fix incorrect JNI signature for PatchAction.FlagConflict constructor

[turix2]

When I do something like the following for an Automerge Document object called _doc with SyncState called _syncState:

    PatchLog patchLog = new PatchLog();
    _doc.receiveSyncMessage(_syncState, patchLog, msg);
    for (Patch p : _doc.makePatches(patchLog)) {
           // determine what types of patch we had here...
    }

I get the following panic if there are any PatchAction.FlagConflict actions among the patches:

thread '<unnamed>' panicked at src/patches.rs:152:56:
called `Result::unwrap()` on an `Err` value: JavaException
stack backtrace:
   0:        0x11759be7b - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h958f6e6821e9b0fb
   1:        0x1175b8073 - core::fmt::write::hb5e3c29742bab55e
   2:        0x11759a65e - std::io::Write::write_fmt::h3f38404afa442946
   3:        0x11759bc59 - std::sys_common::backtrace::print::h1ce04ba6121a0174
   4:        0x11759ce95 - std::panicking::default_hook::{{closure}}::hf2e5fe71523bcace
   5:        0x11759cc14 - std::panicking::default_hook::h4f8cdc98d2dcc8c0
   6:        0x11759d2f3 - std::panicking::rust_panic_with_hook::h28420d44f043d3a5
   7:        0x11759d20e - std::panicking::begin_panic_handler::{{closure}}::h6f886c0e89185cdc
   8:        0x11759c359 - std::sys_common::backtrace::__rust_end_short_backtrace::h7ca2b2ff22d46410
   9:        0x11759cf62 - _rust_begin_unwind
  10:        0x1175cc775 - core::panicking::panic_fmt::h52dad7a658d9bf41
  11:        0x1175ccc15 - core::result::unwrap_failed::h84bbbea2e5d8da9f
  12:        0x117453203 - automerge_jni::patches::to_patch_arraylist::h1da89d0366550522
  13:        0x117468237 - _Java_org_automerge_AutomergeSys_makePatches
fatal runtime error: failed to initiate panic, error 5
Abort trap: 6

I believe this PR fixes that problem. The JNI signature to call the Java FlagConflict constructor from Rust was incorrect: there should only be 1 argument (the Prop), not 2.

[alexjg]

Thanks for catching this, could you also add a test which reproduces the problem so it doesn't occur again

[turix2]

Thanks for catching this, could you also add a test which reproduces the problem so it doesn't occur again

Unfortunately, I don't have the build environment set up, and it's too much work for me to do so right now on my machine. Sorry!

But the test wouldn't be that complicated. I think you just need to set up a merge conflict and then ask for the patches. Something like the following (which I just wrote here and have not even tried to compile) should do it:

    Document doc1 = new Document();
    SyncState ss1 = new SyncState();

    Document doc2 = new Document();
    SyncState ss2 = new SyncState();

    try (Transaction tx1 = doc1.startTransaction()) {
        tx1.set(ObjectId.ROOT, "key", ObjectType.MAP);
        tx1.commit();
    }
    try (Transaction tx2 = doc2.startTransaction()) {
        tx2.set(ObjectId.ROOT, "key", ObjectType.MAP);
        tx2.commit();
    }
    
    byte[] m1to2, m2to1;
    PatchLog pl;
    List<Patch> patches;
    while (true) {
        m1to2 = doc1.generateSyncMessage(ss1).orElse(new byte[0]);

        pl = new PatchLog();
        doc2.receiveSyncMessage(ss2, pl, m1to2);
        patches = doc2.makePatches(pl);    // may panic, but shouldn't after fix

        m2to1 = doc2.generateSyncMessage(ss2).orElse(new byte[0]);

        pl = new PatchLog();
        doc1.receiveSyncMessage(ss1, pl, m2to1);
        patches = doc1.makePatches(pl);    // may panic, but shouldn't after fix

        if (m1to2.length == 0  &&  m2to1.length == 0)
            break;
    }

It may not panic on the first set of messages exchanged in the "handshake" between 1 and 2, but I think it will before they converge.