fix: treat already-fully-promoted target as success (idempotent)

lua/copy_build.lua

@@ -14,10 +14,10 @@ if res.status ~= 200 then
   return
 end
 
--- Check that build_tgt is empty, or allow resuming a partial promote.
+-- Check that build_tgt is empty, or allow resuming a partial/complete promote.
+-- A fully promoted target (.final_status present) is treated as success (idempotent).
 -- A partial target (files present but no .final_status) is resumed by skipping
--- already-copied objects.  A fully promoted target (.final_status present) is
--- rejected immediately.
+-- already-copied objects.
 --
 ngx.say("Checking if the target reference '" .. build_tgt .. "' is empty")
 ngx.flush(true)
@@ -32,7 +32,8 @@ else
     "/force_real_request/download/" .. build_tgt .. "/.final_status"
   )
   if fs_res.status == 200 then
-    ngx.say('FAILED: target already fully promoted')
+    ngx.say('Target already fully promoted, nothing to do')
+    ngx.say('BUILD COPIED')
     ngx.flush(true)
     return
   end
@@ -208,6 +209,18 @@ if next(target_files) ~= nil then
   objects = remaining
 end
 
+-- Remove .final_status from the main copy list so it can be written last.
+-- Its presence on the target is the completion seal: if a promote is interrupted
+-- after some files are copied but before this final write, the target will not
+-- have .final_status and can be safely resumed.
+local main_objects = {}
+for _, obj in ipairs(objects) do
+  if obj ~= '.final_status' then
+    table.insert(main_objects, obj)
+  end
+end
+objects = main_objects
+
 local total_number_of_objects = #objects
 local batch_size = 16
 local current_object = 0
@@ -314,4 +327,17 @@ else
 
 end
 
+-- Copy .final_status last as the completion seal.
+ngx.say("Copying .final_status as completion seal")
+ngx.flush(true)
+local fs_copy_res = ngx.location.capture(
+  "/force_real_request/copy/" .. build_src .. "/" .. build_tgt .. "/.final_status",
+  { method = ngx.HTTP_PUT, body = "" }
+)
+if fs_copy_res.status ~= 200 then
+  ngx.say('FAILED: could not copy .final_status')
+  ngx.flush(true)
+  return
+end
+
 ngx.say("BUILD COPIED")

tests/end2end/test_copy.py

@@ -50,25 +50,19 @@ def test_copy_source_and_target_listings_are_identical(
     assert src.content == tgt.content
 
 
-def test_copy_fails_when_target_already_exists(
+def test_copy_idempotent_when_target_already_fully_promoted(
     session, artifacts_url, upload_file, finish_build
 ):
-    """A second copy to a fully-promoted target is rejected."""
+    """A second copy to a fully-promoted target succeeds (idempotent)."""
     upload_file(STAGING_BUILD, 'file.txt', b'data')
     finish_build(STAGING_BUILD)
 
     session.get(f'{artifacts_url}/copy/{STAGING_BUILD}/{COPY_BUILD}/')
 
-    # Second attempt — target is fully promoted (has .final_status)
+    # Second attempt — target is fully promoted (has .final_status): should succeed.
     resp = session.get(f'{artifacts_url}/copy/{STAGING_BUILD}/{COPY_BUILD}/')
     assert resp.status_code == 200
-    lines = resp.content.splitlines()
-    expected_check_line = (
-        b"Checking if the target reference '%b' is empty"
-        % COPY_BUILD.encode()
-    )
-    assert lines[-2] == expected_check_line
-    assert lines[-1] == b'FAILED: target already fully promoted'
+    assert resp.content.splitlines()[-1] == b'BUILD COPIED'
 
 
 def test_copy_promotes_staging_to_promoted_bucket(
@@ -155,21 +149,47 @@ def test_resume_partial_promote(
 def test_resume_already_complete_promote(
     session, artifacts_url, upload_file, finish_build
 ):
-    """Resuming a fully-promoted target is rejected with a clear error.
+    """Resuming a fully-promoted target succeeds (idempotent).
 
     A target that already has a .final_status is considered complete; a second
-    promote attempt must fail rather than silently overwriting it.
+    promote attempt must succeed so that retrying a failed workflow does not
+    block on this step.
     """
     upload_file(STAGING_BUILD, 'file.txt', b'data')
     finish_build(STAGING_BUILD)
 
     # First promote — completes successfully.
     session.get(f'{artifacts_url}/copy/{STAGING_BUILD}/{COPY_BUILD}/')
 
-    # Second attempt — target is fully promoted.
+    # Second attempt — target is fully promoted: should succeed (idempotent).
+    resp = session.get(f'{artifacts_url}/copy/{STAGING_BUILD}/{COPY_BUILD}/')
+    assert resp.status_code == 200
+    assert resp.content.splitlines()[-1] == b'BUILD COPIED'
+
+
+def test_resume_promote_interrupted_before_final_status(
+    session, artifacts_url, upload_file, finish_build
+):
+    """Promote resumes when interrupted just before .final_status was written.
+
+    .final_status is copied last and acts as the completion seal.  A target that
+    has all artifacts but no .final_status (promote crashed at the final step)
+    must be resumable rather than treated as complete.
+    """
+    n = 3
+    for i in range(n):
+        upload_file(STAGING_BUILD, f'obj-{i}', f'content-{i}'.encode())
+    finish_build(STAGING_BUILD)
+
+    # Pre-populate target with all artifacts but without .final_status.
+    for i in range(n):
+        upload_file(COPY_BUILD, f'obj-{i}', f'content-{i}'.encode())
+
     resp = session.get(f'{artifacts_url}/copy/{STAGING_BUILD}/{COPY_BUILD}/')
     assert resp.status_code == 200
-    assert resp.content.splitlines()[-1] == b'FAILED: target already fully promoted'
+    assert resp.content.splitlines()[-1] == b'BUILD COPIED'
+
+    assert session.get(f'{artifacts_url}/download/{COPY_BUILD}/.final_status').status_code == 200
 
 
 def test_copy_behind_ingress(session, artifacts_url, upload_file, finish_build):