This bug is shared (likely because of code copied from one place to another) between the similar methods in commons IO, codehaus-plexus-utils, and maven-shared-utils.
I don't have an isolated test case because this bug is platform specific and I've only seen it in Travis CI builds on Windows using JDK 7 through 15. I don't have a Windows system handy to test it. However it is reproducible.
Typical code that triggers it is in RestoreBackupPomsPhaseTest in maven-release:
// copy poms so tests are valid without clean
File sourceDir = getTestFile( "src/test/resources" + projectPath );
File testDir = getTestFile( "target/test-classes" + projectPath );
FileUtils.copyDirectoryStructure( sourceDir, testDir );
I don't know whether there might be something weird in the setup of those two directories that's involved.
Typical error message is:
Caused by: java.nio.file.FileSystemException:
F:\jenkins\jenkins-slave\workspace\maven-box_maven-release_windows@2@2\windows-jdk8-m3.6.x_build\maven-release-manager\target\test-classes\projects\restore-backup-poms\basic-pom\pom.xml: The process cannot access the file because it is being used by another process
"The process cannot access the file because it is being used by another process" I think points to the root of the bug. This is a Windows file system error message.
Some history is here where I noticed it:
In this case, I started with plexus-utils 3.1.0 which worked, upgraded t plexus-utils 3.3.0, which didn't. And then tried the FileUtils.copyDirectory from both maven-shared-utils and commons-io, all of which failed in the same way.
I think this is caused by the use of NIO, which doesn't work quite the same when copying files on Windows as on Linux and Mac OS X.