assumeutxo: Get rid of faked nTx and nChainTx values

The `PopulateAndValidateSnapshot` function introduced in
f6e2da5fb7c6406c37612c838c998078ea8d2252 from #19806 has been setting fake
`nTx` and `nChainTx` values that can show up in RPC results (see #29328) and
make `CBlockIndex` state hard to reason about, because it is difficult to know
whether the values are real or fake.

Revert to previous behavior of setting `nTx` and `nChainTx` to 0 when the
values are unknown, instead of faking them.

This commit fixes at least two assert failures in the (pindex->nChainTx ==
pindex->nTx + prev_chain_tx) check that would happen previously. Tests for
these failures are added separately in the next two commits.

Compatibility note: This change could result in -checkblockindex failures if a
snapshot was loaded by a previous version of Bitcoin Core and not fully
validated, because fake nTx values will have been saved to the block index. It
would be pretty easy to avoid these failures by adding some compatibility code
to `LoadBlockIndex` and changing `nTx` values from 1 to 0 when they are fake
(when `(pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TRANSACTIONS`), but a
little simpler not to worry about being compatible in this case.

1 files changed, 5 insertions, 6 deletions

diff --git a/test/functional/feature_assumeutxo.py b/test/functional/feature_assumeutxo.py
index a29ee8be8b..27910c3909 100755
--- a/test/functional/feature_assumeutxo.py
+++ b/test/functional/feature_assumeutxo.py
@@ -244,20 +244,19 @@ class AssumeutxoTest(BitcoinTestFramework):
                 tx = n1.getblockheader(block.hash)["nTx"]
                 chain_tx = n1.getchaintxstats(nblocks=1, blockhash=block.hash)["txcount"]
 
-                # Intermediate nTx of the starting block should be real, but nTx of
-                # later blocks should be fake 1 values set by snapshot loading code.
+                # Intermediate nTx of the starting block should be set, but nTx of
+                # later blocks should be 0 before they are downloaded.
                 if final or height == START_HEIGHT:
                     assert_equal(tx, block.tx)
                 else:
-                    assert_equal(tx, 1)
+                    assert_equal(tx, 0)
 
                 # Intermediate nChainTx of the starting block and snapshot block
-                # should be real, but others will be fake values set by snapshot
-                # loading code.
+                # should be set, but others should be 0 until they are downloaded.
                 if final or height in (START_HEIGHT, SNAPSHOT_BASE_HEIGHT):
                     assert_equal(chain_tx, block.chain_tx)
                 else:
-                    assert_equal(chain_tx, height + 1)
+                    assert_equal(chain_tx, 0)
 
         check_tx_counts(final=False)