<div class="markdown_content"><p>Stefan additionally notes on the old bug (quote):</p>
<p>Here are two examples of queries that still run into infinite loops (work in most corpora)</p>
<div class="codehilite"><pre><span></span>(<p> <s>)* "yyz";
(<s> [::])* "yyz";
</pre></div>
<p>Most such queries are likely to be intentional malicious attacks.</p>
<hr/>
<p><strong> <a class="alink" href="https://sourceforge.net/p/cwb/feature-requests/66/">[feature-requests:#66]</a> Cleanly avoid infinite loop queries</strong></p>
<p><strong>Status:</strong> open<br/>
<strong>Group:</strong> TODO-4.0<br/>
<strong>Labels:</strong> CQP engine <br/>
<strong>Created:</strong> Thu Apr 08, 2021 12:30 AM UTC by Andrew Hardie<br/>
<strong>Last Updated:</strong> Thu Apr 08, 2021 12:32 AM UTC<br/>
<strong>Owner:</strong> Stefan Evert</p>
<p>In version 3, the following (meaningless) query used to get caught in an infinite loop, i.e. CQP runs forever with full CPU power.</p>
<div class="codehilite"><pre><span></span>"<span class="nt"><s></span> * <span class="nt"></s></span>"
</pre></div>
<p>(this was bug #4: <a href="https://sourceforge.net/p/cwb/bugs/4/">https://sourceforge.net/p/cwb/bugs/4/</a> )</p>
<p>The explanation:</p>
<p>(begin quote)==================</p>
<p>What happens is that in the FSA constructed from the query,<br/>
<s> is a transition that does not "consume" a token, so <s>*<br/>
(Kleene star over <s>) effectively generates an eps-loop at<br/>
the start state of the FSA. Normally, such errors are<br/>
caught because the start state is also a final state (the<br/>
query "<s> *" would result in such an error message), but in<br/>
this case, the additional constraint </s> (which can never<br/>
be satisfied) inserts another transition.</p>
<p>There should probably some test for eps-loops in the FSA<br/>
simulation, which can never do anything useful (unless I'm<br/>
mistaken).</p>
<p>(end quote)==================</p>
<p>The bug was fixed long ago, with a check for infinite loops which aborts CQP:</p>
<div class="codehilite"><pre><span></span><span class="nv">CQP</span> <span class="nv">Error</span>:
<span class="nv">Infinite</span> <span class="k">loop</span> <span class="nv">detected</span>: <span class="nv">did</span> <span class="nv">you</span> <span class="nv">quantify</span> <span class="nv">over</span> <span class="nv">a</span> <span class="nv">zero</span><span class="o">-</span><span class="nv">width</span> <span class="nv">element</span> <span class="ss">(</span><span class="nv">XML</span> <span class="nv">tag</span> <span class="nv">or</span> <span class="nv">lookahead</span><span class="ss">)</span>?
<span class="k">If</span> <span class="nv">you</span> <span class="nv">are</span> <span class="nv">reasonably</span> <span class="nv">sure</span> <span class="nv">that</span> <span class="nv">your</span> <span class="nv">query</span> <span class="nv">is</span> <span class="nv">valid</span>, <span class="nv">please</span> <span class="nv">contact</span> <span class="nv">the</span> <span class="nv">CWB</span> <span class="nv">development</span> <span class="nv">team</span> <span class="nv">and</span> <span class="nv">file</span> <span class="nv">a</span> <span class="nv">bug</span> <span class="nv">report</span><span class="o">!</span>
<span class="nv">Query</span> <span class="nv">execution</span> <span class="nv">aborted</span>.
</pre></div>
<p>But, as Stefan noted,</p>
<p>"due to the messy internals of the query implementation, it may still be possible to write CQP queries that trigger an infinite loop. A clean solution will be possible when the query evaluation mechanism is completely overhauled for CWB 4.0. We should keep the ticket pending until then so we remember to consider the case of such "empty loop" queries."</p>
<p>This FR records the need for a clean mechanism to avoid such infinite loops in CWB4.</p>
<hr/>
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