[Bug] Steem blockchain freezes if the SBD supply is 0

Project Information

• Repository: https://github.com/steemit/steem
• Project Name: Steem

This issue is related with the function update_median_feed() in database.cpp

Expected behavior

If the SBD supply is 0 the blockchain takes the median feed price from witnesses.

Actual behavior

When the SBD supply is 0 everyone stop producing blocks.

How to reproduce

• Start a testnet with 21 witnesses (the current version is 0.20.9).
• Set a feed price on at least 7 witnesses.
• Wait 1 hour for the update of the median feed price.
• All witnesses stop producing blocks with this error:

Failed to push new block:
10 assert_exception: Assert Exception
base.amount > share_type(0): 
    {}
    asset.cpp:377 validate

    {"base":{"amount":"0","precision":3,"nai":"@@000000013"},"quote":{"amount":"250037129","precision":3,"nai":"@@000000021"}}
    asset.cpp:380 validate

Got exception while generating block:
10 assert_exception: Assert Exception
base.amount > share_type(0): 
    {}
    asset.cpp:377 validate

    {"base":{"amount":"0","precision":3,"nai":"@@000000013"},"quote":{"amount":"250037129","precision":3,"nai":"@@000000021"}}
    asset.cpp:380 validate

    {}
    database.cpp:3128 update_median_feed

Reason

First, a little background about the variable type price. The prices in steem are not defined with floating point numbers but with fractional numbers, that is, using two numbers: numerator and denominator. In this sense, if the price is 0.3333333... SBD/STEEM, it can be very well defined as 1 SBD/3 STEEM, and we don't need to use floating point numbers and address their accuracy problems.

Then, the conversion of assets needs a multiplication and a division:

• 30 STEEM is equal to: 30 STEEM * 1 SBD / 3 STEEM = 10 SBD
• and, 10 SBD is equal to: 10 SBD * 3 STEEM / 1 SBD = 30 STEEM

As you can see, both numbers can be in the denominator depending the conversion. And, to prevent division by 0, both numbers are required to be greater than zero (validation)

Coming back to update_median_feed(). This function updates the feed price taking into account two things:

• The median price given by the witnesses, and
• The haircut price to force SBD to remain below 10%.

However, if the SBD supply is zero, the haircut price throws an exception because both quote and base have to be greater than zero (to prevent division by 0). No witness can create this new block and therefore the blockchain freezes.

Line in question:

price min_price( asset( 9 * gpo.current_sbd_supply.amount, SBD_SYMBOL ), gpo.current_supply );

Low vulnerability

The unique way to reach 0 SBD in the mainnet is by stopping printing SBD and converting all of them into STEEM, which is very unlikely. However, it becomes more relevant on testnets or private blockchains.

Solution

Only calculate the haircut price if the SBD supply is greater than zero, because in this case there is no risk of debt, the ratio is 0%.

GitHub Account

GitHub account: https://github.com/joticajulian
GitHub issue: https://github.com/steemit/steem/issues/3258
Pull request proposed: https://github.com/steemit/steem/pull/3263