Move alloc_channel_id into sshcommon.c.

That function _did_ depend on ssh.c's internal facilities, namely the
layout of 'struct ssh_channel'. In place of that, it now takes an
extra integer argument telling it where to find the channel id in
whatever data structure you give it a tree of - so now I can split up
the SSH-1 and SSH-2 channel handling without losing the services of
that nice channel-number allocator.

ssh.c

@@ -814,38 +814,6 @@ static int ssh_rportcmp_ssh2(void *av, void *bv)
     return 0;
 }
 
-static unsigned alloc_channel_id(Ssh ssh)
-{
-    const unsigned CHANNEL_NUMBER_OFFSET = 256;
-    search234_state ss;
-
-    /*
-     * First-fit allocation of channel numbers: we always pick the
-     * lowest unused one.
-     *
-     * Every channel before that, and no channel after it, has an ID
-     * exactly equal to its tree index plus CHANNEL_NUMBER_OFFSET. So
-     * we can use the search234 system to identify the length of that
-     * initial sequence, in a single log-time pass down the channels
-     * tree.
-     */
-    search234_start(&ss, ssh->channels);
-    while (ss.element) {
-        struct ssh_channel *c = (struct ssh_channel *)ss.element;
-        if (c->localid == ss.index + CHANNEL_NUMBER_OFFSET)
-            search234_step(&ss, +1);
-        else
-            search234_step(&ss, -1);
-    }
-
-    /*
-     * Now ss.index gives exactly the number of channels in that
-     * initial sequence. So adding CHANNEL_NUMBER_OFFSET to it must
-     * give precisely the lowest unused channel number.
-     */
-    return ss.index + CHANNEL_NUMBER_OFFSET;
-}
-
 static void c_write_stderr(int trusted, const void *vbuf, int len)
 {
     const char *buf = (const char *)vbuf;
@@ -5676,7 +5644,7 @@ static int ssh_is_simple(Ssh ssh)
 static void ssh_channel_init(struct ssh_channel *c)
 {
     Ssh ssh = c->ssh;
-    c->localid = alloc_channel_id(ssh);
+    c->localid = alloc_channel_id(ssh->channels, struct ssh_channel);
     c->closes = 0;
     c->pending_eof = FALSE;
     c->throttling_conn = FALSE;

ssh.h

@@ -1326,3 +1326,11 @@ enum { SSH_IMPL_BUG_LIST(TMP_DECLARE_REAL_ENUM) };
 void write_ttymodes_to_packet_from_conf(
     BinarySink *bs, Frontend *frontend, Conf *conf,
     int ssh_version, int ospeed, int ispeed);
+
+/* Shared system for allocating local SSH channel ids. Expects to be
+ * passed a tree full of structs that have a field called 'localid' of
+ * type unsigned, and will check that! */
+unsigned alloc_channel_id_general(tree234 *channels, size_t localid_offset);
+#define alloc_channel_id(tree, type) \
+    TYPECHECK(&((type *)0)->localid == (unsigned *)0, \
+              alloc_channel_id_general(tree, offsetof(type, localid)))

sshcommon.c

@@ -466,3 +466,40 @@ void write_ttymodes_to_packet_from_conf(
     put_uint32(bs, ospeed);
     put_byte(bs, SSH_TTY_OP_END);
 }
+
+/* ----------------------------------------------------------------------
+ * Routine for allocating a new channel ID, given a means of finding
+ * the index field in a given channel structure.
+ */
+
+unsigned alloc_channel_id_general(tree234 *channels, size_t localid_offset)
+{
+    const unsigned CHANNEL_NUMBER_OFFSET = 256;
+    search234_state ss;
+
+    /*
+     * First-fit allocation of channel numbers: we always pick the
+     * lowest unused one.
+     *
+     * Every channel before that, and no channel after it, has an ID
+     * exactly equal to its tree index plus CHANNEL_NUMBER_OFFSET. So
+     * we can use the search234 system to identify the length of that
+     * initial sequence, in a single log-time pass down the channels
+     * tree.
+     */
+    search234_start(&ss, channels);
+    while (ss.element) {
+        unsigned localid = *(unsigned *)((char *)ss.element + localid_offset);
+        if (localid == ss.index + CHANNEL_NUMBER_OFFSET)
+            search234_step(&ss, +1);
+        else
+            search234_step(&ss, -1);
+    }
+
+    /*
+     * Now ss.index gives exactly the number of channels in that
+     * initial sequence. So adding CHANNEL_NUMBER_OFFSET to it must
+     * give precisely the lowest unused channel number.
+     */
+    return ss.index + CHANNEL_NUMBER_OFFSET;
+}