snap-exec: fix detection if cups interface is connected

* snap-exec: fix detection if `cups` interface is connected

The cups interface needs a way to set the environment variable
`CUPS_SERVER` if the `cups` interface is connected. However we
have no mechanism for setting environment vars based on interface
connections currently. To workaround this, the cups work added
a workaround in `snap-exec` that tried to detect if `cups` is
connected and when it is set the environment. Unfortunately the
code in there was too simplistic because it just checked if
the directory `/var/cups` exists. However ths dir now always
exists because it's needed as the mount point.

This commit fixes the detection by checking if `/var/cups` is
a bind mount. This is checked by looking at the `stat()` data
and the `dev_t` field in there. If they differ it means the
bind mount exists and the only thing that creates this bind
mount is the `cups` `MountConnectedPlug()` code.

This is not great but it fixes the spread failure we see
in the `cups-control` test (which is a real bug) and should
be good enough until we have a proper interface backend that
can set environment variables.

* tests: re-enable interfaces-cups-control test

For unclear reasons the error message on unplug changes when I
run this on my 20.04 system so I updated the tests - it most likely
because the test-snapd-cups-control-consumer snap moved from
core16 to core20 a week ago but that is not 100% confirmed.

* snap-exec: fix unit test for CUPS_SERVER workaround

cmd/snap-exec/export_test.go

@@ -19,6 +19,12 @@
 
 package main
 
+import (
+	"syscall"
+
+	"github.com/snapcore/snapd/testutil"
+)
+
 var (
 	ExpandEnvCmdArgs = expandEnvCmdArgs
 	FindCommand      = findCommand
@@ -58,3 +64,9 @@ func MockOsReadlink(f func(string) (string, error)) func() {
 		osReadlink = realOsReadlink
 	}
 }
+
+func MockSyscallStat(f func(string, *syscall.Stat_t) (err error)) func() {
+	r := testutil.Backup(&syscallStat)
+	syscallStat = f
+	return r
+}

cmd/snap-exec/main.go

@@ -36,6 +36,7 @@ import (
 
 // for the tests
 var syscallExec = syscall.Exec
+var syscallStat = syscall.Stat
 var osReadlink = os.Readlink
 
 // commandline args
@@ -199,10 +200,13 @@ func execApp(snapApp, revision, command string, args []string) error {
 	// variables to plugging snap apps, but this is a lot simpler as a
 	// work-around
 	// we currently only handle the CUPS_SERVER environment variable, setting it
-	// to /var/cups/ if that directory exists - it should not exist anywhere
+	// to /var/cups/ if that dir is a bind-mount - it should not be one
 	// except in a strictly confined snap where we setup the bind mount from the
-	// source cups slot snap to the plugging snap
-	if exists, _, _ := osutil.DirExists(dirs.GlobalRootDir + "/var/cups"); exists {
+	// source cups slot snap to the plugging snap.
+	var stVar, stVarCups syscall.Stat_t
+	err1 := syscallStat(dirs.GlobalRootDir+"/var/", &stVar)
+	err2 := syscallStat(dirs.GlobalRootDir+"/var/cups/", &stVarCups)
+	if err1 == nil && err2 == nil && stVar.Dev != stVarCups.Dev {
 		env["CUPS_SERVER"] = "/var/cups/cups.sock"
 	}
 

cmd/snap-exec/main_test.go

@@ -25,6 +25,8 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
+	"syscall"
 	"testing"
 
 	. "gopkg.in/check.v1"
@@ -193,14 +195,21 @@ func (s *snapExecSuite) TestSnapExecAppIntegrationCupsServerWorkaround(c *C) {
 		Revision: snap.R("42"),
 	})
 
-	// mock the /var/cups dir so that we observe CUPS_SERVER set
-	err := os.MkdirAll(filepath.Join(dir, "/var/cups"), 0755)
-	c.Assert(err, IsNil)
+	// mock the /var/cups dir is a bind-mount
+	restore := snapExec.MockSyscallStat(func(p string, st *syscall.Stat_t) error {
+		if strings.HasSuffix(p, "/var/cups/") {
+			st.Dev = 2
+		} else {
+			st.Dev = 1
+		}
+		return nil
+	})
+	defer restore()
 
 	execArgv0 := ""
 	execArgs := []string{}
 	execEnv := []string{}
-	restore := snapExec.MockSyscallExec(func(argv0 string, argv []string, env []string) error {
+	restore = snapExec.MockSyscallExec(func(argv0 string, argv []string, env []string) error {
 		execArgv0 = argv0
 		execArgs = argv
 		execEnv = env
@@ -209,7 +218,7 @@ func (s *snapExecSuite) TestSnapExecAppIntegrationCupsServerWorkaround(c *C) {
 	defer restore()
 
 	// launch and verify its run the right way
-	err = snapExec.ExecApp("snapname.app", "42", "stop", []string{"arg1", "arg2"})
+	err := snapExec.ExecApp("snapname.app", "42", "stop", []string{"arg1", "arg2"})
 	c.Assert(err, IsNil)
 	c.Check(execArgv0, Equals, fmt.Sprintf("%s/snapname/42/stop-app", dirs.SnapMountDir))
 	c.Check(execArgs, DeepEquals, []string{execArgv0, "arg1", "arg2"})

tests/main/interfaces-cups-control/task.yaml

@@ -72,4 +72,4 @@ execute: |
         echo "Expected error with plug disconnected"
         exit 1
     fi
-    MATCH "scheduler not responding" < print.error
+    MATCH "(scheduler not responding|No default destination)" < print.error