fix(delete): honor If-Match precondition on DeleteObject (#10)

DeleteObject ignored the If-Match request header and always deleted the
object (204). AWS S3 conditional deletes require that when If-Match is
provided and does not match the object's current ETag, the delete is
refused with 412 Precondition Failed and the object is left intact.

The precondition is evaluated in erasureServerPools.DeleteObject, while
the server-pool delete lock is held, before the delete-marker short-circuit
and before any version is removed. It runs against the version that will
actually be deleted: pinfo.ObjInfo for a normal delete, or the specifically
addressed version (read under the held lock) for a version-scoped delete,
since getPoolInfoExistingWithOpts strips VersionID. The check is a pure
function (no ResponseWriter writes) and returns PreConditionFailed, which
toAPIError maps to 412; CheckPrecondFn is cleared before lower layers run
so the precondition is evaluated exactly once.

Semantics:
- If-Match mismatch on a live object -> 412, object preserved.
- If-Match "*" requires a live object; a delete-marker-latest -> 412, and
  an explicitly addressed delete-marker version -> 412 (getObjectInfo
  returns the marker with MethodNotAllowed; the marker is the precondition
  target, not a 405).
- SSE-C/SSE-KMS: compared against the public ETag derived without the
  customer key, so a satisfiable condition is never falsely rejected.
- Explicit versionId -> evaluated against that version; a missing addressed
  version -> NoSuchVersion whether or not the key exists; a missing object
  (no versionId) -> NoSuchKey; no If-Match -> unchanged (including the
  unconditional version-scoped delete's error behavior).

Scope: atomicity is guaranteed for a single erasure set (the default
deployment). Multi-pool conditional-delete atomicity (concurrent writers
across pools, cross-pool version selection) is tracked as a follow-up.

Tests: pure-helper unit test (delete marker, "*", SSE-C without key);
object-layer tests (unversioned match/mismatch/missing, versioned
delete-marker-latest and addressed delete-marker version, explicit-version
selection, missing version on present and absent keys, read-quorum loss);
handler tests (412/204/wildcard/404) across both backends, with red/green
demonstrated per guard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L7qJqWwy8oFA6aCXWRzXQe
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/object-handlers.go

@@ -2947,6 +2947,20 @@ func (api objectAPIHandlers) DeleteObjectHandler(w http.ResponseWriter, r *http.
 		return
 	}
 
+	// If-Match conditional delete (see AWS S3 conditional deletes). Delete the
+	// object only if its current ETag matches the client-supplied value,
+	// otherwise the request is refused with 412 Precondition Failed and the
+	// object is left intact. The precondition is evaluated in
+	// erasureServerPools.DeleteObject, against the version that will actually be
+	// removed, while the delete lock is held, so the object cannot change
+	// between the ETag check and the delete.
+	if ifMatch := r.Header.Get(xhttp.IfMatch); ifMatch != "" {
+		opts.HasIfMatch = true
+		opts.CheckPrecondFn = func(oi ObjectInfo) bool {
+			return deleteIfMatchPreconditionFailed(r.Header, ifMatch, oi)
+		}
+	}
+
 	rcfg, _ := globalBucketObjectLockSys.Get(bucket)
 	if rcfg.LockEnabled && opts.DeletePrefix {
 		apiErr := toAPIError(ctx, errInvalidArgument)
@@ -3012,6 +3026,13 @@ func (api objectAPIHandlers) DeleteObjectHandler(w http.ResponseWriter, r *http.
 			return
 		}
 		if isErrObjectNotFound(err) || isErrVersionNotFound(err) {
+			if opts.HasIfMatch {
+				// A conditional (If-Match) delete cannot satisfy its
+				// precondition against a missing object, so surface the
+				// not-found error instead of the idempotent 204 response.
+				writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)
+				return
+			}
 			// Send an event when the object is not found
 			objInfo.Name = object
 			objInfo.VersionID = opts.VersionID