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80b58f649a
`Chain.GetHeaders` steps by `1 + Skip` in the forward branch but by `1 -
Skip` in the reverse branch. `Skip` is a `uint64`, so the reverse stride
is wrong for every `Skip > 0`:
| Skip | reverse step | result |
| --- | --- | --- |
| 0 | `-= 1` | correct |
| 1 | `-= 0` | block number never moves, the same header is returned
`Amount` times |
| >= 2 | `1 - Skip` underflows | walks *forward* instead of back (`Skip
= 2` on block 100 lands on 101) |
Per the `GetBlockHeadersRequest` definition, `Skip` is "Blocks to skip
between consecutive headers", so the stride is `Skip + 1` in whichever
direction the query runs. Subtracting `1 + Skip` makes the reverse
branch mirror the forward one.
The existing table test covers forward + `Skip: 1` and reverse + `Skip:
0` (which works by accident, since `1 - 0 == 1`). Added the missing
reverse + `Skip: 1` case; it fails on master:
```
--- FAIL: TestChainGetHeaders/3
Test: TestChainGetHeaders/3
FAIL github.com/ethereum/go-ethereum/cmd/devp2p/internal/ethtest
```
and passes with the fix (4/4).
No caller sends a reverse request with `Skip > 0` today, so nothing is
broken in the current suite. The helper computes the expected headers
that responses are checked against, though, so the moment a reverse+skip
case is added it would silently assert the wrong headers rather than
fail loudly.
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| .. | ||
| ethtest | ||
| v4test | ||
| v5test | ||