tar/export: Don't create hardlinks to zero sized files

This is an echo of ostreedev/ostree@558720e

If we hardlink zero sized files, then any modification to them
will result in possibly *many* hardlinks needing to be reproduced
when serializing to tar, which is definitely suboptimal.

But further, it provokes a bug in our import path; when
processing derived layers, we need to handle *both* cases
of having the file in `/sysroot` be the source as well
as the destination of the hardlink in the tar stream.

lib/src/fixture.rs

@@ -128,6 +128,7 @@ static OWNERS: Lazy<Vec<(Regex, &str)>> = Lazy::new(|| {
         ("usr/lib/modules/.*/initramfs", "initramfs"),
         ("usr/lib/modules", "kernel"),
         ("usr/bin/(ba)?sh", "bash"),
+        ("usr/lib.*/emptyfile.*", "bash"),
         ("usr/bin/hardlink.*", "testlink"),
         ("usr/etc/someconfig.conf", "someconfig"),
         ("usr/etc/polkit.conf", "a-polkit-config"),
@@ -146,6 +147,9 @@ m 0 0 755
 r usr/bin/bash the-bash-shell
 l usr/bin/sh bash
 m 0 0 644
+# Some empty files
+r usr/lib/emptyfile 
+r usr/lib64/emptyfile2 
 # Should be the same object
 r usr/bin/hardlink-a testlink
 r usr/bin/hardlink-b testlink
@@ -163,8 +167,8 @@ m 0 0 1755
 d tmp
 "## };
 pub const CONTENTS_CHECKSUM_V0: &str =
-    "3af747e156c34d08a3a2fb85b94de6999205a1d1c1c7b1993d6ce534a8918cd9";
-pub static CONTENTS_V0_LEN: Lazy<usize> = Lazy::new(|| OWNERS.len());
+    "5e41de82f9f861fa51e53ce6dd640a260e4fb29b7657f5a3f14157e93d2c0659";
+pub static CONTENTS_V0_LEN: Lazy<usize> = Lazy::new(|| OWNERS.len().checked_sub(1).unwrap());
 
 #[derive(Debug, PartialEq, Eq)]
 enum SeLabel {

lib/src/tar/export.rs

@@ -419,9 +419,9 @@ impl<'a, W: std::io::Write> OstreeTarWriter<'a, W> {
         h.set_gid(meta.attribute_uint32("unix::gid") as u64);
         let mode = meta.attribute_uint32("unix::mode");
         h.set_mode(self.filter_mode(mode));
-        let mut target_header = h.clone();
-        target_header.set_size(0);
-
+        if instream.is_some() {
+            h.set_size(meta.size() as u64);
+        }
         if !self.wrote_content.contains(checksum) {
             let inserted = self.wrote_content.insert(checksum.to_string());
             debug_assert!(inserted);
@@ -464,7 +464,7 @@ impl<'a, W: std::io::Write> OstreeTarWriter<'a, W> {
             }
         }
 
-        Ok((path, target_header))
+        Ok((path, h))
     }
 
     /// Write a directory using the provided metadata.
@@ -488,9 +488,21 @@ impl<'a, W: std::io::Write> OstreeTarWriter<'a, W> {
         mut h: tar::Header,
         dest: &Utf8Path,
     ) -> Result<()> {
-        h.set_entry_type(tar::EntryType::Link);
-        h.set_link_name(srcpath)?;
-        self.out.append_data(&mut h, dest, &mut std::io::empty())?;
+        // Query the original size first
+        let size = h.size().context("Querying size for hardlink append")?;
+        // Don't create hardlinks to zero-sized files, it's much more likely
+        // to result in generated tar streams from container builds resulting
+        // in a modified linked-to file in /sysroot, which we don't currently handle.
+        // And in the case where the input is *not* zero sized, we still output
+        // a hardlink of size zero, as this is what is normal.
+        h.set_size(0);
+        if h.entry_type() == tar::EntryType::Regular && size == 0 {
+            self.out.append_data(&mut h, dest, &mut std::io::empty())?;
+        } else {
+            h.set_entry_type(tar::EntryType::Link);
+            h.set_link_name(srcpath)?;
+            self.out.append_data(&mut h, dest, &mut std::io::empty())?;
+        }
         Ok(())
     }
 

lib/tests/it/main.rs

@@ -306,10 +306,12 @@ fn common_tar_structure() -> impl Iterator<Item = TarExpected> {
 
 // Find various expected files
 fn common_tar_contents_all() -> impl Iterator<Item = TarExpected> {
-    use tar::EntryType::{Directory, Link};
+    use tar::EntryType::{Directory, Link, Regular};
     [
         ("boot", Directory, 0o755),
         ("usr", Directory, 0o755),
+        ("usr/lib/emptyfile", Regular, 0o644),
+        ("usr/lib64/emptyfile2", Regular, 0o644),
         ("usr/bin/bash", Link, 0o755),
         ("usr/bin/hardlink-a", Link, 0o644),
         ("usr/bin/hardlink-b", Link, 0o644),