Add a loopback block device test for blkdiscard (#166)

### What does this PR do

This adds a unit test that uses a loopback device to verify that the
blkdiscard code
does indeed wipe a block device.

### What version of tooling - vendor specific or opensource does this
change depend on (if applicable)

This test depends on `losetup` `sha256sum` and `od`. All appear to be
present and working in CI.

### How can this change be tested by a PR reviewer?

The new test is now run in CI.

### Description for changelog/release notes
N/A (just testing improvement)

.github/workflows/push-pr-lint.yaml

@@ -20,7 +20,7 @@ jobs:
       run: make check-go-generated
 
     - name: Run go tests
-      run: make go-test
+      run: sudo ENABLE_PRIV_TESTS=true make go-test
 
     - name: Set up docker buildx
       uses: docker/setup-buildx-action@v3

utils/blkdiscard_test.go

@@ -1,8 +1,10 @@
 package utils
 
 import (
+	"bytes"
 	"context"
 	"os"
+	"os/exec"
 	"testing"
 
 	"github.com/bmc-toolbox/common"
@@ -27,3 +29,68 @@ func Test_blkdiscard(t *testing.T) {
 	// fake-block-device isn't a blockdevice that supports TRIM so we expect an error
 	assert.Error(t, err)
 }
+
+// Test blkdiscard using a file mounted as a loopback device. This test requires
+// root privileges, and is only run if the ENABLE_PRIV_TESTS environment variable
+// is set to true.
+func Test_blkdiscard_loopback(t *testing.T) {
+	if os.Getenv("ENABLE_PRIV_TESTS") != "true" {
+		t.Skip("Skipping privileged test blkdiscard_loopback")
+	}
+
+	tempDir := t.TempDir()
+
+	// Create a 10 MB temporary file
+	fileSize := int64(10 * 1024 * 1024)
+	fileName := tempDir + "testfile"
+	knownOdOutput := `0000000 000000 000000 000000 000000 000000 000000 000000 000000
+*
+50000000
+`
+	f, err := os.Create(fileName)
+	assert.NoError(t, err)
+
+	// Fill the file with 1's
+	_, err = f.WriteAt(bytes.Repeat([]byte{1}, int(fileSize)), 0)
+	f.Close()
+	assert.NoError(t, err)
+	t.Cleanup(func() { os.Remove(fileName) })
+
+	// Create a loopback device using losetup
+	losetupOutput, err := exec.Command("losetup", "--show", "-f", fileName).Output()
+	loopbackDevice := string(bytes.TrimSpace(losetupOutput))
+	t.Logf("loopbackDevice is: %s", loopbackDevice)
+	assert.NoError(t, err)
+	t.Cleanup(func() {
+		cleanerr := exec.Command("losetup", "--detach", loopbackDevice).Run()
+		if cleanerr != nil {
+			println("WARNING: trouble cleaning up loopback device")
+		}
+	})
+
+	// Make note of the sha256 checksum of the block device
+	sha256Output, err := exec.Command("sha256sum", loopbackDevice).Output()
+	t.Logf("sha256sum output before wipe is: %s", string(sha256Output))
+	assert.NoError(t, err)
+	sha256Before := string(bytes.Fields(sha256Output)[0])
+
+	// Run blkdiscard on the loopback device
+	d := NewBlkdiscardCmd(false)
+	err = d.Discard(context.TODO(), &common.Drive{Common: common.Common{LogicalName: loopbackDevice}})
+	assert.NoError(t, err)
+
+	// Make note of the sha256 checksum of the block device after running blkdiscard
+	sha256Output, err = exec.Command("sha256sum", loopbackDevice).Output()
+	assert.NoError(t, err)
+	t.Logf("sha256sum output after wipe is: %s", string(sha256Output))
+	sha256After := string(bytes.Fields(sha256Output)[0])
+
+	// The sha256 checksum should be different after running blkdiscard
+	assert.NotEqual(t, sha256Before, sha256After)
+
+	// od the file for funsies
+	odOutput, err := exec.Command("od", loopbackDevice).Output()
+	assert.NoError(t, err)
+	t.Logf("od output after wipe is: %s", string(odOutput))
+	assert.Equal(t, string(odOutput), knownOdOutput)
+}