libct/int: improve TestExecInEnvironment

This is a slight refactor of TestExecInEnvironment, making it more
strict wrt checking the exec output.

1. Explain why DEBUG is added twice to the env.
2. Reuse the execEnv for the check.
3. Make the check more strict -- instead of looking for substrings,
   check line by line.
4. Add a check for extra environment variables.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/execin_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
 	"testing"
@@ -345,16 +346,20 @@ func TestExecInEnvironment(t *testing.T) {
 	defer stdinW.Close() //nolint: errcheck
 	ok(t, err)
 
+	execEnv := []string{
+		"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
+		// The below line is added to check the deduplication feature:
+		// if a variable with the same name appears more than once,
+		// only the last value should be added to the environment.
+		"DEBUG=true",
+		"DEBUG=false",
+		"ENV=test",
+	}
 	buffers := newStdBuffers()
 	process2 := &libcontainer.Process{
-		Cwd:  "/",
-		Args: []string{"env"},
-		Env: []string{
-			"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
-			"DEBUG=true",
-			"DEBUG=false",
-			"ENV=test",
-		},
+		Cwd:    "/",
+		Args:   []string{"/bin/env"},
+		Env:    execEnv,
 		Stdin:  buffers.Stdin,
 		Stdout: buffers.Stdout,
 		Stderr: buffers.Stderr,
@@ -366,14 +371,24 @@ func TestExecInEnvironment(t *testing.T) {
 	_ = stdinW.Close()
 	waitProcess(process, t)
 
-	out := buffers.Stdout.String()
-	// check execin's process environment
-	if !strings.Contains(out, "DEBUG=false") ||
-		!strings.Contains(out, "ENV=test") ||
-		!strings.Contains(out, "HOME=/root") ||
-		!strings.Contains(out, "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin") ||
-		strings.Contains(out, "DEBUG=true") {
-		t.Fatalf("unexpected running process, output %q", out)
+	// Check exec process environment.
+	t.Logf("exec output:\n%s", buffers.Stdout.String())
+	out := strings.Fields(buffers.Stdout.String())
+	// If not present in the Process.Env, runc should add $HOME,
+	// which is deduced by parsing container's /etc/passwd.
+	// We use a known image in the test, so we know its value.
+	for _, e := range append(execEnv, "HOME=/root") {
+		if e == "DEBUG=true" { // This one should be dedup'ed out.
+			continue
+		}
+		if !slices.Contains(out, e) {
+			t.Errorf("Expected env %s not found in exec output", e)
+		}
+	}
+	// Check that there are no extra variables. We have 1 env ("DEBUG=true")
+	// removed and 1 env ("HOME=root") added, resulting in the same number.
+	if got, want := len(out), len(execEnv); want != got {
+		t.Errorf("Mismatched number of variables: want %d, got %d", want, got)
 	}
 }