194c/plan9-arm
2021-05-17T16:02:12-b1aff42/plan9-arm
2021-05-14T19:27:25-02699f8/plan9-arm
2021-02-01T16:54:54-0b6cfea/plan9-arm
2021-02-01T15:52:59-26e29aa/plan9-arm
2021-01-26T00:02:49-cf263e9/plan9-arm
2021-01-25T19:40:02-54514c6/plan9-arm
2021-01-22T23:03:58-25c39e4/plan9-arm
2021-01-22T21:16:10-a2cef9b/plan9-arm
2021-01-22T18:58:08-dab3e5a/plan9-arm
2021-01-22T16:23:59-7ece3a7/plan9-arm
2021-01-21T20:47:45-e2b4f1f/plan9-arm
2021-01-21T19:15:21-3c2f11b/plan9-arm
2021-01-21T19:08:04-46e2e2e/plan9-arm
2021-01-20T22:16:54-ecf4ebf/plan9-arm
2021-01-20T17:37:54-6e243ce/plan9-arm
2021-01-20T14:59:52-be28e5a/plan9-arm
2021-01-14T21:55:29-eb33002/plan9-arm
2021-01-14T17:23:11-d9b79e5/plan9-arm
2021-01-13T23:37:31-7eb31d9/plan9-arm
2021-01-12T21:39:59-ba76567/plan9-arm
2021-01-11T21:34:46-81ea89a/plan9-arm
2021-01-08T17:08:44-e65c543/plan9-arm
2021-01-08T01:33:44-54bd1cc/plan9-arm
2021-01-07T22:28:57-9b55088/plan9-arm
2021-01-07T18:56:09-7cee66d/plan9-arm
2021-01-05T20:54:17-1b85e7c/plan9-arm

[millerresearch]

May be related to #48416 which also involves locking a file on /tmp.

I'll try moving /tmp off ramfs on the plan9-arm builders for a while and see whether the failures stop.