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commit d6fedac352a618123d3bd75c57528da16d8ff6c7
Author: Dridi Boukelmoune <dridi.boukelmoune@gmail.com>
Date: Mon Jan 29 10:59:22 2024 +0100

http2_send: Explain the winup-open discrepancy

diff --git a/bin/varnishd/http2/cache_http2_send.c b/bin/varnishd/http2/cache_http2_send.c
index ca6151fb2..c1e03a5dc 100644
--- a/bin/varnishd/http2/cache_http2_send.c
+++ b/bin/varnishd/http2/cache_http2_send.c
@@ -286,6 +286,18 @@ h2_do_window(struct worker *wrk, struct h2_req *r2,
assert (w > 0);
}

+ /* If all streams ran out of control flow window credits
+ * upon triggering h2_window_timeout, declare bankruptcy
+ * for the entire connection.
+ *
+ * But streams may be closed from the h2_sess thread while
+ * waiting for a window update. So the open_streams counter
+ * may be decremented in a different critical section than
+ * winup_streams, right before signalling the stream thread.
+ * So there may be more streams awaiting a window updates
+ * than streams officially open, hence the "lower-equal"
+ * comparison.
+ */
if (r2->error == H2SE_BROKE_WINDOW &&
h2->open_streams <= h2->winup_streams)
h2->error = r2->error = H2CE_BANKRUPT;
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