[channels] Added ssh_channel_window_size() and avoided reentrancy in channel_write_common()

(cherry picked from commit 27313334578f4acb7ccf0687bba7f76b2febd569)

include/libssh/libssh.h

@@ -359,6 +359,7 @@ LIBSSH_API int ssh_channel_select(ssh_channel *readchans, ssh_channel *writechan
         timeval * timeout);
 LIBSSH_API void ssh_channel_set_blocking(ssh_channel channel, int blocking);
 LIBSSH_API int ssh_channel_write(ssh_channel channel, const void *data, uint32_t len);
+LIBSSH_API uint32_t ssh_channel_window_size(ssh_channel channel);
 
 LIBSSH_API int ssh_try_publickey_from_file(ssh_session session, const char *keyfile,
     ssh_string *publickey, int *type);

src/channels.c

@@ -1173,15 +1173,12 @@ int channel_write_common(ssh_channel channel, const void *data,
           "Remote window is %d bytes. going to write %d bytes",
           channel->remote_window,
           len);
-      ssh_log(session, SSH_LOG_PROTOCOL,
-          "Waiting for a growing window message...");
       /* What happens when the channel window is zero? */
-      while(channel->remote_window == 0) {
-        /* parse every incoming packet */
-        if (ssh_handle_packets(session,-1) == SSH_ERROR) {
-          leave_function();
-          return SSH_ERROR;
-        }
+      if(channel->remote_window == 0) {
+          /* nothing can be written */
+          ssh_log(session, SSH_LOG_PROTOCOL,
+                "Wait for a growing window message...");
+          return 0;
       }
       effectivelen = len > channel->remote_window ? channel->remote_window : len;
     } else {
@@ -1230,6 +1227,10 @@ error:
   return SSH_ERROR;
 }
 
+uint32_t ssh_channel_window_size(ssh_channel channel) {
+    return channel->remote_window;
+}
+
 /**
  * @brief Blocking write on a channel.
  *