pkg/systemd: don't require LISTEN_FDNAMES for socket activation

LISTEN_FDNAMES is optional, the docs for sd_listen_fds() says:

    This information is read from the $LISTEN_FDNAMES variable, which
    **may** contain a colon-separated list of names.

emphasis mine (indeed, the cited coreos code also suggests it is optional).

This actually results in bug, since the default
/contrib/systemd/system/podman.socket file doesn't set a
FileDescriptorName=. podman when run with this systemd configuration
*always* starts in unix socket mode since SocketActivated() will return
false because the name is missing.

The bug is a race with a very small window: between when podman does the
unlink() and when it re-binds the socket later in the code, requests made
during this time will fail since nothing is listening. There's another
small race when the service stops and systemd realizes it and starts
listening again.

However, small this window we managed to hit it :).

Let's fix this by ignoring LISTEN_FDNAMES. Since the code in
cmd/podman/system/service_abi.go:restService() ignores this value anyway
when setting up the socket activated stuff, there's no real loss here.

Signed-off-by: Tycho Andersen <tycho@tycho.pizza>

pkg/systemd/activation.go

@@ -25,11 +25,5 @@ func SocketActivated() bool {
 	if err != nil || nfds == 0 {
 		return false
 	}
-
-	// "github.com/coreos/go-systemd/v22/activation" will use and validate this variable's
-	// value. We're just providing a fast fail
-	if _, found = os.LookupEnv("LISTEN_FDNAMES"); !found {
-		return false
-	}
 	return true
 }

pkg/systemd/activation_test.go

@@ -0,0 +1,32 @@
+package systemd
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSocketActivated(t *testing.T) {
+	assert := assert.New(t)
+
+	assert.False(SocketActivated())
+
+	// different pid
+	assert.NoError(os.Setenv("LISTEN_PID", "1"))
+	assert.False(SocketActivated())
+
+	// same pid no fds
+	assert.NoError(os.Setenv("LISTEN_PID", fmt.Sprintf("%d", os.Getpid())))
+	assert.NoError(os.Setenv("LISTEN_FDS", "0"))
+	assert.False(SocketActivated())
+
+	// same pid some fds
+	assert.NoError(os.Setenv("LISTEN_FDS", "1"))
+	assert.True(SocketActivated())
+
+	// FDNAME is ok too (but not required)
+	assert.NoError(os.Setenv("LISTEN_FDNAMES", "/meshuggah/rocks"))
+	assert.True(SocketActivated())
+}