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<a href="https://gitlab.com/rwmjones">Richard W.M. Jones</a>
<a href="https://gitlab.com/gnutls/gnutls/-/issues/1470#note_1308821019">commented</a>:
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">I think I may understand the problem. I collected an <code style='font-size: 0.875rem; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>strace</code> of the server when the desynchronisation happens, and the pattern of syscalls is odd. We see:</p>
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<pre lang="plaintext" class="code highlight js-syntax-highlight language-plaintext" data-canonical-lang="" v-pre="true" style='display: block; font-size: 0.875rem; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: 0.875rem; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">sendmsg(7, {msg_name=NULL, msg_namelen=0, msg_iov=[{... iov_len=262144}], msg_iovlen=1, msg_control=[{cmsg_len=17, cmsg_</span>
<span id="LC2" class="line" lang="plaintext">level=SOL_TLS, cmsg_type=0x1}], msg_controllen=17, msg_flags=0}, MSG_DONTWAIT = 245760</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">In other words, a short write. We wanted to send 262144 bytes, but only sent 245760 (16K less).</p>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">Then I see:</p>
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<pre lang="plaintext" class="code highlight js-syntax-highlight language-plaintext" data-canonical-lang="" v-pre="true" style='display: block; font-size: 0.875rem; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: 0.875rem; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">sendmsg(7, {msg_name=NULL, msg_namelen=0, msg_iov=[{... iov_len=16384}], msg_iovlen=1, msg_control=[{cmsg_len=17, cmsg_level=SOL_TLS, cmsg_type=0x1}], msg_controllen=17, msg_flags=0}, MSG_DONTWAIT = -1 EAGAIN (Resource temporarily unavailable)</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">It's trying to follow on with the remaining 16K of data, and gets EAGAIN. But immediately after:</p>
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<pre lang="plaintext" class="code highlight js-syntax-highlight language-plaintext" data-canonical-lang="" v-pre="true" style='display: block; font-size: 0.875rem; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: 0.875rem; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">sendmsg(7, {msg_name=NULL, msg_namelen=0, msg_iov=[{... iov_len=262144}], msg_iovlen=1, msg_control=[{cmsg_len=17, cmsg_level=SOL_TLS, cmsg_type=0x1}], msg_controllen=17, msg_flags=0}, MSG_DONTWAIT= -1 EAGAIN (Resource temporarily unavailable)</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">It's trying to send the full block (the data is the same as before) again, which is definitely wrong.</p>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">The documentation for <code style='font-size: 0.875rem; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>GNUTLS_E_INTERRUPTED</code> is quite confusing to be honest:</p>
<blockquote dir="auto" style="font-size: inherit; color: #535158; padding-top: 0.5rem; padding-bottom: 0.5rem; padding-left: 1.5rem; box-shadow: inset 4px 0 0 0 #dcdcde; border-top-width: 0; border-bottom-width: 0; border-right-width: 0; margin: 0.5rem 0;" align="initial">
<p style="color: inherit; line-height: 1.5; margin: 0;">If the EINTR is returned by the internal push function then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again with the exact same parameters, or provide a NULL pointer for data and 0 for data_size , in order to write the same data as before. If you wish to discard the previous data instead of retrying, you must call gnutls_record_discard_queued() before calling this function with different parameters. Note that the latter works only on special transports (e.g., UDP). cf. gnutls_record_get_direction().</p>
</blockquote>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">Our code is here, and I think it's doing the right thing but I'm not certain of it:</p>
<p dir="auto" style="color: #333238; margin: 0;" align="initial"><a href="https://gitlab.com/nbdkit/nbdkit/-/blob/45b72f5bd8fc1b475fa130d06c86cd877bf595d5/server/crypto.c#L399" style="margin-top: 0;">https://gitlab.com/nbdkit/nbdkit/-/blob/45b72f5bd8fc1b475fa130d06c86cd877bf595d5/server/crypto.c#L399</a></p>
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