SCP: let the application start the transfer

An application that binds an "scp ..." command to a channel itself has no
way to run the transfer; wolfSSH_accept() did it through a WS_SCP_INIT
re-entry only that state machine can drive.

- add wolfSSH_SCP_accept(), a wrapper over DoScpRequest() reporting
  WS_SCP_COMPLETE for any non-negative result, as accept() does
- a receive-side want reaches the wrapper as a generic error with the
  want in ssh->error, so report the want itself and let the caller retry
- state that resume contract beside the prototype, and clear a stale want
  on entry the way the other re-entrant entry points do

src/wolfscp.c

@@ -883,6 +883,40 @@ int DoScpSource(WOLFSSH* ssh)
     return ret;
 }
 
+/* Contract is in wolfssh/wolfscp.h. */
+int wolfSSH_SCP_accept(WOLFSSH* ssh)
+{
+    int ret;
+
+    if (ssh == NULL)
+        return WS_BAD_ARGUMENT;
+
+    /* Clear a want left by the previous call so the retry starts clean,
+     * the way the other re-entrant entry points do. */
+    if (ssh->error == WS_WANT_READ || ssh->error == WS_WANT_WRITE)
+        ssh->error = WS_SUCCESS;
+
+    ret = DoScpRequest(ssh);
+
+    if (ret >= WS_SUCCESS) {
+        /* The tail of DoScpRequest() passes a read count through, so treat
+         * anything non-negative as done the way wolfSSH_accept() does. */
+        ret = WS_SCP_COMPLETE;
+    }
+    else {
+        /* A non-blocking want on a read path surfaces as a generic error
+         * with the want recorded in ssh->error (see GetInputData), so
+         * report it as the want the caller is told to retry on. */
+        int err = wolfSSH_get_error(ssh);
+
+        if (err == WS_WANT_READ || err == WS_WANT_WRITE)
+            ret = err;
+    }
+
+    return ret;
+}
+
+
 int DoScpRequest(WOLFSSH* ssh)
 {
     int ret = WS_SUCCESS;

wolfssh/wolfscp.h

@@ -158,6 +158,17 @@ WOLFSSH_API int   wolfSSH_SCP_to(WOLFSSH* ssh, const char* src,
         const char* dst);
 WOLFSSH_API int   wolfSSH_SCP_from(WOLFSSH* ssh, const char* src,
         const char* dst);
+/* Server side. Drives an SCP transfer on a channel whose "exec scp ..."
+ * command is already bound. This is the same work wolfSSH_accept() does
+ * through its WS_SCP_INIT re-entry, exposed so an application can start the
+ * transfer itself; use one or the other, not both. Call it once
+ * wolfSSH_accept() has returned and the exec channel-request callback has
+ * reported an SCP command, not from inside that callback.
+ *
+ * Returns WS_SCP_COMPLETE when the transfer is done. On a non-blocking
+ * socket it returns WS_WANT_READ or WS_WANT_WRITE with the transfer part
+ * done; call it again on the same session until it completes. */
+WOLFSSH_API int   wolfSSH_SCP_accept(WOLFSSH* ssh);
 
 
 #ifdef __cplusplus