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<a href="https://gitlab.com/CendioOssman" style="color: #1068bf; text-decoration: none;">Pierre Ossman (Work account)</a>
<a href="https://gitlab.com/gnutls/gnutls/-/issues/1251#note_902836387" style="color: #1068bf; text-decoration: none;">commented</a>:
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<p dir="auto" style="color: #303030; margin: 0 0 16px;" align="initial">I agree and we've been bitten by this in TigerVNC as well. A brief glance of the GnuTLS code suggests this can happen in a lot of different scenarios, so fixing the implementation seems to be non-trivial.</p>
<p dir="auto" style="color: #303030; margin: 0 0 16px;" align="initial">A more reasonable short term goal is to update the documentation to warn about this. Right now the documentation implies that you will only get <code style='font-size: 90%; color: #1f1f1f; word-wrap: break-word; background-color: #f0f0f0; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: "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_AGAIN</code> if the underlying pull function sets <code style='font-size: 90%; color: #1f1f1f; word-wrap: break-word; background-color: #f0f0f0; border-radius: 4px; font-weight: inherit; font-family: "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;'>errno</code> to <code style='font-size: 90%; color: #1f1f1f; word-wrap: break-word; background-color: #f0f0f0; border-radius: 4px; font-weight: inherit; font-family: "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;'>EAGAIN</code>, which is obviously not true.</p>
<p dir="auto" style="color: #303030; margin: 0 0 16px;" align="initial">In our case we got this problem when interoperating with x11vnc, using OpenSSL. The issue seems to be reception of a session ticket that contains no data. Debug output from GnuTLS for this:</p>
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<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true" style='display: block; font-size: 13px; color: #303030; text-shadow: none; line-height: 1.6em; overflow-x: auto; border-radius: 2px; position: relative; font-family: "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: #fafafa; margin: 0 0 16px; padding: 12px; border: 1px solid #dbdbdb;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: "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;">gnutls[10]: READ: Got 5 bytes from 0x2765530</span>
<span id="LC2" class="line" lang="plaintext">gnutls[10]: READ: read 5 bytes from 0x2765530</span>
<span id="LC3" class="line" lang="plaintext">gnutls[10]: RB: Have 0 bytes into buffer. Adding 5 bytes.</span>
<span id="LC4" class="line" lang="plaintext">gnutls[10]: RB: Requested 5 bytes</span>
<span id="LC5" class="line" lang="plaintext">gnutls[5]: REC[0x2756ee0]: SSL 3.3 Application Data packet received. Epoch 2, length: 250</span>
<span id="LC6" class="line" lang="plaintext">gnutls[5]: REC[0x2756ee0]: Expected Packet Application Data(23)</span>
<span id="LC7" class="line" lang="plaintext">gnutls[5]: REC[0x2756ee0]: Received Packet Application Data(23) with length: 250</span>
<span id="LC8" class="line" lang="plaintext">gnutls[10]: READ: Got 250 bytes from 0x2765530</span>
<span id="LC9" class="line" lang="plaintext">gnutls[10]: READ: read 250 bytes from 0x2765530</span>
<span id="LC10" class="line" lang="plaintext">gnutls[10]: RB: Have 5 bytes into buffer. Adding 250 bytes.</span>
<span id="LC11" class="line" lang="plaintext">gnutls[10]: RB: Requested 255 bytes</span>
<span id="LC12" class="line" lang="plaintext">gnutls[5]: REC[0x2756ee0]: Decrypted Packet[1] Handshake(22) with length: 233</span>
<span id="LC13" class="line" lang="plaintext">gnutls[3]: ASSERT: buffers.c[get_last_packet]:1169</span>
<span id="LC14" class="line" lang="plaintext">gnutls[4]: HSK[0x2756ee0]: NEW SESSION TICKET (4) was received. Length 229[229], frag offset 0, frag length: 229, sequence: 0</span>
<span id="LC15" class="line" lang="plaintext">gnutls[3]: ASSERT: buffers.c[_gnutls_handshake_io_recv_int]:1429</span>
<span id="LC16" class="line" lang="plaintext">gnutls[4]: HSK[0x2756ee0]: parsing session ticket message</span>
<span id="LC17" class="line" lang="plaintext">gnutls[3]: ASSERT: record.c[_gnutls_recv_in_buffers]:1577</span>
<span id="LC18" class="line" lang="plaintext">gnutls[3]: ASSERT: record.c[_gnutls_recv_int]:1775</span></code></pre>
<copy-code></copy-code>
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<p dir="auto" style="color: #303030; margin: 0 0 16px;" align="initial">At this point we go back to doing <code style='font-size: 90%; color: #1f1f1f; word-wrap: break-word; background-color: #f0f0f0; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: "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;'>select()</code> on the socket, which never completes as we've already read the data and we have more records in our buffer ready to be pulled by GnuTLS.</p>
<p dir="auto" style="color: #303030; margin: 0 0 16px;" align="initial">Downstream bug report, with references to workaround:</p>
<p dir="auto" style="color: #303030; margin: 0;" align="initial"><a href="https://github.com/TigerVNC/tigervnc/issues/1450" rel="nofollow noreferrer noopener" target="_blank" style="color: #1068bf; margin-top: 0; text-decoration: none;">https://github.com/TigerVNC/tigervnc/issues/1450</a></p>
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