xl: Heal empty parts (#7860)

posix.VerifyFile() doesn't know how to check if a file
is corrupted if that file is empty. We do have the part
size in xl.json so we pass it to VerifyFile to return
an error so healing empty parts can work properly.

cmd/xl-v1-healing_test.go

@@ -107,7 +107,7 @@ func TestHealObjectCorrupted(t *testing.T) {
 		t.Fatalf("Failed to complete multipart upload - %v", err)
 	}
 
-	// Remove the object backend files from the first disk.
+	// Test 1: Remove the object backend files from the first disk.
 	xl := obj.(*xlObjects)
 	firstDisk := xl.storageDisks[0]
 	err = firstDisk.DeleteFile(bucket, filepath.Join(object, xlMetaJSONFile))
@@ -125,7 +125,34 @@ func TestHealObjectCorrupted(t *testing.T) {
 		t.Errorf("Expected xl.json file to be present but stat failed - %v", err)
 	}
 
-	// Delete xl.json from more than read quorum number of disks, to create a corrupted situation.
+	// Test 2: Heal when part.1 is empty
+	partSt1, err := firstDisk.StatFile(bucket, filepath.Join(object, "part.1"))
+	if err != nil {
+		t.Errorf("Expected part.1 file to be present but stat failed - %v", err)
+	}
+	err = firstDisk.DeleteFile(bucket, filepath.Join(object, "part.1"))
+	if err != nil {
+		t.Errorf("Failure during part.1 removal - %v", err)
+	}
+	err = firstDisk.AppendFile(bucket, filepath.Join(object, "part.1"), []byte{})
+	if err != nil {
+		t.Errorf("Failure during creating part.1 - %v", err)
+	}
+	_, err = obj.HealObject(context.Background(), bucket, object, false, true, madmin.HealDeepScan)
+	if err != nil {
+		t.Errorf("Expected nil but received %v", err)
+	}
+	partSt2, err := firstDisk.StatFile(bucket, filepath.Join(object, "part.1"))
+	if err != nil {
+		t.Errorf("Expected from part.1 file to be present but stat failed - %v", err)
+	}
+	if partSt1.Size != partSt2.Size {
+		t.Errorf("part.1 file size is not the same before and after heal")
+	}
+
+	// Test 3: checks if HealObject returns an error when xl.json is not found
+	// in more than read quorum number of disks, to create a corrupted situation.
+
 	for i := 0; i <= len(xl.storageDisks)/2; i++ {
 		xl.storageDisks[i].DeleteFile(bucket, filepath.Join(object, xlMetaJSONFile))
 	}