journal: add StderrIsJournalStream function

This function can be used for automatic protocol upgrade described in
[1].

Both unit tests and runnable example are included. Only the latter
requires systemd, as unit tests are self-sufficient, and only test
that JOURNAL_STREAM environment variable is checked properly.

[1] https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading

examples/journal/main.go

@@ -0,0 +1,37 @@
+// Copyright 2022 CoreOS, Inc.
+//
+// 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 main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/coreos/go-systemd/v22/journal"
+)
+
+func main() {
+	ok, err := journal.StderrIsJournalStream()
+	if err != nil {
+		panic(err)
+	}
+
+	if ok {
+		// use journal native protocol
+		journal.Send("this is a message logged through the native protocol", journal.PriInfo, nil)
+	} else {
+		// use stderr
+		fmt.Fprintln(os.Stderr, "this is a message logged through stderr")
+	}
+}

examples/journal/run.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -e
+
+go build
+
+echo "Running directly"
+./journal
+
+echo "Running through systemd"
+unit_name="run-$(systemd-id128 new)"
+systemd-run -u "$unit_name" --user --wait --quiet ./journal
+journalctl --user -u "$unit_name"

journal/journal_unix.go

@@ -69,6 +69,44 @@ func Enabled() bool {
 	return true
 }
 
+// StderrIsJournalStream returns whether the process stderr is connected
+// to the Journal's stream transport.
+//
+// This can be used for automatic protocol upgrading described in [Journal Native Protocol].
+//
+// Returns true if JOURNAL_STREAM environment variable is present,
+// and stderr's device and inode numbers match it.
+//
+// Error is returned if unexpected error occurs: e.g. if JOURNAL_STREAM environment variable
+// is present, but malformed, fstat syscall fails, etc.
+//
+// [Journal Native Protocol]: https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading
+func StderrIsJournalStream() (res bool, err error) {
+	journalStream := os.Getenv("JOURNAL_STREAM")
+	if journalStream == "" {
+		return
+	}
+
+	var expectedStat syscall.Stat_t
+	_, err = fmt.Sscanf(journalStream, "%d:%d", &expectedStat.Dev, &expectedStat.Ino)
+	if err != nil {
+		err = fmt.Errorf("failed to parse JOURNAL_STREAM=%q: %w", journalStream, err)
+		return
+	}
+
+	var stat syscall.Stat_t
+	err = syscall.Fstat(syscall.Stderr, &stat)
+	if err != nil {
+		return
+	}
+
+	if stat.Dev != expectedStat.Dev || stat.Ino != expectedStat.Ino {
+		return
+	}
+
+	return true, nil
+}
+
 // Send a message to the local systemd journal. vars is a map of journald
 // fields to values.  Fields must be composed of uppercase letters, numbers,
 // and underscores, but must not start with an underscore. Within these

journal/journal_unix_test.go

@@ -0,0 +1,142 @@
+// Copyright 2022 CoreOS, Inc.
+//
+// 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.
+
+//go:build unix
+// +build unix
+
+package journal_test
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"testing"
+
+	"github.com/coreos/go-systemd/v22/journal"
+)
+
+func TestStderrIsJournalStream(t *testing.T) {
+	if _, ok := os.LookupEnv("JOURNAL_STREAM"); ok {
+		t.Fatal("unset JOURNAL_STREAM before running this test")
+	}
+
+	t.Run("Missing", func(t *testing.T) {
+		ok, err := journal.StderrIsJournalStream()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if ok {
+			t.Error("stderr shouldn't be connected to journal stream")
+		}
+	})
+	t.Run("Present", func(t *testing.T) {
+		f, stat := getUnixStreamSocket(t)
+		defer f.Close()
+		os.Setenv("JOURNAL_STREAM", fmt.Sprintf("%d:%d", stat.Dev, stat.Ino))
+		defer os.Unsetenv("JOURNAL_STREAM")
+		replaceStderr(int(f.Fd()), func() {
+			ok, err := journal.StderrIsJournalStream()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !ok {
+				t.Error("stderr should've been connected to journal stream")
+			}
+		})
+	})
+	t.Run("NotMatching", func(t *testing.T) {
+		f, stat := getUnixStreamSocket(t)
+		defer f.Close()
+		os.Setenv("JOURNAL_STREAM", fmt.Sprintf("%d:%d", stat.Dev+1, stat.Ino))
+		defer os.Unsetenv("JOURNAL_STREAM")
+		replaceStderr(int(f.Fd()), func() {
+			ok, err := journal.StderrIsJournalStream()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if ok {
+				t.Error("stderr shouldn't be connected to journal stream")
+			}
+		})
+	})
+	t.Run("Malformed", func(t *testing.T) {
+		f, stat := getUnixStreamSocket(t)
+		defer f.Close()
+		os.Setenv("JOURNAL_STREAM", fmt.Sprintf("%d-%d", stat.Dev, stat.Ino))
+		defer os.Unsetenv("JOURNAL_STREAM")
+		replaceStderr(int(f.Fd()), func() {
+			_, err := journal.StderrIsJournalStream()
+			if err == nil {
+				t.Fatal("JOURNAL_STREAM is malformed, but no error returned")
+			}
+		})
+	})
+}
+
+func ExampleStderrIsJournalStream() {
+	// NOTE: this is just an example. Production code
+	// will likely use this to setup a logging library
+	// to write messages to either journal or stderr.
+	ok, err := journal.StderrIsJournalStream()
+	if err != nil {
+		panic(err)
+	}
+
+	if ok {
+		// use journal native protocol
+		journal.Send("this is a message logged through the native protocol", journal.PriInfo, nil)
+	} else {
+		// use stderr
+		fmt.Fprintln(os.Stderr, "this is a message logged through stderr")
+	}
+}
+
+func replaceStderr(fd int, cb func()) {
+	savedStderr, err := syscall.Dup(syscall.Stderr)
+	if err != nil {
+		panic(err)
+	}
+	defer syscall.Close(savedStderr)
+	err = syscall.Dup2(fd, syscall.Stderr)
+	if err != nil {
+		panic(err)
+	}
+	defer func() {
+		err := syscall.Dup2(savedStderr, syscall.Stderr)
+		if err != nil {
+			panic(err)
+		}
+	}()
+	cb()
+}
+
+// getUnixStreamSocket returns a unix stream socket obtained with
+// socketpair(2), and its fstat result. Only one end of the socket pair
+// is returned, and the other end is closed immediately: we don't need
+// it for our purposes.
+func getUnixStreamSocket(t *testing.T) (*os.File, *syscall.Stat_t) {
+	fds, err := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_STREAM, 0)
+	if err != nil {
+		t.Fatal(os.NewSyscallError("socketpair", err))
+	}
+	// we don't need the remote end for our tests
+	syscall.Close(fds[1])
+
+	file := os.NewFile(uintptr(fds[0]), "unix-stream")
+	stat, err := file.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return file, stat.Sys().(*syscall.Stat_t)
+}