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

[Eirik Bjørsnøs]

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"

> > java/net/URLConnection/UNCTest.java

> This test wants to verify that any URLConnection to an UNC path is unconnected. Instead, since the UNC path does not exists, it just tests that the FtpURLConnection is unconnected, which it always is and has been and which in any case seems unrelated to JDK-4401485

Files https://bugs.openjdk.org/browse/JDK-8355475 to track an enhancement to make UNCTest use an existing Administrative Shares UNC path.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/24657#issuecomment-2826923618