On Mon, 8 Nov 2021 15:53:54 GMT, Jaikiran Pai <j...@openjdk.org> wrote:
>>> printStackTrace interacts with locking of the streams to avoid garbled >>> output when many threads are printing to standard output output/error at >>> the same time. If we change dumpStack to use StackWalker then it will need >>> to do the same. >> >> Indeed. I have updated the PR to use a lock while writing out to the >> `System.err`. >> I had a look at the `printStackTrace()` implementation and it ends up >> locking the `PrintStream` (`System.err`) or `PrintWriter` for the duration >> of the entire stacktrace printing of each stacktrace element. The updated PR >> thus uses `System.err` as the lock to match that semantic. > >> The recursive initialisation issue will require discussion to see if we can >> avoid StackWalker.getInstance return null (which I assume is masking the >> issue). > > For a better context, here's the stacktrace of such a call to > `Thread.dumpStack` during the class initialization of `StackWalker`: > > Caused by: java.lang.NullPointerException: Cannot invoke > "java.lang.StackWalker.forEach(java.util.function.Consumer)" because the > return value of "java.lang.StackWalker.getInstance()" is null > at java.base/java.lang.Thread.dumpStack(Thread.java:1383) > at > java.base/java.security.AccessController.checkPermission(AccessController.java:1054) > at > java.base/java.lang.SecurityManager.checkPermission(SecurityManager.java:411) > at > java.base/java.lang.reflect.AccessibleObject.checkPermission(AccessibleObject.java:91) > at java.base/java.lang.reflect.Method.setAccessible(Method.java:193) > at java.base/java.lang.Class$3.run(Class.java:3864) > at java.base/java.lang.Class$3.run(Class.java:3862) > at > java.base/java.security.AccessController.doPrivileged(AccessController.java:318) > at java.base/java.lang.Class.getEnumConstantsShared(Class.java:3861) > at java.base/java.lang.System$2.getEnumConstantsShared(System.java:2295) > at java.base/java.util.EnumSet.getUniverse(EnumSet.java:408) > at java.base/java.util.EnumSet.noneOf(EnumSet.java:111) > at java.base/java.lang.StackWalker.<clinit>(StackWalker.java:291) > > As you will notice, this call comes from the security (permission check) > layer when `StackWalker` class is being `clinit`ed. The check for > `StackWalker.getInstance()` being `null`, in the `Thread.dumpStack()` > implementation is indeed almost a hackish way to identify this case where > `StackWalker`'s `clinit` is in progress (in the current thread). I can't > think of a different way to handle this use case, so looking forward to any > suggestions. > The updated PR thus uses System.err as the lock to match that semantic. The test case has also been updated to add a new test which invokes Thread.dumpStack() from multiple threads and verifies that the stacktrace written out isn't garbled. Running the test _without_ the addition of the lock on System.err in the Thread.dumpStack implementation does indeed show the garbled output and causes the test failure (which is a good thing). ------------- PR: https://git.openjdk.java.net/jdk/pull/6292
