feat(ecstore): add on-demand migration SourceClient

Add bucket/on_demand_migration/source_client.rs on top of the shared
remote S3 builder: HEAD, ranged streaming GET, ListObjectsV2 with
source-prefix mapping, GetObjectTagging and an admin probe. Every request
carries the x-rustfs-/x-minio-source-proxy-request anti-loop markers and
a RustFS-OnDemandMigration/<version> User-Agent suffix; SSE-C source
objects are rejected as unsupported. SourceError classifies SDK failures
(not found, access denied, throttled, timeout, connect, server error)
with retryability and a stable metrics label. Debug output redacts
credentials.

Refs rustfs/backlog#2149

crates/ecstore/src/api/mod.rs

@@ -166,6 +166,16 @@ pub mod bucket {
         pub use crate::bucket::migration::{LegacyBlobDecryptFn, try_migrate_bucket_metadata, try_migrate_iam_config};
     }
 
+    pub mod on_demand_migration {
+        pub mod source_client {
+            pub use crate::bucket::on_demand_migration::source_client::{
+                SourceClient, SourceClientSpec, SourceError, SourceGet, SourceHead, SourceObject, SourcePage, SourceProbe,
+                SourceProvider, SourceSse, SourceTimeouts, USER_AGENT_SUFFIX, is_multipart_etag, range_header_value,
+                resolve_path_style,
+            };
+        }
+    }
+
     pub mod object_lock {
         pub use crate::bucket::object_lock::{ObjectLockApi, ObjectLockStatusExt};
 

crates/ecstore/src/bucket/mod.rs

@@ -26,6 +26,7 @@ mod metadata_test;
 pub mod migration;
 mod msgp_decode;
 pub mod object_lock;
+pub mod on_demand_migration;
 pub mod policy_sys;
 pub mod quota;
 pub mod remote_s3_client;

crates/ecstore/src/bucket/on_demand_migration/mod.rs

@@ -0,0 +1,18 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! On-demand migration (ODM): serve and back-fill objects from an external
+//! S3-compatible source bucket.
+
+pub mod source_client;