Split seat_banner from seat_output.

Previously, SSH authentication banners were displayed by calling the
ordinary seat_output function, and passing it a special value in the
SeatOutputType enumeration indicating an auth banner.

The awkwardness of this was already showing a little in SshProxy's
implementation of seat_output, where it had to check for that special
value and do totally different things for SEAT_OUTPUT_AUTH_BANNER and
everything else. Further work in that area is going to make it more
and more awkward if I keep the two output systems unified.

So let's split them up. Now, Seat has separate output() and banner()
methods, which each implementation can override differently if it
wants to.

All the 'end user' Seat implementations use the centralised
implementation function nullseat_banner_to_stderr(), which turns
banner text straight back into SEAT_OUTPUT_STDERR and passes it on to
seat_output. So I didn't have to tediously implement a boring version
of this function in GTK, Windows GUI, consoles, file transfer etc.

proxy/sshproxy.c

@@ -262,20 +262,22 @@ static size_t sshproxy_output(Seat *seat, SeatOutputType type,
                               const void *data, size_t len)
 {
     SshProxy *sp = container_of(seat, SshProxy, seat);
-    if (type == SEAT_OUTPUT_AUTH_BANNER) {
-        if (sp->clientseat) {
-            /*
-             * If we have access to the outer Seat, pass the SSH login
-             * banner on to it.
-             */
-            return seat_output(sp->clientseat, type, data, len);
-        } else {
-            return 0;
-        }
+    bufchain_add(&sp->ssh_to_socket, data, len);
+    try_send_ssh_to_socket(sp);
+    return bufchain_size(&sp->ssh_to_socket);
+}
+
+static size_t sshproxy_banner(Seat *seat, const void *data, size_t len)
+{
+    SshProxy *sp = container_of(seat, SshProxy, seat);
+    if (sp->clientseat) {
+        /*
+         * If we have access to the outer Seat, pass the SSH login
+         * banner on to it.
+         */
+        return seat_banner(sp->clientseat, data, len);
     } else {
-        bufchain_add(&sp->ssh_to_socket, data, len);
-        try_send_ssh_to_socket(sp);
-        return bufchain_size(&sp->ssh_to_socket);
+        return 0;
     }
 }
 
@@ -452,6 +454,7 @@ static const SeatVtable SshProxy_seat_vt = {
     .output = sshproxy_output,
     .eof = sshproxy_eof,
     .sent = sshproxy_sent,
+    .banner = sshproxy_banner,
     .get_userpass_input = sshproxy_get_userpass_input,
     .notify_session_started = sshproxy_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

pscp.c

@@ -67,6 +67,7 @@ static const SeatVtable pscp_seat_vt = {
     .output = pscp_output,
     .eof = pscp_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = filexfer_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

psftp.c

@@ -48,6 +48,7 @@ static const SeatVtable psftp_seat_vt = {
     .output = psftp_output,
     .eof = psftp_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = filexfer_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

putty.h

@@ -925,7 +925,7 @@ typedef enum SeatInteractionContext {
 } SeatInteractionContext;
 
 typedef enum SeatOutputType {
-    SEAT_OUTPUT_STDOUT, SEAT_OUTPUT_STDERR, SEAT_OUTPUT_AUTH_BANNER
+    SEAT_OUTPUT_STDOUT, SEAT_OUTPUT_STDERR
 } SeatOutputType;
 
 /*
@@ -941,11 +941,10 @@ struct Seat {
 struct SeatVtable {
     /*
      * Provide output from the remote session. 'type' indicates the
-     * type of the output (stdout, stderr or SSH auth banner), which
-     * can be used to split the output into separate message channels,
-     * if the seat wants to handle them differently. But combining the
-     * channels into one is OK too; that's what terminal-window based
-     * seats do.
+     * type of the output (stdout or stderr), which can be used to
+     * split the output into separate message channels, if the seat
+     * wants to handle them differently. But combining the channels
+     * into one is OK too; that's what terminal-window based seats do.
      *
      * The return value is the current size of the output backlog.
      */
@@ -971,6 +970,14 @@ struct SeatVtable {
      */
     void (*sent)(Seat *seat, size_t new_sendbuffer);
 
+    /*
+     * Provide authentication-banner output from the session setup.
+     * End-user Seats can treat this as very similar to 'output', but
+     * intermediate Seats in complex proxying situations will want to
+     * implement this and 'output' differently.
+     */
+    size_t (*banner)(Seat *seat, const void *data, size_t len);
+
     /*
      * Try to get answers from a set of interactive login prompts. The
      * prompts are provided in 'p'.
@@ -1227,6 +1234,8 @@ static inline bool seat_eof(Seat *seat)
 { return seat->vt->eof(seat); }
 static inline void seat_sent(Seat *seat, size_t bufsize)
 { seat->vt->sent(seat, bufsize); }
+static inline size_t seat_banner(Seat *seat, const void *data, size_t len)
+{ return seat->vt->banner(seat, data, len); }
 static inline int seat_get_userpass_input(Seat *seat, prompts_t *p)
 { return seat->vt->get_userpass_input(seat, p); }
 static inline void seat_notify_session_started(Seat *seat)
@@ -1293,10 +1302,10 @@ static inline size_t seat_stderr(Seat *seat, const void *data, size_t len)
 { return seat_output(seat, SEAT_OUTPUT_STDERR, data, len); }
 static inline size_t seat_stderr_pl(Seat *seat, ptrlen data)
 { return seat_output(seat, SEAT_OUTPUT_STDERR, data.ptr, data.len); }
-static inline size_t seat_banner(Seat *seat, const void *data, size_t len)
-{ return seat_output(seat, SEAT_OUTPUT_AUTH_BANNER, data, len); }
+
+/* Alternative API for seat_banner taking a ptrlen */
 static inline size_t seat_banner_pl(Seat *seat, ptrlen data)
-{ return seat_output(seat, SEAT_OUTPUT_AUTH_BANNER, data.ptr, data.len); }
+{ return seat->vt->banner(seat, data.ptr, data.len); }
 
 /* In the utils subdir: print a message to the Seat which can't be
  * spoofed by server-supplied auth-time output such as SSH banners */
@@ -1313,6 +1322,8 @@ size_t nullseat_output(
     Seat *seat, SeatOutputType type, const void *data, size_t len);
 bool nullseat_eof(Seat *seat);
 void nullseat_sent(Seat *seat, size_t bufsize);
+size_t nullseat_banner(Seat *seat, const void *data, size_t len);
+size_t nullseat_banner_to_stderr(Seat *seat, const void *data, size_t len);
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p);
 void nullseat_notify_session_started(Seat *seat);
 void nullseat_notify_remote_exit(Seat *seat);

ssh/server.c

@@ -108,6 +108,7 @@ static const SeatVtable server_seat_vt = {
     .output = nullseat_output,
     .eof = nullseat_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner,
     .get_userpass_input = nullseat_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

ssh/sesschan.c

@@ -187,6 +187,7 @@ static const SeatVtable sesschan_seat_vt = {
     .output = sesschan_seat_output,
     .eof = sesschan_seat_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner,
     .get_userpass_input = nullseat_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = sesschan_notify_remote_exit,
@@ -614,11 +615,6 @@ static size_t sesschan_seat_output(
     Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
     sesschan *sess = container_of(seat, sesschan, seat);
-
-    /* We don't expect anything but stdout and stderr to come here,
-     * because the pty backend doesn't generate auth banners */
-    assert(type != SEAT_OUTPUT_AUTH_BANNER);
-
     return sshfwd_write_ext(sess->c, type == SEAT_OUTPUT_STDERR, data, len);
 }
 

unix/plink.c

@@ -391,6 +391,7 @@ static const SeatVtable plink_seat_vt = {
     .output = plink_output,
     .eof = plink_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = plink_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

unix/window.c

@@ -390,6 +390,7 @@ static const SeatVtable gtk_seat_vt = {
     .output = gtk_seat_output,
     .eof = gtk_seat_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = gtk_seat_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = gtk_seat_notify_remote_exit,

utils/nullseat.c

@@ -8,6 +8,9 @@ size_t nullseat_output(
     Seat *seat, SeatOutputType type, const void *data, size_t len) {return 0;}
 bool nullseat_eof(Seat *seat) { return true; }
 void nullseat_sent(Seat *seat, size_t bufsize) {}
+size_t nullseat_banner(Seat *seat, const void *data, size_t len) {return 0;}
+size_t nullseat_banner_to_stderr(Seat *seat, const void *data, size_t len)
+{ return seat_output(seat, SEAT_OUTPUT_STDERR, data, len); }
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p) { return 0; }
 void nullseat_notify_session_started(Seat *seat) {}
 void nullseat_notify_remote_exit(Seat *seat) {}

utils/tempseat.c

@@ -224,6 +224,11 @@ static int tempseat_get_userpass_input(Seat *seat, prompts_t *p)
     unreachable("get_userpass_input should never be called on TempSeat");
 }
 
+static size_t tempseat_banner(Seat *seat, const void *data, size_t len)
+{
+    unreachable("banner should never be called on TempSeat");
+}
+
 static int tempseat_confirm_ssh_host_key(
     Seat *seat, const char *host, int port, const char *keytype,
     char *keystr, const char *keydisp, char **key_fingerprints, bool mismatch,
@@ -299,6 +304,7 @@ static const struct SeatVtable tempseat_vt = {
     .output = tempseat_output,
     .eof = tempseat_eof,
     .sent = nullseat_sent,
+    .banner = tempseat_banner,
     .get_userpass_input = tempseat_get_userpass_input,
     .notify_session_started = tempseat_notify_session_started,
     .notify_remote_exit = tempseat_notify_remote_exit,

windows/plink.c

@@ -85,6 +85,7 @@ static const SeatVtable plink_seat_vt = {
     .output = plink_output,
     .eof = plink_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = plink_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,

windows/window.c

@@ -332,6 +332,7 @@ static const SeatVtable win_seat_vt = {
     .output = win_seat_output,
     .eof = win_seat_eof,
     .sent = nullseat_sent,
+    .banner = nullseat_banner_to_stderr,
     .get_userpass_input = win_seat_get_userpass_input,
     .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = win_seat_notify_remote_exit,