kubelet: Expose simple journald and Get-WinEvent shims on the logs endpoint

Provide an administrator a streaming view of journal logs on Linux
systems using journalctl, and event logs on Windows systems using the
Get-WinEvent PowerShell cmdlet without them having to implement a client
side reader.

Only available to cluster admins.

The implementation for journald on Linux was originally done by Clayton
Coleman.

Introduce a heuristics approach to query logs

The logs query for node objects will follow a heuristics approach
when asked to query for logs from a service. If asked to get the
logs from a service foobar, it will first check if foobar logs to the
native OS service log provider. If unable to get logs from these, it
will attempt to get logs from /var/foobar, /var/log/foobar.log or
/var/log/foobar/foobar.log in that order.
The logs sub-command can also directly serve a file if the query looks
like a file.

Co-authored-by: Clayton Coleman <ccoleman@redhat.com>
Co-authored-by: Christian Glombek <cglombek@redhat.com>

pkg/kubelet/kubelet_server_journal_linux.go

@@ -0,0 +1,74 @@
+//go:build linux
+
+/*
+Copyright 2022 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package kubelet
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// getLoggingCmd returns the journalctl cmd and arguments for the given nodeLogQuery and boot. Note that
+// services are explicitly passed here to account for the heuristics
+func getLoggingCmd(n *nodeLogQuery, services []string) (string, []string, error) {
+	args := []string{
+		"--utc",
+		"--no-pager",
+		"--output=short-precise",
+	}
+	if n.SinceTime != nil {
+		args = append(args, fmt.Sprintf("--since=%s", n.SinceTime.Format(dateLayout)))
+	}
+	if n.UntilTime != nil {
+		args = append(args, fmt.Sprintf("--until=%s", n.SinceTime.Format(dateLayout)))
+	}
+	if n.TailLines != nil {
+		args = append(args, "--pager-end", fmt.Sprintf("--lines=%d", *n.TailLines))
+	}
+	for _, service := range services {
+		if len(service) > 0 {
+			args = append(args, "--unit="+service)
+		}
+	}
+	if len(n.Pattern) > 0 {
+		args = append(args, "--grep="+n.Pattern)
+	}
+
+	if n.Boot != nil {
+		args = append(args, "--boot", fmt.Sprintf("%d", *n.Boot))
+	}
+
+	return "journalctl", args, nil
+}
+
+// checkForNativeLogger checks journalctl output for a service
+func checkForNativeLogger(ctx context.Context, service string) bool {
+	// This will return all the journald units
+	cmd := exec.CommandContext(ctx, "journalctl", []string{"--field", "_SYSTEMD_UNIT"}...)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		// Returning false to allow checking if the service is logging to a file
+		return false
+	}
+
+	// journalctl won't return an error if we try to fetch logs for a non-existent service,
+	// hence we search for it in the list of services known to journalctl
+	return strings.Contains(string(output), service+".service")
+}