fix: retransmit existing SSE-C replicas instead of metadata-copying them

The replication sender's target HEAD carries no SSE-C customer key, so for
an SSE-C object the target answers 400 and replicateAll fell into a
metadata-only CopyObject that fails on any non-empty SSE-C object (the
undecryptable source checksum makes the target recompute one and rewrite
the data with a plaintext-sized reader). Once a non-empty SSE-C replica
existed, tag, retention and legal-hold changes never reached it, a heal
never retransmitted, and a resync neither repaired the replica nor
counted it correctly. Forcing a full retransmit alone was not enough:
checkPreconditionsPUT rejects a write whose PreserveETag and VersionID
match the stored version, only the single-part sealed ETag is truncated
before that comparison, so a multipart SSE-C retransmit answered 412,
which the sender turns into success. Inherited from upstream ad04afe38.

Select replicateAll when the SSE-C HEAD cannot answer (the two previous
assignments were dead: rAction still forced the metadata path), exempt an
authenticated replica write that carries an SSE-C seal from the duplicate
version and ETag rejection (the predicate is the incoming write's
restored SSE-C metadata, not what the destination holds), and send the
internal replication marker on the resync accounting HEAD for SSE-C
objects so a peer answers with the replica metadata instead of 400.

Tests: TestAPISSECReplicaRetransmitOverExistingVersion (multipart replica
initiation over the same version and ETag answered 412 on main, now 200
with parts sent and plaintext readback; single-part and zero-byte writes
unchanged), TestAPISSECReplicaWriteExemptionIsKeyedOnTheIncomingWrite
(plaintext replica over an SSE-C version still 412; SSE-C replica over a
plaintext version exempted and readable), and
TestAPISSECReplicationTargetHead (keyless HEAD 400, missing key 404,
marked HEAD 200 with metadata, metadata CopyObject ExcessData on a
non-empty object) on ErasureSD and Erasure. Compatibility: every update
of an SSE-C object now retransmits its bytes; a peer that rejects the
internal marker fails the accounting HEAD as before; the #109 destination
fix must be deployed first or a retransmitted replica is transformed
again.

Fixes pgsty/silo#120

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L7qJqWwy8oFA6aCXWRzXQe
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/object-handlers-common.go

@@ -28,6 +28,7 @@ import (
 
 	"github.com/minio/minio/internal/amztime"
 	"github.com/minio/minio/internal/bucket/lifecycle"
+	"github.com/minio/minio/internal/crypto"
 	"github.com/minio/minio/internal/event"
 	"github.com/minio/minio/internal/hash"
 	xhttp "github.com/minio/minio/internal/http"
@@ -193,7 +194,15 @@ func checkPreconditionsPUT(ctx context.Context, w http.ResponseWriter, r *http.R
 
 	etagMatch := opts.PreserveETag != "" && isETagEqual(objInfo.ETag, opts.PreserveETag)
 	vidMatch := opts.VersionID != "" && opts.VersionID == objInfo.VersionID
-	if etagMatch && vidMatch {
+	// A matching version and ETag normally mean the destination already holds
+	// this version, so the write is skipped. They do not establish that for an
+	// authenticated SSE-C replica write: the destination cannot decrypt or
+	// re-encrypt the body without the customer key, so it cannot verify the
+	// replica, and this retransmission is how such a replica is repaired or
+	// updated. The predicate is the incoming request's restored SSE-C metadata,
+	// not what the destination happens to hold.
+	ssecReplica := isReplicaTrusted(r.Context()) && crypto.SSEC.IsEncrypted(opts.UserDefined)
+	if etagMatch && vidMatch && !ssecReplica {
 		writeHeaders()
 		writeErrorResponse(ctx, w, errorCodes.ToAPIErr(ErrPreconditionFailed), r.URL)
 		return true