[spencer-tb]

Description

calculate_excess_blob_gas only reads the parent's blob gas fields when the parent is the current fork's own Header. At a fork boundary the parent is the previous fork's header type, so chain validation zeroes any accumulated excess blob gas at the transition block. Clients carry it across the boundary, so EELS rejects correct chains at every post-Cancun transition. The path is unreachable through t8n, which builds the parent from the env as the current fork's class, so fills and fixtures were always correct.

The fix widens the check to also accept the previous fork's header, on every fork whose predecessor carries the fields (prague through amsterdam). Cancun keeps the zeroing, since its shanghai parent has no blob gas fields.

Demonstration at the BPO5 to Amsterdam boundary, with a parent at excess_blob_gas=10000000 and blob_gas_used=786432:

• before: calculate_excess_blob_gas(parent) returns 0
• after: it returns 8951424

Receipts:

• Existing coverage already pins the carried value in fixtures: test_fork_transition_excess_blob_gas_post_blob_genesis runs at the transition to Prague and every later fork and verifies the fork block header. Those fixtures were always correct. This change fixes the chain validation half so EELS accepts them.
• 2706 blob fixtures fill byte for byte identically before and after the fix.
• ethereum-spec-lint, full mypy and just static are clean.

Found while reviewing projects/zkevm, which carries the same fix for amsterdam.

Adding @jsign for credit, added as a co-author.

Related Issues or PRs

N/A.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

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[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.53%. Comparing base (5b2b22c) to head (4d5c575).

Files with missing lines	Patch %	Lines
src/ethereum/forks/amsterdam/vm/gas.py	0.00%	0 Missing and 1 partial ⚠️
src/ethereum/forks/osaka/vm/gas.py	66.66%	0 Missing and 1 partial ⚠️
src/ethereum/forks/prague/vm/gas.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #3352   +/-   ##
================================================
  Coverage            93.53%   93.53%           
================================================
  Files                  624      624           
  Lines                37070    37072    +2     
  Branches              3394     3394           
================================================
+ Hits                 34675    34677    +2     
  Misses                1645     1645           
  Partials               750      750           

Flag	Coverage Δ
unittests	93.53% <57.14%> (+<0.01%)	⬆️

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[spencer-tb]

Don't merge without ack from @jsign please!