odb--daemon: implement get-nth-ancestor command

Implement a basic version of the get-nth-ancestor command in the ODB
daemon that will simply call the `get_nth_ancestor` function that is
being routed over the IPC channel with the same arguments.

This will get us a simple working proof of concept.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

builtin/odb--daemon.c

@@ -255,6 +255,50 @@ static int odb_ipc_cb__get_parent(struct my_odb_ipc_state *state,
 	return 0;
 }
 
+static int odb_ipc_cb__get_nth_ancestor(struct my_odb_ipc_state *state,
+					const char *command, size_t command_len,
+					ipc_server_reply_cb *reply_cb,
+					struct ipc_server_reply_data *reply_data)
+{
+	struct odb_over_ipc__get_nth_ancestor__request *req;
+	struct odb_over_ipc__get_nth_ancestor__response *resp;
+	const char *name;
+	size_t name_len;
+	int ret;
+
+	if (command_len < sizeof(*req))
+		BUG("incorrect size for binary data");
+
+	req = (struct odb_over_ipc__get_nth_ancestor__request *)command;
+
+	name = command + sizeof(*req);
+	name_len = command_len - sizeof(*req);
+
+	if (req->name_len != name_len)
+		BUG("incorrect data length");
+
+	resp = xmalloc(sizeof(*resp));
+	memcpy(&resp->key.key, "get-nth-ancestor", 11);
+
+	ret = get_nth_ancestor(the_repository, name, name_len, &resp->oid,
+			       req->generation);
+
+	if (ret != FOUND)
+		goto fail;
+
+	reply_cb(reply_data, (const char *)resp, sizeof(*resp));
+
+	return 0;
+
+fail:
+	/*
+	 * Send the client an error response to force it to do
+	 * the work itself.
+	 */
+	reply_cb(reply_data, "error", 6);
+	return 0;
+}
+
 /*
  * This callback handles IPC requests from clients.  We run on an
  * arbitrary thread.
@@ -333,6 +377,18 @@ static int odb_ipc_cb(void *data,
 		return 0;
 	}
 
+	if (!strcmp(command, "get-nth-ancestor")) {
+		/*
+		 * A client has requested that we find the nth ancestpr of a
+		 * given object.
+		 */
+		trace2_region_enter("odb-daemon", "get-nth-ancestor", NULL);
+		ret = odb_ipc_cb__get_nth_ancestor(state, command, command_len,
+						   reply_cb, reply_data);
+		trace2_region_leave("odb-daemon", "get-nth-ancestor", NULL);
+		return 0;
+	}
+
 	// TODO respond to other requests from client.
 	//
 	// TODO decide how to return an error for unknown commands.

object-name.c

@@ -1125,6 +1125,9 @@ enum get_oid_result get_nth_ancestor(struct repository *r,
 	struct commit *commit;
 	int ret;
 
+	if (odb_over_ipc__get_nth_ancestor(r, name, len, generation, result) == 0)
+		return FOUND;
+
 	ret = get_oid_1(r, name, len, &oid, GET_OID_COMMITTISH);
 	if (ret)
 		return ret;

odb-over-ipc.c

@@ -311,4 +311,52 @@ int odb_over_ipc__get_parent(struct repository *r, const char *name, int len,
 	return ret;
 }
 
+int odb_over_ipc__get_nth_ancestor(struct repository *r, const char *name,
+				   int len, int generation,
+				   struct object_id *result)
+{
+	struct odb_over_ipc__get_nth_ancestor__request req;
+	struct odb_over_ipc__get_nth_ancestor__response *resp;
+	struct strbuf msg = STRBUF_INIT;
+	struct strbuf answer = STRBUF_INIT;
+	int ret;
+
+	if (is_daemon)
+		return -1;
+
+	if (!core_use_odb_over_ipc)
+		return -1;
+
+	if (r != the_repository)	// TODO not dealing with this
+		return -1;
+
+	memset(&req, 0, sizeof(req));
+	memcpy(req.key.key, "get-nth-ancestor", 16);
+	req.generation = generation;
+	req.name_len = len;
+
+	/* Append the name at the end of the request */
+	strbuf_init(&msg, sizeof(req) + len);
+	strbuf_add(&msg, &req, sizeof(req));
+	strbuf_add(&msg, name, len);
+
+	ret = odb_over_ipc__command((const char *)msg.buf, msg.len, &answer);
+	if (ret)
+		return ret;
+
+	if (!strncmp(answer.buf, "error", 5)) {
+		trace2_printf("odb-over-ipc: failed");
+		return -1;
+	}
+
+	if (answer.len != sizeof(*resp))
+		BUG("incorrect size for binary data");
+	resp = (struct odb_over_ipc__get_nth_ancestor__response *)answer.buf;
+
+	oidcpy(result, &resp->oid);
+
+	strbuf_release(&answer);
+	return ret;
+}
+
 #endif /* SUPPORTS_SIMPLE_IPC */

odb-over-ipc.h

@@ -95,6 +95,19 @@ struct odb_over_ipc__get_parent__response
 	struct object_id oid;
 };
 
+struct odb_over_ipc__get_nth_ancestor__request
+{
+	struct odb_over_ipc__key key;
+	int generation;
+	int name_len;
+};
+
+struct odb_over_ipc__get_nth_ancestor__response
+{
+	struct odb_over_ipc__key key;
+	struct object_id oid;
+};
+
 /*
  * Connect to an existing `git odb--daemon` process and ask it for
  * an object.  This is intended to be inserted into the client
@@ -116,6 +129,10 @@ int odb_over_ipc__hash_object(struct repository *r, struct object_id *oid,
 int odb_over_ipc__get_parent(struct repository *r, const char *name, int len,
 			     int idx, struct object_id *result);
 
+int odb_over_ipc__get_nth_ancestor(struct repository *r, const char *name,
+				   int len, int generation,
+				   struct object_id *result);
+
 /*
  * Explicitly shutdown IPC connection to the `git odb--daemon` process.
  * The connection is implicitly created upon the first request and we