daemon: add SdNotifyMonotonicUsec helper function

daemon/sdnotify_linux_test.go

@@ -0,0 +1,78 @@
+// Copyright 2025
+//
+// 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 daemon
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+// TestSdNotifyMonotonicUsec checks that SdNotifyMonotonicUsec is probably not returning complete garbage.
+func TestSdNotifyMonotonicUsec(t *testing.T) {
+	var resolution unix.Timespec
+	if err := unix.ClockGetres(unix.CLOCK_MONOTONIC, &resolution); err != nil {
+		if err == unix.EINVAL {
+			t.Log("CLOCK_MONOTONIC is not supported on this system")
+			if got := SdNotifyMonotonicUsec(); got != "" {
+				t.Errorf("SdNotifyMonotonicUsec() = %q; want empty string", got)
+			}
+			return
+		}
+		t.Fatalf("ClockGetres(CLOCK_MONOTONIC) failed: %v", err)
+	}
+
+	now := func() uint64 {
+		got := SdNotifyMonotonicUsec()
+		t.Logf("SdNotifyMonotonicUsec() = %q", got)
+		if got == "" {
+			t.Fatal("SdNotifyMonotonicUsec() returned empty string on system which supports CLOCK_MONOTONIC")
+		}
+		fields := strings.SplitN(got, "=", 2)
+		if len(fields) != 2 {
+			t.Fatal("string is not a well-formed variable assignment")
+		}
+		tag, val := fields[0], fields[1]
+		if tag != "MONOTONIC_USEC" {
+			t.Errorf("expected tag MONOTONIC_USEC, got %q", tag)
+		}
+		if val[len(val)-1] != '\n' {
+			t.Errorf("expected value to end with newline, got %q", val)
+		}
+		ts, err := strconv.ParseUint(val[:len(val)-1], 10, 64)
+		if err != nil {
+			t.Fatalf("value %q is not well-formed: %v", val, err)
+		}
+		if ts == 0 {
+			// CLOCK_MONOTONIC is defined on Linux as the number of seconds
+			// since boot, per clock_gettime(2). A timestamp of zero is
+			// almost certainly bogus.
+			t.Fatal("timestamp is zero")
+		}
+		return ts
+	}
+
+	start := now()
+	time.Sleep(time.Duration(resolution.Nano()) * 3)
+	ts := now()
+	if ts < start {
+		t.Errorf("timestamp went backwards: %d < %d", ts, start)
+	} else if ts == start {
+		t.Errorf("timestamp did not advance: %d == %d", ts, start)
+	}
+}

daemon/sdnotify_other.go

@@ -0,0 +1,22 @@
+// Copyright 2025
+//
+// 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
+
+package daemon
+
+// SdNotifyMonotonicUsec returns the empty string on unsupported platforms.
+func SdNotifyMonotonicUsec() string {
+	return ""
+}

daemon/sdnotify_unix.go

@@ -0,0 +1,36 @@
+// Copyright 2025
+//
+// 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
+
+package daemon
+
+import (
+	"strconv"
+
+	"golang.org/x/sys/unix"
+)
+
+// SdNotifyMonotonicUsec returns a MONOTONIC_USEC=... assignment for the current time
+// with a trailing newline included. This is typically used with [SdNotifyReloading].
+//
+// If the monotonic clock is not available on the system, the empty string is returned.
+func SdNotifyMonotonicUsec() string {
+	var ts unix.Timespec
+	if err := unix.ClockGettime(unix.CLOCK_MONOTONIC, &ts); err != nil {
+		// Monotonic clock is not available on this system.
+		return ""
+	}
+	return "MONOTONIC_USEC=" + strconv.FormatInt(ts.Nano()/1000, 10) + "\n"
+}

go.mod

@@ -2,4 +2,7 @@ module github.com/coreos/go-systemd/v22
 
 go 1.23
 
-require github.com/godbus/dbus/v5 v5.1.0
+require (
+	github.com/godbus/dbus/v5 v5.1.0
+	golang.org/x/sys v0.1.0
+)

go.sum

@@ -1,2 +1,4 @@
 github.com/godbus/dbus/v5 v5.1.0 h1:4KLkAxT3aOY8Li4FRJe/KvhoNFFxo0m6fNuFUO8QJUk=
 github.com/godbus/dbus/v5 v5.1.0/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
+golang.org/x/sys v0.1.0 h1:kunALQeHf1/185U1i0GOB/fy1IPRDDpuoOOqRReG57U=
+golang.org/x/sys v0.1.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=