Bug fix: guard against left recursion in DTDs.

[svenemtell]

This is my suggested fix to the problem discussed in #9.
I don't have a good understanding of the function patched but have merely tried to copy the essence of the fix mentioned by ruricolist and made some personal tests.

[luismbo]

@svenemtell could you add a test case as well?

[svenemtell]

@luismbo Is there a test suite that I can add to?
The tests in /test seem to look for the local path "/home/david/2001/...". Are the instructions regarding downloading test suites at https://common-lisp.net/project/cxml/installation.html#tests up to date?
I tried to use the test suite mentioned in #4 but got an error:

(cxml-tests:run-tests)

Running test suite CXML
 Running test suite XMLCONF
  Running test SAX valid/sa/012.xml: test applies to parsers without namespace support, skipping

Error: duplicate internal subset
  1 (continue) Skip this test
  2 Rerun the test #<IT.BESE.FIVEAM::TEST-CASE SAX 41110EB403>
  3 Signal an exceptional test failure and abort the test #<IT.BESE.FIVEAM::TEST-CASE SAX 41110EB403>.
  4 Ignore the rest of the tests and explain current results
  5 (abort) Return to top loop level 0.

[svenemtell]

I forked the test suite from @ruricolist and added a test case svenemtell/cxml-tests@f4210da.

Running the test suite without this pull request produces a Stack overflow while running the test suite with the pull request doesn't produce a Stack overflow.

This is how I run the test suite in LispWorks Professional 7.1.2 64-bit on macOS:

(pushnew "/Volumes/source/" ql:*local-project-directories*)
(ql:quickload :cxml)
(ql:quickload :cxml-tests)
(cxml-tests:run-tests)

The output without the pull request
...
xmltest/valid/sa/120.xml [not validating:] input/output [validating:] FAILED:
Stack overflow (stack size 15998).
[
Left recursive.]
...
7/1842 tests failed; 744 tests were skipped.
...

The output with the pull request does not contain the Stack overflow
...
6/1842 tests failed; 744 tests were skipped.
...

[luismbo]

@svenemtell cool, maybe you'll want to submit your new test to cxml-tests and at some point someone should merge cxml-tests into this project, yes, no, maybe?

[svenemtell]

Great!
I added this pull request ruricolist/cxml-tests#1.
Is some work needed to have this fix in the next quicklisp dist, or is it done automatically?