cmd,tests: fix classic confinement confusing re-execution code (#3598)

cmd/cmd.go

@@ -20,6 +20,7 @@
 package cmd
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -136,6 +137,7 @@ func InternalToolPath(tool string) string {
 	}
 
 	if !strings.HasPrefix(exe, dirs.SnapMountDir) {
+		logger.Noticef("exe doesn't have snap mount dir prefix: %q vs %q", exe, dirs.SnapMountDir)
 		return distroTool
 	}
 

tests/lib/snaps.sh

@@ -17,6 +17,10 @@ install_local_devmode() {
     install_local "$1" --devmode
 }
 
+install_local_classic() {
+    install_local "$1" --classic
+}
+
 # mksnap_fast creates a snap using a faster compress algorithm (gzip)
 # than the regular snaps (which are lzma)
 mksnap_fast() {

tests/main/interfaces-avahi-observe/task.yaml

@@ -16,7 +16,7 @@ prepare: |
         restart avahi-daemon
     else
         systemctl daemon-reload
-        systemctl restart avahi-daemon.{service,socket}
+        systemctl restart avahi-daemon.{socket,service}
     fi
 
 restore: |

tests/regression/lp-1704860/snap-env-query.sh

@@ -0,0 +1 @@
+env

tests/regression/lp-1704860/task.yaml

@@ -0,0 +1,22 @@
+summary: Work-in-progress on reproducing lp:1704860
+systems: [ubuntu-16.04-64]
+details: |
+    In this bug, an app belonging go a snap using classic confinement confuses
+    the re-execution system in a way that causes distribution version of
+    snap-confine to be used, instead of the one from the core snap. If the
+    version outside and inside are different and incompatible the classily
+    confined snap will malfunction.
+
+    This specifically happens when the distribution uses snapd 2.25 and the
+    core snap has snapd 2.26.9
+
+    Testing is somewhat complex but we can approximate by observing the value
+    of SNAP_DID_REEXEC as set inside the environment set up by snap run
+    --shell.  Since neither snap-confine nor snap-exec re-execute themselves
+    (instead they rely on snap run to run the right tool in the first place)
+    this is safe to do.
+execute: |
+    . $TESTSLIB/snaps.sh
+    install_local_classic test-snapd-classic-confinement
+    # We don't want to see SNAP_DID_REEXEC being set.
+    snap run --shell test-snapd-classic-confinement ./snap-env-query.sh | MATCH -v 'SNAP_DID_REEXEC='