Fix memory corruption in `writeCopyTo`.

[mandrigin]

What, How & Why?

We found an issue that it isn't possible to open a realm database if it was created as a copy using writeCopyTo and providing an encryption key.
The error looked like that:

1) RealmTests : testWriteCopyTo
   Error: Unable to open a realm at path '/Users/mandrigin/golang/src/github.com/realm/realm-js/tests/default.realm.copy.realm': Invalid mnemonic. top_ref[0]: 528EB59E00000001, top_ref[1]: E008049A66C646D4, mnemonic: D1 77 E4 5C, fmt[0]: 64, fmt[1]: 63, flags: F4.
       at Error (native)

After investigation, we noticed a memory corruption in the key, when it was sent to write_copy.

writeCopyTo / key data is: 
[0]=31 [1]=31 [2]=31 [3]=31 [4]=31 [5]=31 [6]=31 [7]=31 [8]=31 [9]=31 [10]=31 [11]=31 [12]=31 [13]=31 [14]=31 [15]=31 [16]=31 [17]=31 [18]=31 [19]=31 [20]=31 [21]=31 [22]=31 [23]=31 [24]=31 [25]=31 [26]=31 [27]=31 [28]=31 [29]=31 [30]=31 [31]=31 [32]=31 [33]=31 [34]=31 [35]=31 [36]=31 [37]=31 [38]=31 [39]=31 [40]=31 [41]=31 [42]=31 [43]=31 [44]=31 [45]=31 [46]=31 [47]=31 [48]=31 [49]=31 [50]=31 [51]=31 [52]=31 [53]=31 [54]=31 [55]=31 [56]=31 [57]=31 [58]=31 [59]=31 [60]=31 [61]=31 [62]=31 [63]=31

ERROR: Connection[1]: Failed to connect to endpoint '::1:9080': Connection refused
write_copy / key data is: [0]=ffffffad [1]=ffffffbe [2]=ffffffab [3]=fffffff9 [4]=68 [5]=6a [6]=ffffffdd [7]=ffffffba [8]=ffffff80 [9]=ffffff82 [10]=07 [11]=00 [12]=00 [13]=60 [14]=00 [15]=00 [16]=31 [17]=31 [18]=31 [19]=31 [20]=31 [21]=31 [22]=31 [23]=31 [24]=31 [25]=31 [26]=31 [27]=31 [28]=31 [29]=31 [30]=31 [31]=31 [32]=31 [33]=31 [34]=31 [35]=31 [36]=31 [37]=31 [38]=31 [39]=31 [40]=31 [41]=31 [42]=31 [43]=31 [44]=31 [45]=31 [46]=31 [47]=31 [48]=31 [49]=31 [50]=31 [51]=31 [52]=31 [53]=31 [54]=31 [55]=31 [56]=31 [57]=31 [58]=31 [59]=31 [60]=31 [61]=31 [62]=31 [63]=31

Gotcha: this was only reproducible on iOS 10+ and Android, tests on macos always passed. Probably iOS 8 and 9 and macos have more relaxed memory optimizations.

After a closer look, it seems like this code was responsible for that (my comments):

    BinaryData key;
    
    if (args.count == 2) {
        ValueType key_value = args[1];
        if (!Value::is_binary(ctx, key_value)) {
            throw std::runtime_error("Encryption key for 'writeCopyTo' must be a Binary.");
        }
        auto key_data = Value::validated_to_binary(ctx, key_value); // we create a local variable here
        key = { static_cast<const char *>(key_data.data()), key_data.size() }; // it's a pointer to the `key_data`
     } // here, `key_data` is freed and it's memory is up for grabs

     realm->write_copy(path, key); // key.data() points to a unused memory that can be overriden
}

I restructured the code a little bit and this bug doesn't appear anymore.

Steps to verify the problem

0. Override tests/js/realm-tests.js with the version from this PR (required to work well on a mobile device).
1. Tweak scripts/test.sh to run on a modern iOS (I hardcoded IOS_RUNTIME='com.apple.CoreSimulator.SimRuntime.iOS-11-0' locally)
2. Remove and update dependencies of the react-test-app:
   pushd tests/react-test-app && rm -rf node_modules && npm install && popd
3. scripts/test.sh react-tests

Expected: testWriteCopyTo fails with something like Invalid mnemonic. top_ref[0]: 528EB59E00000001, top_ref[1]: E008049A66C646D4, mnemonic: D1 77 E4 5C, fmt[0]: 64, fmt[1]: 63, flags: F4.

Steps to verify the solution

1. Tweak scripts/test.sh to run on a modern iOS (I hardcoded IOS_RUNTIME='com.apple.CoreSimulator.SimRuntime.iOS-11-0' locally)
2. Remove and update dependencies of the react-test-app:
   pushd tests/react-test-app && rm -rf node_modules && npm install && popd
3. scripts/test.sh react-tests

Expected:testWriteCopyTo should pass successfully.

☑️ ToDos

• 🚦 Tests

[kneth]

@mandrigin Thanks for the fix. It looks good to me. I believe that you have fixed #1748 for us! I remember that writeCopyTo had some problems on some platforms when it was introduced but I didn't have the time to fix it - and created #1748 to remind me to come back to it.

Can you update CHANGELOG.md? Just something like "Fixed a memory corruption in writeCopyTo ".

[kneth]

👍 for the code. Waiting for an update of CHANGELOG.md.

[kneth]

👍 for the code. Waiting for an update of CHANGELOG.md.

[mandrigin]

@kneth cool, happy to help!
Updated CHANGELOG.md.

[mandrigin]

@kneth yay! kudos for such swift responses! 🎉 top-notch contributor experience 👍