RFR: 8353440: Disable FTP fallback for non-local file URLs by default [v5]

[Eirik Bjørsnøs]

On Wed, 23 Apr 2025 12:29:03 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:
> 
>   Move file URL properties out of the Proxy section, replace URI with URL

src/java.base/share/classes/sun/net/www/protocol/file/FileURLConnection.java line 242:

> 240:      static void requireFtpFallbackEnabled() throws MalformedURLException {
> 241:         if (!FTP_FALLBACK_ENABLED) {
> 242:             throw new MalformedURLException("Unsupported non-local file URL");

I aimed to keep this error message terse but informative, without revealing any parts of the URL spec.

A user stumbling on a rejected URL will not have a lot to work from here though..

If anyone have suggestions to improve the message, please chime in!

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PR Review Comment: https://git.openjdk.org/jdk/pull/24657#discussion_r2056556441