Starting Java 17, System.setSecurityManager() usage is deprecated (for removal) as noted in https://openjdk.java.net/jeps/411.
The code here was just calling getSecurityManager() and then in a finally block setting it back using setSecurityManager(). However, we aren't setting any other security manager in between these calls.
Looking at the code history, it appears that at one point back in 2001, we were indeed setting a custom security manager (commit 41893fdb30) but that was then reverted in commit ff4e823ee2. So this current piece of code is practically doing nothing.

see https://bz.apache.org/bugzilla/show_bug.cgi?id=65316

This closes #147 pull request at github/apache/ant repo.

Related to https://bz.apache.org/bugzilla/show_bug.cgi?id=64733

The main "writer" thread which initiates the writes to the PipedOutputStream (via the switched System.out/System.err) would exit/complete but would leave the SysOutErrStreamReader thread alive thus causing the PipedInputStream.read()s to continue happening in that SysOutErrStreamReader thread. This would cause the following IOException because the writer thread has exited and is no longer alive:

[junitlauncher] Caused by: java.io.IOException: Pipe broken
[junitlauncher] 	at java.base/java.io.PipedInputStream.read(PipedInputStream.java:321)
[junitlauncher] 	at java.base/java.io.PipedInputStream.read(PipedInputStream.java:377)
[junitlauncher] 	at java.base/java.io.InputStream.read(InputStream.java:205)
[junitlauncher] 	at org.apache.tools.ant.taskdefs.optional.junitlauncher.LauncherSupport$SysOutErrStreamReader.run(LauncherSupport.java:525)
[junitlauncher] 	... 1 more

Bugzilla Report 65105

Closes #125 pull request at github/apache/ant repo

also avoid double-encoding of sysout/err

Bugzilla Reports 63436 and 65030

#64855 Add filterbeforeconcat option to Concat

#64854 Add preserveduplicates option to ResourceList

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Merge tag 'rel/1.10.9'

Tagging version 1.10.9 of Ant

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and add a magic property to disable it again

People using the JRE won't have it and it has been removed from
OpenJDK with Java 9, it seems.

https://bz.apache.org/bugzilla/show_bug.cgi?id=40455

Suggested by Vladimir Sitnikov

Property enumerator