Repository: pulp/pulpcore

Issue:
File System  Exporter needs to use the correct relative path.
**Version**
Please provide the versions of the pulpcore and plugin packages in use, and how they are installed.  If you are using Pulp via Katello, please provide the Katello version.

pulpcore  3.16 and above.

**Describe the bug**
Note: This is not exactly a bug since this functionality is listed as "Tech Preview", but Katello and customers now need it work. More over this is a feature that's already there in the code so marking it as bug seems appropriate to me .

File system exporter is not exporting the publications correctly.

**To Reproduce**
Steps to reproduce the behavior:
- Sync a RH repo that has packages with the following structure (example repo here is `openstack-tools`)
```bash
$ ls content/dist/rhel/server/7/7.5/x86_64/openstack-tools/10/os/
Packages  repodata

$ ls content/dist/rhel/server/7/7.5/x86_64/openstack-tools/10/os/Packages/
b  listing  o  p

$ ls content/dist/rhel/server/7/7.5/x86_64/openstack-tools/10/os/Packages/b/
babel-2.3.4-1.el7ost.noarch.rpm  listing
```
- Create a publication. 
- Use the file system exporter to export this somewhere like `/var/lib/pulp/exports/foo` and give the publication href.
- Check the exported result 
```
$ ls -l /var/lib/pulp/exports/foo
.....
babel-2.3.4-1.el7ost.noarch.rpm
.....
repodata
....
```
- The Packages directory is not retained while the primary xml in the repodata points to `Packages/b/babel-2.3.4-1.el7ost.noarch.rpm


**Expected behavior**
The Packages directory is correctly maintained when exporting.

**Additional context**
The relevant code is at https://github.com/pulp/pulpcore/blob/main/pulpcore/app/tasks/export.py#L63 and https://github.com/pulp/pulpcore/blob/main/pulpcore/app/tasks/export.py#L82.

The actual paths are correct in  core_publishedartifact. So just need to make sure the relative_paths are correct in case of a publication.

```sql
# select relative_path from  core_publishedartifact;
.....
Packages/b/babel-2.3.4-1.el7ost.noarch.rpm
 ....
```


Discussion:

Resolving patch:
diff --git a/pulpcore/app/tasks/export.py b/pulpcore/app/tasks/export.py
--- a/pulpcore/app/tasks/export.py
+++ b/pulpcore/app/tasks/export.py
@@ -37,31 +37,38 @@
 log = logging.getLogger(__name__)
 
 
-def _export_to_file_system(path, content_artifacts, method=FS_EXPORT_METHODS.WRITE):
+def _validate_fs_export(content_artifacts):
     """
-    Export a set of ContentArtifacts to the filesystem.
-
     Args:
-        path (str): A path to export the ContentArtifacts to
         content_artifacts (django.db.models.QuerySet): Set of ContentArtifacts to export
 
     Raises:
-        ValidationError: When path is not in the ALLOWED_EXPORT_PATHS setting
         RuntimeError: If Artifacts are not downloaded or when trying to link non-fs files
     """
     if content_artifacts.filter(artifact=None).exists():
         RuntimeError(_("Cannot export artifacts that haven't been downloaded."))
 
+
+def _export_to_file_system(path, relative_paths_to_artifacts, method=FS_EXPORT_METHODS.WRITE):
+    """
+    Export a set of artifacts to the filesystem.
+
+    Args:
+        path (str): A path to export the ContentArtifacts to
+        relative_paths_to_artifacts: A dict with {relative_path: artifact} mapping
+
+    Raises:
+        ValidationError: When path is not in the ALLOWED_EXPORT_PATHS setting
+    """
+
     if (
         settings.DEFAULT_FILE_STORAGE != "pulpcore.app.models.storage.FileSystem"
         and method != FS_EXPORT_METHODS.WRITE
     ):
         raise RuntimeError(_("Only write is supported for non-filesystem storage."))
 
-    for ca in content_artifacts.select_related("artifact").iterator():
-        artifact = ca.artifact
-        dest = os.path.join(path, ca.relative_path)
-
+    for relative_path, artifact in relative_paths_to_artifacts.items():
+        dest = os.path.join(path, relative_path)
         os.makedirs(os.path.split(dest)[0], exist_ok=True)
 
         if method == FS_EXPORT_METHODS.SYMLINK:
@@ -74,7 +81,6 @@ def _export_to_file_system(path, content_artifacts, method=FS_EXPORT_METHODS.WRI
             with open(dest, "wb") as f, artifact.file as af:
                 for chunk in af.chunks(1024 * 1024):
                     f.write(chunk)
-
         else:
             raise RuntimeError(_("Unsupported export method '{}'.").format(method))
 
@@ -112,7 +118,21 @@ def fs_publication_export(exporter_pk, publication_pk):
             content__in=publication.repository_version.content
         )
 
-    _export_to_file_system(exporter.path, content_artifacts)
+    _validate_fs_export(content_artifacts)
+
+    relative_path_to_artifacts = {}
+    if publication.pass_through:
+        relative_path_to_artifacts = {
+            ca.relative_path: ca.artifact
+            for ca in content_artifacts.select_related("artifact").iterator()
+        }
+
+    for pa in publication.published_artifact.select_related(
+        "content_artifact", "content_artifact__artifact"
+    ).iterator():
+        relative_path_to_artifacts[pa.relative_path] = pa.content_artifact.artifact
+
+    _export_to_file_system(exporter.path, relative_path_to_artifacts, exporter.method)
 
 
 def fs_repo_version_export(exporter_pk, repo_version_pk):
@@ -140,8 +160,14 @@ def fs_repo_version_export(exporter_pk, repo_version_pk):
     )
 
     content_artifacts = ContentArtifact.objects.filter(content__in=repo_version.content)
+    _validate_fs_export(content_artifacts)
+
+    relative_path_to_artifacts = {
+        ca.relative_path: ca.artifact
+        for ca in content_artifacts.select_related("artifact").iterator()
+    }
 
-    _export_to_file_system(exporter.path, content_artifacts)
+    _export_to_file_system(exporter.path, relative_path_to_artifacts, exporter.method)
 
 
 def _get_versions_to_export(the_exporter, the_export):


Test patch: