RFR: 8353440: Disable FTP fallback for non-local file URLs by default [v6]

[Daniel Fuchs]

On Wed, 23 Apr 2025 17:34:33 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

>> Please help review this PR which disables the unspecified but long-standing feature where an `FtpURLConnection` is opened as a fallback for non-local file URLs.
>> 
>> Before this change, if a file URL has a non-local host component, say `file://remotehost/folder/data.txt`, then the  implementation would attempt opening an FTP connection to `remotehost`. After this change, such URLs will be rejected with a `MalformedURLException`, unless the FTP fallback feature is explicitly re-enabled via a system property.
>> 
>> This change was initially discussed here: https://mail.openjdk.org/pipermail/net-dev/2025-March/025988.html
>> 
>> See the above discussion and CSR draft JDK-8354678 for the motivation for this change. I plan to update the CSR pending an initial round of review of this PR.
>> 
>> This PR:
>> 
>> * Changes file URL `Handler::openConnection` implementation for unix/windows to throw `MalformedURLException`, unless the FTP fallback feature is explicitly enabled by configuration.
>> * Introduces a new system property `sun.net.www.protocol.file.ftp-enabled` which when set to `true` re-enables the feature.
>> * Updates the existing test `NonLocalFtpFallback` to enable the feature via said system property.
>> * Adds a new test `NonLocalFtpFallbackDisabled` verifying that a `MalformedURLException` is thrown by default for a non-local URL host component.
>> 
>> I have added a Release Note as a subtask in the JBS issue, this also needs a review.
>
> Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix repeated "should should"

OK - it seems the tests are just missing the system property. These two tests are skipped if the OS is not windows. 
The `java/net/URL/OpenStream.java` test however shows a potential compatibility issue: it was expecting `UnknownHostException` and gets `MalformedURLException` instead.

This should probably be mentioned in the compatibility risk section of the CSR, and in the release note too.

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PR Comment: https://git.openjdk.org/jdk/pull/24657#issuecomment-2825183643