issue 1841/S15: put the booted kernel first when GRUB's menu is generated again, so the fallback kernel, sorted first by version, never becomes the default entry (ADR 0508 §9)

modules/kernel/cmd/kernel-tools/bootentries_test.go

@@ -78,7 +78,8 @@ func newBootBed(t *testing.T) bootBed {
 	}
 	write(t, b.script, content, 0o755)
 	write(t, b.root+"/proc/cmdline", bootedCmdline, 0o644)
-	write(t, filepath.Join(b.bin, "grub-mkconfig"), "#!/bin/sh\necho \"$@\" >>"+b.mkconfig+"\n", 0o755)
+	// the stand-in grub-mkconfig records the kernel it was told to put first, then its arguments.
+	write(t, filepath.Join(b.bin, "grub-mkconfig"), "#!/bin/sh\necho \"top=$GRUB_TOP_LEVEL $@\" >>"+b.mkconfig+"\n", 0o755)
 	return b
 }
 
@@ -164,11 +165,37 @@ func TestAGrubMachineIsGeneratedAgainOnlyWhenItsKernelsChanged(t *testing.T) {
 		t.Fatalf("changed: %v %s", err, out)
 	}
 	raw, _ := os.ReadFile(b.mkconfig)
-	if string(raw) != "-o "+filepath.Join(b.root, "boot/grub/grub.cfg")+"\n" {
+	if string(raw) != "top=/boot/vmlinuz-linux -o "+filepath.Join(b.root, "boot/grub/grub.cfg")+"\n" {
 		t.Fatalf("grub-mkconfig was asked %q", raw)
 	}
 }
 
+// ADR 0508 §9: GRUB's menu lists kernels in reverse version order, so linux-lts would come first and become the
+// default entry (GRUB_DEFAULT=0); the kernel the machine booted is put first, and the fallback kernel never.
+func TestGrubKeepsTheBootedKernelFirstAndNeverTheFallback(t *testing.T) {
+	for name, c := range map[string]struct{ cmdline, want string }{
+		"booted from the default kernel":  {"BOOT_IMAGE=/vmlinuz-linux root=UUID=x rw\n", "top=/boot/vmlinuz-linux"},
+		"booted from a grub device path":  {"BOOT_IMAGE=(hd0,msdos1)/vmlinuz-linux-zen root=UUID=x rw\n", "top=/boot/vmlinuz-linux-zen"},
+		"booted from the fallback kernel": {"BOOT_IMAGE=/vmlinuz-linux-lts root=UUID=x rw\n", "top=/boot/vmlinuz-linux"},
+		"no image on the command line":    {"root=UUID=x rw\n", "top=/boot/vmlinuz-linux"},
+		"an image no longer in /boot":     {"BOOT_IMAGE=/vmlinuz-linux-gone root=UUID=x rw\n", "top=/boot/vmlinuz-linux"},
+	} {
+		b := newBootBed(t)
+		b.put(t, "boot/grub/grub.cfg", grubMenu)
+		b.put(t, "proc/cmdline", c.cmdline)
+		for _, k := range []string{"vmlinuz-linux", "vmlinuz-linux-lts", "vmlinuz-linux-zen"} {
+			b.put(t, "boot/"+k, "k")
+		}
+		if out, err := b.run(t); err != nil {
+			t.Fatalf("%s: %v %s", name, err, out)
+		}
+		raw, _ := os.ReadFile(b.mkconfig)
+		if !strings.HasPrefix(string(raw), c.want+" -o ") {
+			t.Errorf("%s: grub-mkconfig was asked %q, want %s first", name, raw, c.want)
+		}
+	}
+}
+
 func TestAMachineWithNoLoaderFailsLoudly(t *testing.T) {
 	b := newBootBed(t)
 	out, err := b.run(t)