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I have a contract that uses Oraclize to query some data. When the query is sent, a global variable address is set to the msg.sender. This is to be able to interact with the query sender in the __callback() function.

However, I have a doubt that, while waiting for the __callback() to be executed, another address may initiate the query, and the global state variable address will be set to that new address, and when the __callback() of the first will be executed, it will interact with a wrong address.

Is this a possible scenario? And, if so, how to prevent it? Maybe by linking the __callback() to the query?

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Is this a possible scenario?

Yes, it's possible.

And, if so, how to prevent it?

You have to associate each msg.sender value with the ID returned by the oraclize_query() call using a mapping, for example:

mapping(bytes32 => address) senderAddresses;

Then when you call oraclize_query():

bytes32 queryId = oraclize_query(<query>);
senderAddresses[queryId] = msg.sender;

When the callback eventually arrives, you can get the original msg.sender value from the mapping:

function __callback(bytes32 myid, string result) {
    require(msg.sender == oraclize_cbAddress());

    address originalSender = senderAddresses[myid];

    // Do other stuff with originalSender...

}
  • Thank you, I have tried this, but in order to test it I have created a global state variable address and assigned it the value of senderAddresses[myid]. The result was an address 0x0. I suspect that for some reason the queryId and myid are different. – Ruham Oct 16 '18 at 19:04
  • I've summarized the issue here, if you could take a look. ethereum.stackexchange.com/questions/60692/… – Ruham Oct 16 '18 at 19:42

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