fees: prevent divide by zero in CFeeRate #36712

1. 
   ismaelsadeeq commented at 4:33 PM on October 30, 2026: member

   A CFeeRate with size 0 GetFeePerK() and CFeeRate+= evaluated EvaluateFeeDown(1000) unconditionally, dividing by that zero size and crashing.

   Return 0 for an empty fee rate, matching GetFee() behaviour, and add minimal unit test coverage.

   This was reported by Jacob F Cudney (aragondragon)

2. fees: prevent divide by zero in CFeeRate

   A CFeeRate with size 0. GetFeePerK() evaluated EvaluateFeeDown(1000)
   unconditionally, dividing by that zero size and crashing.
   
   Return 0 for an empty fee rate, matching GetFee(), and add coverage.

   40647a2057
3. DrahtBot added the label TX fees and policy on Oct 30, 2026
4. 
   DrahtBot commented at 4:33 PM on October 30, 2026: contributor

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   • Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36712
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5. 
   ismaelsadeeq commented at 4:33 PM on October 30, 2026: member

   cc @polespinasa

6. in src/test/amount_tests.cpp:96 in 40647a2057

     91 | @@ -92,6 +92,11 @@ BOOST_AUTO_TEST_CASE(GetFeeTest)
     92 |      BOOST_CHECK(CFeeRate(CAmount(27), 789) < CFeeRate(35));
     93 |      // Maximum size in bytes, should not crash
     94 |      CFeeRate(MAX_MONEY, std::numeric_limits<int32_t>::max()).GetFeePerK();
     95 | +    BOOST_CHECK_EQUAL(CFeeRate(0, 0).GetFeePerK(), CAmount{0});
     96 | +    BOOST_CHECK_EQUAL(CFeeRate(CAmount{5}, 0).GetFeePerK(), CAmount{0});
   

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   maflcko commented at 4:43 PM on October 30, 2026:

   nit: Could move the tests up to where GetFee() returns zero, so that the same tests are grouped? For the next one, could have a newline, as this test is unrelated from the one about "Maximum size in bytes"?

7. maflcko approved
8. 
   maflcko commented at 4:43 PM on October 30, 2026: member

   lgtm

9. 
   pablomartin4btc commented at 8:14 PM on October 30, 2026: member

   ACK 40647a20572a06b0643e28d92b471a2ab3954de6

10. 
    darosior commented at 8:48 PM on October 30, 2026: member

    Concept ACK

11. 
    polespinasa commented at 9:48 PM on October 30, 2026: member

    ACK 40647a20572a06b0643e28d92b471a2ab3954de6

12. DrahtBot requested review from darosior on Oct 30, 2026

Review Requested
darosior