Failed multihop doesn't play nice with source_replica_expression
Motivation
----------

Assume a transfer request with `source_replica_expression = A`. The transfer is scheduled as a multihop `A -> B -> Destination`. If the transfer fails for some reason after a successful first hop `A->B`; The transfer `B->Destination` will not be able to schedule because it doesn't match the source replica expression. 


Modification
------------

Fix this issue


diff --git a/lib/rucio/core/transfer.py b/lib/rucio/core/transfer.py
index 707c6e7f1..f0ac849ad 100644
--- a/lib/rucio/core/transfer.py
+++ b/lib/rucio/core/transfer.py
@@ -1192,27 +1192,35 @@ def get_dsn(scope, name, dsn):
     return 'other'
 
 
-def __filter_unwanted_paths(candidate_paths: "Iterable[List[DirectTransferDefinition]]") -> "Generator[List[DirectTransferDefinition]]":
-
+def __filter_multihops_with_intermediate_tape(candidate_paths: "Iterable[List[DirectTransferDefinition]]") -> "Generator[List[DirectTransferDefinition]]":
     # Discard multihop transfers which contain a tape source as an intermediate hop
-    filtered_candidate_paths = []
     for path in candidate_paths:
         if any(transfer.src.rse.is_tape_or_staging_required() for transfer in path[1:]):
-            continue
-        filtered_candidate_paths.append(path)
-    candidate_paths = filtered_candidate_paths
+            pass
+        else:
+            yield path
 
-    # Discard multihop transfers which contain other candidate as part of itself For example:
-    # if A->B->C and B->C are both candidates, discard A->B->C because it includes B->C. Doing B->C is enough.
-    source_rses = {path[0].src.rse.id for path in candidate_paths}
-    filtered_candidate_paths = []
+
+def __compress_multihops(
+        candidate_paths: "Iterable[List[DirectTransferDefinition]]",
+        sources: "Iterable[TransferSource]",
+) -> "Generator[List[DirectTransferDefinition]]":
+    # Compress multihop transfers which contain other sources as part of itself.
+    # For example: multihop A->B->C and B is a source, compress A->B->C into B->C
+    source_rses = {s.rse.id for s in sources}
+    seen_source_rses = set()
     for path in candidate_paths:
-        if any(hop.src.rse.id in source_rses for hop in path[1:]):
-            continue
-        filtered_candidate_paths.append(path)
-    candidate_paths = filtered_candidate_paths
+        if len(path) > 1:
+            # find the index of the first hop starting from the end which is also a source. Path[0] will always be a source.
+            last_source_idx = next((idx for idx, hop in reversed(list(enumerate(path))) if hop.src.rse.id in source_rses), (0, None))
+            if last_source_idx > 0:
+                path = path[last_source_idx:]
 
-    yield from candidate_paths
+        # Deduplicate paths from same source
+        src_rse_id = path[0].src.rse.id
+        if src_rse_id not in seen_source_rses:
+            seen_source_rses.add(src_rse_id)
+            yield path
 
 
 def __sort_paths(candidate_paths: "Iterable[List[DirectTransferDefinition]]") -> "Generator[List[DirectTransferDefinition]]":
@@ -1397,6 +1405,9 @@ def __build_transfer_paths(
     unavailable_read_rse_ids = __get_unavailable_rse_ids(operation='read', session=session)
     unavailable_write_rse_ids = __get_unavailable_rse_ids(operation='write', session=session)
 
+    # Disallow multihop via blocklisted RSEs
+    multihop_rses = list(set(multihop_rses).difference(unavailable_write_rse_ids).difference(unavailable_read_rse_ids))
+
     candidate_paths_by_request_id, reqs_no_source, reqs_only_tape_source, reqs_scheme_mismatch = {}, set(), set(), set()
     for rws in requests_with_sources:
 
@@ -1488,7 +1499,8 @@ def __build_transfer_paths(
         if len(filtered_sources) != len(candidate_paths):
             logger(logging.DEBUG, 'Sources after path computation for %s: %s', rws, [str(path[0].src.rse) for path in candidate_paths])
 
-        candidate_paths = __filter_unwanted_paths(candidate_paths)
+        candidate_paths = __filter_multihops_with_intermediate_tape(candidate_paths)
+        candidate_paths = __compress_multihops(candidate_paths, rws.sources)
         candidate_paths = list(__sort_paths(candidate_paths))
 
         if not candidate_paths:
['lib/rucio/core/transfer.py']