feat(d5-daemon-motd): daemon MOTD display + --no-motd

src/shared/motd.h

@@ -0,0 +1,54 @@
+#ifndef MOTD_H
+#define MOTD_H
+
+#include <stdbool.h>
+
+/* Daemon Message-Of-The-Day (Wave C).
+ *
+ * The daemon listener (fastsync-server --daemon) may advertise a `motd file`
+ * configured in its globals.  When a client connects with a host::module/path
+ * destination and the module gate accepts the connection, the server sends the
+ * MOTD as a single string frame BEFORE any transfer data (rsync sends its MOTD
+ * as the first thing from the server at the start of a daemon connection).
+ * The client reads that frame right after the config/status handshake and
+ * displays it on stdout unless --no-motd was given.
+ *
+ * The MOTD is ordinary display text, never a secret, so it uses the normal
+ * (non-redacted) string primitive.  The exchange is strictly server->client
+ * and happens on the daemon listener path only; the --stdio SSH path has no
+ * MOTD.
+ *
+ * No PROTOCOL_VERSION bump is involved: the frame is sent and read
+ * symmetrically by every 2.15.0 daemon build (the strict same-version
+ * handshake rejects any other version before the frame), so it cannot
+ * desynchronize a peer. */
+
+/* Upper bound on the MOTD bytes the server will read from disk and put on the
+ * wire.  Kept far below MAX_STRING_SIZE (64 KB) so a huge/hostile motd file
+ * can never produce an unbounded frame or allocation. */
+#define MOTD_MAX_BYTES 4096
+
+/* Read a daemon MOTD file, bounded to MOTD_MAX_BYTES.  Returns a malloc'd
+ * NUL-terminated copy of the file content (bytes beyond the bound are
+ * truncated) or NULL when path is NULL/empty, the file cannot be opened or
+ * read, or allocation fails.  An absent or unreadable motd file is NOT an
+ * error: the caller simply sends an empty MOTD frame and continues. */
+char* motd_read_file(const char* path);
+
+/* Render MOTD text for terminal display.  Newlines and tabs are preserved so
+ * a multi-line motd still reads naturally, while every other non-printable /
+ * control byte (ESC included) is escaped with FastSync's `\NNN` octal
+ * convention, so a hostile server cannot inject terminal escape sequences
+ * through the MOTD.  eight_bit_output keeps bytes >= 0x80 verbatim (matching
+ * --8-bit-output).  Returns a malloc'd string or NULL on allocation failure. */
+char* motd_render(const char* motd, bool eight_bit_output);
+
+/* Send/receive the MOTD string frame.  These wrap the normal string
+ * primitive: the MOTD is not a credential, so no redaction is used.  The
+ * receiver additionally rejects an over-bound frame (> MOTD_MAX_BYTES) as a
+ * hostile input guard; the frame itself is always fully consumed first, so the
+ * stream stays framed. */
+bool motd_send(int file_descriptor, const char* motd);
+char* motd_receive(int file_descriptor);
+
+#endif