xdrgen: Stream optional-data aggregate lists during encode

The optional-data aggregate encoder loops cursor.count times, so the
begin hook must report the element count before the first element is
written.  A producer that reads its elements while the reply is
encoded -- NFSv2 READDIR streaming directory entries straight into the
send buffer -- does not know that count in advance, and would have to
materialize the whole list just to satisfy the loop bound.

An optional-data list carries no count on the wire, so nothing forces
one on the encoder.  Drive it from the item hook instead: a false
return ends the list, and the framing writes the terminator.

The counted-array form and its hook contract are unchanged.  No
in-tree spec marks an optional-data member yet, so no generated code
changes here.

Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260908134234.512312-11-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

1 files changed, 18 insertions, 9 deletions

diff --git a/include/linux/sunrpc/xdrgen/_defs.h b/include/linux/sunrpc/xdrgen/_defs.h
index f4375d73c5ca..d767206feea4 100644
--- a/include/linux/sunrpc/xdrgen/_defs.h
+++ b/include/linux/sunrpc/xdrgen/_defs.h
@@ -30,15 +30,24 @@ typedef struct {
 } opaque;
 
 /*
- * Cursor a hook-driven aggregate codec hands to its application hooks,
- * one element at a time, in place of a materialized C array. The
- * generated framing owns it. @xdr is the RPC layer's stream; @ctx is
- * that stream's xdrgen_ctx, the svc_rqst on the server. @index is
- * the current element and @count the wire array length: a decoder
- * fills @count from the wire before the begin hook runs, an
- * encoder's begin hook sets it. @member_id selects among a type's
- * marked members. The begin/item/end contract is under "Pragma
- * aggregate" in tools/net/sunrpc/xdrgen/README.
+ * Cursor a hook-driven aggregate codec hands to its begin/item/end
+ * hooks, one element at a time, in place of a materialized C array.
+ * The generated framing owns it. @xdr is the RPC layer's stream and
+ * @ctx its xdrgen_ctx, the svc_rqst on the server. @index is the
+ * current element; @member_id selects among a type's marked members.
+ * For a counted array @count is the wire length: a decoder fills it
+ * before the begin hook runs, an encoder's begin hook sets it. The
+ * hook contract is under "Pragma aggregate" in
+ * tools/net/sunrpc/xdrgen/README.
+ *
+ * An optional-data list ("type *name") carries no count. Its encoder
+ * pulls elements until the item hook returns false. False ends the
+ * list rather than failing the encode: the framing writes the
+ * value-follows terminator, so a producer that stops early leaves a
+ * valid truncated list on the wire. A producer that stops for a
+ * mid-list error records it in the application state @ctx reaches,
+ * and either returns false from the end hook, which fails the
+ * encode, or reports it in the procedure status.
  */
 struct xdrgen_aggregate_cursor {
 	struct xdr_stream	*xdr;