1 files changed, 24 insertions, 0 deletions
diff --git a/packages/meshbay-common/src/meshbay_common/protocol.py b/packages/meshbay-common/src/meshbay_common/protocol.py
index 8a521bb..5bd2903 100644
--- a/packages/meshbay-common/src/meshbay_common/protocol.py
+++ b/packages/meshbay-common/src/meshbay_common/protocol.py
@@ -501,6 +501,22 @@ def file_upload_payload(gek: bytes, group_id: str, msg: dict) -> dict:
     return unseal(gek, PURPOSE_UPLOAD, MNP.FILE_UPLOAD, group_id, msg)
 
 
+# "Where am I?", asked as an ordinary sealed upload chunk rather than as a new
+# message.
+#
+# The node identifies an upload by (member, directory, filename), so a client
+# resuming one has to name the file — and `transfer_open`, the obvious place to
+# ask, travels in clear. Naming it there would undo exactly what sealing the
+# upload path bought: before MNP 2.0 the same file was ciphertext leaving a node
+# and plaintext arriving at one.
+#
+# So the question is asked inside the seal that already exists, as a chunk with
+# no bytes and this index. The node writes nothing, changes nothing, and answers
+# with `resume_from`. A node that predates this refuses the index, which the
+# client reads as "start from the beginning" — the behaviour it had anyway.
+UPLOAD_PROBE_INDEX = -1
+
+
 def file_upload_ack_wire(
     gek: bytes,
     group_id: str,
@@ -510,6 +526,7 @@ def file_upload_ack_wire(
     filename: str,
     stored_as: str,
     dir: str = "",
+    resume_from: int | None = None,
 ) -> dict:
     """
     The node's answer to one chunk, sealed the same way.
@@ -518,8 +535,15 @@ def file_upload_ack_wire(
     replacing anything — and `dir` is where it landed. Both name the operator's
     content, so both belong inside the seal; only `upload_id` and `chunk_index`
     stay out, because the client matches on them.
+
+    `resume_from` answers the probe chunk (`UPLOAD_PROBE_INDEX`): how many
+    chunks of this file the node already holds. Inside the seal like the rest —
+    it is a fact about the operator's disk — and absent from an ordinary ack, so
+    a client can tell the two apart without looking at `chunk_index`.
     """
     payload = {"filename": filename, "stored_as": stored_as, "dir": dir}
+    if resume_from is not None:
+        payload["resume_from"] = int(resume_from)
     return {
         "type": MNP.FILE_UPLOAD_ACK,
         "v": MNP_VERSION,