write/elf: Fix writing of aligned empty sections (gimli-rs#540)

This was previously causing an assertion failure because the alignment
padding was not written to the buffer if the section was empty, which
caused subsequent sections to be placed at the wrong offsets.

src/write/elf/object.rs

@@ -379,12 +379,9 @@ impl<'a> Object<'a> {
             }
         }
         for (index, section) in self.sections.iter().enumerate() {
-            let len = section.data.len();
-            if len != 0 {
-                writer.write_align(section.align as usize);
-                debug_assert_eq!(section_offsets[index].offset, writer.len());
-                writer.write(&section.data);
-            }
+            writer.write_align(section.align as usize);
+            debug_assert_eq!(section_offsets[index].offset, writer.len());
+            writer.write(&section.data);
         }
 
         // Write symbols.

tests/aligned_sections.rs

@@ -0,0 +1,23 @@
+#![cfg(feature = "write")]
+
+use object::{
+    write::{Object, StreamingBuffer},
+    Architecture, BinaryFormat, Endianness, SectionKind,
+};
+
+#[test]
+fn aligned_sections() {
+    let mut object = Object::new(BinaryFormat::Elf, Architecture::X86_64, Endianness::Little);
+
+    let text_section_id = object.add_section(vec![], b".text".to_vec(), SectionKind::Text);
+    let text_section = object.section_mut(text_section_id);
+    text_section.set_data(&[][..], 4096);
+
+    let data_section_id = object.add_section(vec![], b".data".to_vec(), SectionKind::Data);
+    let data_section = object.section_mut(data_section_id);
+    data_section.set_data(&b"1234"[..], 16);
+
+    let mut buffer = StreamingBuffer::new(vec![]);
+    object.emit(&mut buffer).unwrap();
+    buffer.result().unwrap();
+}