commit 074230876d05bdf3fe33893889b326da14ab8ae9
Author: Christopher Faulet <cfaulet@haproxy.com>
Date:   Thu Oct 24 10:31:01 2019 +0200

    BUG/MINOR: mux-h2: Don't pretend mux buffers aren't full anymore if nothing sent
    
    In h2_send(), when something is sent, we remove the flags
    (H2_CF_MUX_MFULL|H2_CF_DEM_MROOM) on the h2 connection. This way, we are able to
    wake up all streams waiting to send data. Unfortunatly, these flags are
    unconditionally removed, even when nothing was sent. So if the h2c is blocked
    because the mux buffers are full and we are unable to send anything, all streams
    in the send_list are woken up for nothing. Now, we only remove these flags if at
    least a send succeeds.
    
    This patch must be backport to 2.0.
    
    (cherry picked from commit 69fe5cea213afd0c7465094e9dfead93143dcf3f)
    Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

diff --git a/src/mux_h2.c b/src/mux_h2.c
index afa68e80..ac34a723 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2943,7 +2943,8 @@ static int h2_send(struct h2c *h2c)
 			offer_buffers(NULL, tasks_run_queue);
 
 		/* wrote at least one byte, the buffer is not full anymore */
-		h2c->flags &= ~(H2_CF_MUX_MFULL | H2_CF_DEM_MROOM);
+		if (sent)
+			h2c->flags &= ~(H2_CF_MUX_MFULL | H2_CF_DEM_MROOM);
 	}
 
 	if (conn->flags & CO_FL_SOCK_WR_SH) {