JNI: Fixed local-refs overflow in Document.insertRevisionWithHistory

[snej]

This function can use an arbitrary number of local JNI refs (one for
each item in the history array), so preallocate a new local frame with
enough room for them.

(Also clarified a comment about critical allocation just below, to make
it more clear that JNI allocations are illegal until the critical ref
is released.)

Fixes couchbase/couchbase-lite-android#782 (I think; needs testing)

[snej]

This builds, but I don't have anything set up to run/test the JNI interface. Could you try this out, @hideki ?

[hideki]

Unfortunately PushLocalFrame(n+1) solution still causes JNI ERROR (app bug): local reference table overflow (max=512)

Note: I cherry-picked the commit to 1.2.0-java branch to test.

[snej]

Crap. Well, it looks like, at least on Android, PushLocalFrame doesn't actually guarantee you the capacity you asked for. Look at the function PushLocalFrame here:

    if (!ensureLocalCapacity(ts.self(), capacity) ||
            !dvmPushLocalFrame(ts.self(), dvmGetCurrentJNIMethod()))

So if there isn't enough capacity in the current frame it pushes a new one ... but with whatever the default capacity is, not the capacity you asked for.

[hideki]

Thank you for link to the code.

It seems 512 is maximum local reference table size.

#define kJniLocalRefMax         512     /* arbitrary; should be plenty */

https://android.googlesource.com/platform/dalvik/+/kitkat-release/vm/Thread.h#74

[snej]

OK, here's another try. I've added a new jstringSlice method copyAndReleaseRef() that copies the string data to the heap, then releases the JNI local ref.

[hideki]

New solution throws RuntimeException.

java.lang.RuntimeException: too many PopLocalFrame calls
at com.couchbase.cbforest.Document.insertRevisionWithHistory(Native Method)
at com.couchbase.cbforest.Document.insertRevisionWithHistory(Document.java:73)
at com.couchbase.cbforest.C4DatabaseTest._testInsertRevisionWithHistory(C4DatabaseTest.java:264)
at com.couchbase.cbforest.C4DatabaseTest.testInsertRevisionWith512History(C4DatabaseTest.java:234)

[hideki]

All Unit tests can pass. Thank you for fix.