IO-475: Fix UNC path prefix validation in FilenameUtils

[MiriamCohenDev]

Fixes a bug in FilenameUtils#normalizeNoEndSeparator when handling UNC paths that do not end with a slash.

What:

• The function normalizeNoEndSeparator incorrectly returned null for valid UNC paths like //server because it treated them as invalid.
• This was caused by FilenameUtils#getPrefixLength returning -1 for UNC paths without a trailing slash.

How:

• Updated getPrefixLength to correctly handle UNC paths without a trailing slash, returning the proper prefix length instead of -1.
• Updated and added unit tests to reflect the correct behavior, since many existing tests assumed null would be returned for these paths.

Why:

• Previous behavior caused valid UNC paths to be treated as invalid, which could break downstream logic that relies on normalizeNoEndSeparator.
• This change ensures proper handling of UNC paths and maintains consistency across FilenameUtils methods.

Testing:

• Updated tests in FilenameUtilsTest to cover UNC paths without trailing slashes.
• Verified that all tests pass with the new logic.

Files changed:

• src/main/java/org/apache/commons/io/FilenameUtils.java
• src/test/java/org/apache/commons/io/FilenameUtilsTest.java

[garydgregory]

Hello @MiriamCohenDev

Thank you for your PR. Please see my comments. Run mvn by itself before you push and fix any issues that come up.

[garydgregory]

Jira ticket is https://issues.apache.org/jira/browse/IO-475

[garydgregory]

Hello @MiriamCohenDev

Thank you for your updates. You missed a few spots. It's simpler to review a PR if you don't delete assertions. Instead, change them to reflect the new behavior.

[garydgregory]

See my previous comments, and run mvn without anything else and fix:

[INFO] --- checkstyle:3.6.0:check (default-cli) @ commons-io ---
[INFO] There are 16 errors reported by Checkstyle 13.0.0 with /Users/garygregory/git/commons/commons-io/src/conf/checkstyle.xml ruleset.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[940,13] (coding) FinalLocalVariable: Variable 'pos' should be declared final.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[941,16] (coding) FinalLocalVariable: Variable 'hostnamePart' should be declared final.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,9] (whitespace) WhitespaceAfter: 'if' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,9] (whitespace) WhitespaceAround: 'if' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,56] (whitespace) WhitespaceAround: '{' is not preceded with whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[945,9] (blocks) RightCurly: '}' at column 9 should be on the same line as the next part of a multi-block statement (one that directly contains multiple blocks: if/else-if/else, do/while or try/catch/finally).
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,9] (whitespace) WhitespaceAfter: 'else' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,9] (whitespace) WhitespaceAround: 'else' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,13] (whitespace) WhitespaceAround: '{' is not preceded with whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[410,24] (whitespace) WhitespaceAfter: ',' is not followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[457,24] (whitespace) WhitespaceAfter: ',' is not followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[561,54] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[780,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[919,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[992,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[1132,21] (whitespace) ParenPad: '(' is followed by whitespace.

[MiriamCohenDev]

Is that good now?

[garydgregory]

The build is green and I'll check the patch later today or tomorrow.