Fixes for reading journald logs #38859

Clean up a deferred function call in the journal reading logic. Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Minor code simplification. Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

In case Tail=N parameter is requested, we need to show N lines. It does not make sense to walk backwards one by one if we can do it at once. Now, if Since=T is also provided, make sure we haven't jumped too far (before T), and if we did, move forward. The primary motivation for this was to make the code simpler. This also fixes a tiny bug in the "since" implementation. Before this commit: > $ docker logs -t --tail=6000 --since="2019-03-10T03:54:25.00" $ID | head > 2019-03-10T03:54:24.999821000Z 95981 After: > $ docker logs -t --tail=6000 --since="2019-03-10T03:54:25.00" $ID | head > 2019-03-10T03:54:25.000013000Z 95982 Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

In case the LogConsumer is gone, the code that sends the message can stuck forever. Wrap the code in select case, as all other loggers do. Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

1. The journald client library initializes inotify watch(es) during the first call to sd_journal_get_fd(), and it make sense to open it earlier in order to not lose any journal file rotation events. 2. It only makes sense to call this if we're going to use it later on -- so add a check for config.Follow. 3. Remove the redundant call to sd_journal_get_fd(). NOTE that any subsequent calls to sd_journal_get_fd() return the same file descriptor, so there's no real need to save it for later use in wait_for_data_cancelable(). Based on earlier patch by Nalin Dahyabhai <nalin@redhat.com>. Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

TL;DR: simplify the code, fix --follow hanging indefinitely Do the following to simplify the followJournal() code: 1. Use Go-native select instead of C-native polling. 2. Use Watch{Producer,Consumer}Gone(), eliminating the need to have journald.closed variable, and an extra goroutine. 3. Use sd_journal_wait(). In the words of its own man page: > A synchronous alternative for using sd_journal_get_fd(), > sd_journal_get_events(), sd_journal_get_timeout() and > sd_journal_process() is sd_journal_wait(). Unfortunately, the logic is still not as simple as it could be; the reason being, once the container has exited, journald might still be writing some logs from its internal buffers onto journal file(s), and there is no way to figure out whether it's done so we are guaranteed to read all of it back. This bug can be reproduced with something like > $ ID=$(docker run -d busybox seq 1 150000); docker logs --follow $ID > ... > 128123 > $ (The last expected output line should be `150000`). To avoid exiting from followJournal() early, add the following logic: once the container is gone, keep trying to drain the journal until there's no new data for at least `waitTimeout` time period. Should fix docker/for-linux#575 Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

If we take a long time to process log messages, and during that time journal file rotation occurs, the journald client library will keep those rotated files open until sd_journal_process() is called. By periodically calling sd_journal_process() during the processing loop we shrink the window of time a client instance has open file descriptors for rotated (deleted) journal files. This code is modelled after that of journalctl [1]; the above explanation as well as the value of 1024 is taken from there. [v2: fix CErr() argument] [1] https://github.com/systemd/systemd/blob/dc16327c48d/src/journal/journalctl.c#L2676 Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

From the first glance, `docker logs --tail 0` does not make sense, as it is supposed to produce no output, but `tail -n 0` from GNU coreutils is working like that, plus there is even a test case (`TestLogsTail` in integration-cli/docker_cli_logs_test.go). Now, something like `docker logs --follow --tail 0` makes total sense, so let's make it work. (NOTE if --tail is not used, config.Tail is set to -1) Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

1. Use "in-place" variables for if statements to limit their scope to the respectful `if` block. 2. Report the error returned from sd_journal_* by using CErr(). 3. Use errors.New() instead of fmt.Errorf(). Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

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Fixes for reading journald logs #38859

Fixes for reading journald logs #38859

Commits on Aug 2, 2019