Fixes to eXist-db regarding circular imports

[adamretter]

These errors although mostly in eXist-db itself were easy to manifest through using RESTXQ.

Closes #3448
Closes #1010
Closes #4699

1. The first thing that this PR deals with is a Library Module that attempts to import itself. Whilst BaseX raises an error, and Saxon allows it. With this PR eXist-db now follows Saxon in basically skipping the superfluous import and continues. This is inline with the W3C XQuery 3.1 specification which states:

   The namespace prefix specified in a module declaration must not be xml or xmlns [err:XQST0070], and must not be the same as any namespace prefix bound in the same module by a schema import, by a namespace declaration, or by a module import with a different target namespace [err:XQST0033]. ¹

   and:

   A module may import its own target namespace (this is interpreted as importing an implementation-defined set of other modules that share its target namespace.) ²

2. The second thing that this PR deals with is cyclic imports of Library Modules. i.e. M1 -> M2 -> M1. Before this PR an XQuery executed on eXist-db with a cyclic set of imports in its library modules would crash with a Java StackOverflowError. The expected behaviour has changed here between XQuery 1.0 and XQuery 3.1. In XQuery 1.0 cyclic imports were prohibited:

   It is a static error [err:XQST0093] to import a module M1 if there exists a sequence of modules M1 ... Mi ... M1 such that each module directly depends on the next module in the sequence (informally, if M1 depends on itself through some chain of module dependencies.) ³

   however, in XQuery 3.1 cyclic imports are allowed:

   Implementations must resolve cycles in the import graph, either at the level of target namespace URIs or at the level of location URIs, and ensure that each module is imported only once. ⁴

   In this PR, the XQuery version is now checked. If the XQuery is 1.0 then XQST0093 is raised, if the XQuery is newer (i.e. 3.0 or 3.1) then the cyclic import is resolved and allowed without issue.

---

This open source contribution to the eXist-db project was commissioned by the Office of the Historian, U.S. Department of State, https://history.state.gov/.

Footnotes

1. https://www.w3.org/TR/xquery-31/#id-module-declaration ↩

2. https://www.w3.org/TR/xquery-31/#dt-module-import ↩

3. https://www.w3.org/TR/2010/REC-xquery-20101214/#id-module-import ↩

4. https://www.w3.org/TR/xquery-31/#id-module-handling-cycles ↩

[joewiz]

@adamretter To aid folks reviewing the PR, could you share your assessment of the CI failures and whether they are due to the problems in the PR or issues specific to the CI environment?

[adamretter]

@joewiz All resolved now.

[duncdrum]

@adamretter looks like the license header checker doesn't like one of the license headers
https://github.com/eXist-db/exist/actions/runs/5751766777/job/15611509940?pr=4996#step:4:1871

Error:  Failed to execute goal com.mycila:license-maven-plugin:4.2:check (default-cli) on project exist-restxq: Some files do not have the expected license header -> [Help 1]

Could you please explain the effect of these changes to #4699, is it implementing switching off restxq by default and thereby introducing a breaking change?

[adamretter]

looks like the license header checker doesn't like one of the license headers

@duncdrum I have fixed that now.

Could you please explain the effect of these changes to #4699

PR #4699 was never the correct approach and is now redundant due to the fixes in this PR. This PR does not break any backwards compatibility.

[sonarqubecloud]

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[dizzzz]

[reinhapa]

@adamretter could you resolve the existing conflicts in order to merge your're PR?

[adamretter]

I have just rebased this now. @dizzzz @reinhapa I believe this is now ready to merge please

[sonarqubecloud]

SonarCloud Quality Gate failed.   

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