I changed libguac/display.c(around line 238).
Before:
void guac_display_dup(guac_display* display, guac_socket* socket) {
guac_client* client = display->client;
guac_rwlock_acquire_read_lock(&display->last_frame.lock);
….
After:
void guac_display_dup(guac_display* display, guac_socket* socket) {
guac_client* client = display->client;
guac_rwlock_acquire_read_lock(&display->pending_frame.lock);
guac_rwlock_acquire_read_lock(&display->last_frame.lock);
…
Best regards,
Markus Timpl
Von: Dilip Modi via user <[email protected]>
Gesendet: Donnerstag, 29. Januar 2026 10:43
An: [email protected]
Cc: Dilip Modi <[email protected]>
Betreff: Re: [External⚠️] AW: EXT: VNC : User joins existing session gets stuck
in guac_display_dup
EXTERNAL SENDER
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Thanks for sharing your thoughts.
If you don't mind, would you be able to share the code changes which helped to
fix this ? I found an issue where avoiding processessing cursor buffers helped
me avoid the deadlock. Something like below code.
/* Sync the state of all layers/buffers */
guac_display_layer*
current
= display->last_frame.layers;
while (current
!=
NULL) {
/*
* The cursor buffer is handled separately. If it ends up in the main
* layer list, just skip it.
*/
if (current
==
display->cursor_buffer) {
current
= current->last_frame.next;
continue;
}
On Thu, Jan 29, 2026 at 1:38 PM Timpl, Markus
<[email protected]<mailto:[email protected]>> wrote:
Hello,
I had a lot of deadlocks/issues with users joining existing connections as
well(see https://lists.apache.org/thread/3bjbtstq1qln9vgcly3qhkdzvhjzxbbl),
although it was specifically for RDP but we are also using VNC connections with
multiple users with the fix.
What fixed it for me was to acquire the pending_frame.lock in guac_display_dup
before getting the last_frame.lock and to have the owner of the connection
never disconnect from the connection. With those changes I don’t get any
deadlocks on multiple clients connecting and disconnecting from the same
connection.
I sadly didn’t yet have time to create a JIRA user/issue + MR for the fix.
Best regards,
Markus Timpl
Von: Dilip Modi via user
<[email protected]<mailto:[email protected]>>
Gesendet: Mittwoch, 28. Januar 2026 18:26
An: [email protected]<mailto:[email protected]>
Cc: Dilip Modi <[email protected]<mailto:[email protected]>>
Betreff: EXT: VNC : User joins existing session gets stuck in guac_display_dup
EXTERNAL SENDER
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be cautious when clicking links or opening attachments. Never enter username
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Hello Guacamole Developers,
I hope this email finds you well.
I am using VNC with guacd 1.6.0 version.
I am finding an issue where user joins VNC session and doesn’t get display in
browser. This is happening when we run https://giphy.com<https://giphy.com/>
for display frame update.
I'm currently looking into the guacamole-server source code, specifically the
display synchronization logic in src/libguac/display.c. I have a question about
the behavior of the guac_display_dup() function.
In this function, there is a call to guac_flag_wait_and_lock() to wait for any
in-progress frame rendering to complete before synchronizing the display for a
new user. The comment just above this call says:
/* ... A timeout is used here to avoid blocking indefinitely if a frame is
already
* in progress. */
guac_flag_wait_and_lock(&display->render_state,
GUAC_DISPLAY_RENDER_STATE_FRAME_NOT_IN_PROGRESS);
However, from what I can tell, guac_flag_wait_and_lock() seems to be a blocking
call without a timeout. This could potentially cause the guac_display_dup()
function to hang indefinitely if the
GUAC_DISPLAY_RENDER_STATE_FRAME_NOT_IN_PROGRESS flag is never set for some
reason during an active connection.
Could you please clarify if this is the intended behavior? Perhaps I am missing
something regarding a timeout mechanism that is implemented elsewhere.
Need your input to fix this issue.
Thank you for your time and for all the great work on Guacamole.
Best regards,
Dilip
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