dockerTools: dereference hard links in tar archives

This is to improve image creation reproducibility. Since the nar
format doesn't support hard link, the tar stream of a layer can be
different if a dependency of a layer has been built locally or if it
has been fetched from a binary cache.

If the dependency has been build locally, it can contain hard links
which are encoded in the tar stream. If the dependency has been
fetched from a binary cache, the tar stream doesn't contain any hard
link. So even if the content is the same, tar streams are different.

1 files changed, 5 insertions, 5 deletions

diff --git a/pkgs/build-support/docker/default.nix b/pkgs/build-support/docker/default.nix
index c40b096e2088..328419d639f9 100644
--- a/pkgs/build-support/docker/default.nix
+++ b/pkgs/build-support/docker/default.nix
@@ -212,7 +212,7 @@ rec {
 
       postMount = ''
         echo "Packing raw image..."
-        tar -C mnt --mtime="@$SOURCE_DATE_EPOCH" -cf $out .
+        tar -C mnt --hard-dereference --mtime="@$SOURCE_DATE_EPOCH" -cf $out .
       '';
     };
 
@@ -277,7 +277,7 @@ rec {
       # Tar up the layer and throw it into 'layer.tar'.
       echo "Packing layer..."
       mkdir $out
-      tar -C layer --mtime="@$SOURCE_DATE_EPOCH" --owner=${toString uid} --group=${toString gid} -cf $out/layer.tar .
+      tar -C layer --hard-dereference --mtime="@$SOURCE_DATE_EPOCH" --owner=${toString uid} --group=${toString gid} -cf $out/layer.tar .
 
       # Compute a checksum of the tarball.
       echo "Computing layer checksum..."
@@ -359,7 +359,7 @@ rec {
 
         echo "Packing layer..."
         mkdir $out
-        tar -C layer --mtime="@$SOURCE_DATE_EPOCH" -cf $out/layer.tar .
+        tar -C layer --hard-dereference --mtime="@$SOURCE_DATE_EPOCH" -cf $out/layer.tar .
 
         # Compute the tar checksum and add it to the output json.
         echo "Computing checksum..."
@@ -489,7 +489,7 @@ rec {
         comm <(sort -n baseFiles|uniq) \
              <(sort -n layerFiles|uniq|grep -v ${layer}) -1 -3 > newFiles
         # Append the new files to the layer.
-        tar -rpf temp/layer.tar --mtime="@$SOURCE_DATE_EPOCH" \
+        tar -rpf temp/layer.tar --hard-dereference --mtime="@$SOURCE_DATE_EPOCH" \
           --owner=0 --group=0 --no-recursion --files-from newFiles
 
         echo "Adding meta..."
@@ -537,7 +537,7 @@ rec {
         chmod -R a-w image
 
         echo "Cooking the image..."
-        tar -C image --mtime="@$SOURCE_DATE_EPOCH" --owner=0 --group=0 --xform s:'./':: -c . | pigz -nT > $out
+        tar -C image --hard-dereference --mtime="@$SOURCE_DATE_EPOCH" --owner=0 --group=0 --xform s:'./':: -c . | pigz -nT > $out
 
         echo "Finished."
       '';