runc exec -p: fix adding HOME to nil env

Before commit 7dc24868, when process.env was nil, prepareEnv
returned a flag telling HOME is not set, and it was added.

Commit 7dc24868 moved the functionality of adding HOME into
prepareEnv but did not properly handle nil case. As a result,
runc exec -p with process.json having no env set resulted in
an exec with no HOME set.

Fix this, and add unit and integration tests.

Fixes: 7dc24868 ("libct: switch to numeric UID/GID/groups")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/env.go

@@ -22,10 +22,7 @@ import (
 //
 // Returns the prepared environment.
 func prepareEnv(env []string, uid int) ([]string, error) {
-	if env == nil {
-		return nil, nil
-	}
-	var homeIsSet bool
+	homeIsSet := false
 
 	// Deduplication code based on dedupEnv from Go 1.22 os/exec.
 

libcontainer/env_test.go

@@ -21,6 +21,10 @@ func TestPrepareEnv(t *testing.T) {
 	tests := []struct {
 		env, wantEnv []string
 	}{
+		{
+			env:     nil,
+			wantEnv: []string{home},
+		},
 		{
 			env:     []string{},
 			wantEnv: []string{home},