refactor: improve compaction logic for AddObject

Previously, an add operation would simply append the data object to the
end of the data section. If one or more object(s) had previously been
deleted from the image, this could result in wasted space. Now, we trim
the data section based on in-use objects before adding the new object.
Update tests to reflect this new behaviour.

pkg/sif/create.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2018-2023, Sylabs Inc. All rights reserved.
+// Copyright (c) 2018-2024, Sylabs Inc. All rights reserved.
 // Copyright (c) 2017, SingularityWare, LLC. All rights reserved.
 // Copyright (c) 2017, Yannick Cote <yhcote@gmail.com> All rights reserved.
 // This software is licensed under a 3-clause BSD license. Please consult the
@@ -55,6 +55,20 @@ func writeDataObjectAt(ws io.WriteSeeker, offsetUnaligned int64, di DescriptorIn
 	return nil
 }
 
+// calculatedDataSize calculates the size of the data section based on the in-use descriptors.
+func (f *FileImage) calculatedDataSize() int64 {
+	dataEnd := f.DataOffset()
+
+	f.WithDescriptors(func(d Descriptor) bool {
+		if objectEnd := d.Offset() + d.Size(); dataEnd < objectEnd {
+			dataEnd = objectEnd
+		}
+		return false
+	})
+
+	return dataEnd - f.DataOffset()
+}
+
 var (
 	errInsufficientCapacity = errors.New("insufficient descriptor capacity to add data object(s) to image")
 	errPrimaryPartition     = errors.New("image already contains a primary partition")
@@ -80,6 +94,8 @@ func (f *FileImage) writeDataObject(i int, di DescriptorInput, t time.Time) erro
 	d := &f.rds[i]
 	d.ID = uint32(i) + 1
 
+	f.h.DataSize = f.calculatedDataSize()
+
 	if err := writeDataObjectAt(f.rw, f.h.DataOffset+f.h.DataSize, di, t, d); err != nil {
 		return err
 	}