logger: fix follow logs for max-file=1

[kolyshkin]

In case jsonlogfile is used with max-file=1 and max-size set,
the log rotation is not performed; instead, the log file is closed
and re-opened with O_TRUNC.

This situation is not handled by the log reader in follow mode,
leading to an issue of log reader being stuck forever.

This situation (file close/reopen) could be handled in waitRead(),
but fsnotify library chose to not listen to or deliver this event
(IN_CLOSE_WRITE in inotify lingo) as it's not portable,
see fsnotify/fsnotify#252.

So, we have to handle this by checking the file size upon receiving
io.EOF from the log reader, and comparing the size with the one received
earlier. In case the new size is less than the old one, the file was
truncated and we need to seek to its beginning.

Fixes #39235. See the steps to verify in that issue.

Description for the changelog

jsonfile logger: Fix lollow logs stuck when max-size is set and max-file=1

[kolyshkin]

Did some manual checking using a locally compiled daemon and
cd integration-cli; go test -v -run TestDockerSuite/TestLogs .

Got the same failures before and after the commit, meaning this PR is not breaking anything (still looking at those failures...)

Update: the above was mistakenly done using journald log driver :) Using json-file reveals no failures with or without this PR

[kolyshkin]

kir@kd:~/go/src/github.com/docker/docker/integration-cli$ go test -v -run TestDockerSuite/TestLogs .
INFO: Testing against a local daemon
=== RUN   TestDockerSuite
=== RUN   TestDockerSuite/TestLogsAPIContainerNotFound
=== RUN   TestDockerSuite/TestLogsAPIFollowEmptyOutput
=== RUN   TestDockerSuite/TestLogsAPINoStdoutNorStderr
=== RUN   TestDockerSuite/TestLogsAPIUntil
=== RUN   TestDockerSuite/TestLogsAPIUntilDefaultValue
=== RUN   TestDockerSuite/TestLogsAPIUntilFutureFollow
=== RUN   TestDockerSuite/TestLogsAPIWithStdout
=== RUN   TestDockerSuite/TestLogsCLIContainerNotFound
=== RUN   TestDockerSuite/TestLogsContainerBiggerThanPage
=== RUN   TestDockerSuite/TestLogsContainerMuchBiggerThanPage
=== RUN   TestDockerSuite/TestLogsContainerSmallerThanPage
=== RUN   TestDockerSuite/TestLogsFollowGoroutinesNoOutput
=== RUN   TestDockerSuite/TestLogsFollowGoroutinesWithStdout
=== RUN   TestDockerSuite/TestLogsFollowSlowStdoutConsumer
=== RUN   TestDockerSuite/TestLogsFollowStopped
=== RUN   TestDockerSuite/TestLogsSeparateStderr
=== RUN   TestDockerSuite/TestLogsSince
=== RUN   TestDockerSuite/TestLogsSinceFutureFollow
=== RUN   TestDockerSuite/TestLogsStderrInStdout
=== RUN   TestDockerSuite/TestLogsTail
=== RUN   TestDockerSuite/TestLogsTimestamps
=== RUN   TestDockerSuite/TestLogsWithDetails
--- PASS: TestDockerSuite (48.19s)
    --- PASS: TestDockerSuite/TestLogsAPIContainerNotFound (0.21s)
    --- PASS: TestDockerSuite/TestLogsAPIFollowEmptyOutput (1.17s)
    --- PASS: TestDockerSuite/TestLogsAPINoStdoutNorStderr (0.99s)
    --- PASS: TestDockerSuite/TestLogsAPIUntil (4.16s)
    --- PASS: TestDockerSuite/TestLogsAPIUntilDefaultValue (1.16s)
    --- PASS: TestDockerSuite/TestLogsAPIUntilFutureFollow (5.09s)
    --- PASS: TestDockerSuite/TestLogsAPIWithStdout (1.15s)
    --- PASS: TestDockerSuite/TestLogsCLIContainerNotFound (0.22s)
    --- PASS: TestDockerSuite/TestLogsContainerBiggerThanPage (1.14s)
    --- PASS: TestDockerSuite/TestLogsContainerMuchBiggerThanPage (1.33s)
    --- PASS: TestDockerSuite/TestLogsContainerSmallerThanPage (1.19s)
    --- PASS: TestDockerSuite/TestLogsFollowGoroutinesNoOutput (1.48s)
    --- PASS: TestDockerSuite/TestLogsFollowGoroutinesWithStdout (1.38s)
    --- PASS: TestDockerSuite/TestLogsFollowSlowStdoutConsumer (1.98s)
    --- PASS: TestDockerSuite/TestLogsFollowStopped (1.12s)
    --- PASS: TestDockerSuite/TestLogsSeparateStderr (1.10s)
    --- PASS: TestDockerSuite/TestLogsSince (7.13s)
    --- PASS: TestDockerSuite/TestLogsSinceFutureFollow (5.99s)
    --- PASS: TestDockerSuite/TestLogsStderrInStdout (1.13s)
    --- PASS: TestDockerSuite/TestLogsTail (1.36s)
    --- PASS: TestDockerSuite/TestLogsTimestamps (6.46s)
    --- PASS: TestDockerSuite/TestLogsWithDetails (1.03s)
PASS
ok  	github.com/docker/docker/integration-cli	48.238s

[thaJeztah]

SGTM

should we have a test for this?

[thaJeztah]

ah, I see your comment above; guess that means we can't add a test?

[thaJeztah]

@cpuguy83 ptal

[cpuguy83]

LGTM

[cpuguy83]

I think a test that's actually testing the problem would be rather difficult.
Maybe if the logger code was instrumented for the test it could work but even then it still seems like it would be flaky.