#36712 fees: prevent divide by zero in CFeeRate

Prevents a division-by-zero crash when calling CFeeRate::GetFeePerK() on a zero-sized fee rate. If the fee rate is empty, GetFeePerK() now returns zero instead of evaluating fees down to 1000 vbytes, matching the behavior of GetFee().

Problem: Constructing a CFeeRate with zero size causes subsequent calls to GetFeePerK() or CFeeRate::operator+= to unconditionally call EvaluateFeeDown(1000), resulting in a division by zero that crashes the node.

Evidence of importance:

• A CFeeRate with size 0 GetFeePerK() and CFeeRate+= evaluated EvaluateFeeDown(1000) unconditionally, dividing by that zero size and crashing (PR description)
• This was reported by Jacob F Cudney (aragondragon) (PR description)

Risk of a mistake: medium (mempool_policy). Incorrect return values or logic errors in CFeeRate could corrupt fee calculations or comparisons across mempool policy and wallet operations.

Ready

Nothing known that would invalidate a review now.

Waiting on (from the thread read):

• nothing

Positive

support without stated reasons, no open objection (darosior, polespinasa, maflcko, pablomartin4btc)

Thread notes: Initial record. maflcko approved with a test grouping and newline nit (rc:4426094942), which is a style preference rather than a blocking claim. pablomartin4btc, darosior, and polespinasa also supported. / Initial record. Multiple reviewers support the change with ACKs and a Concept ACK; maflcko posted an inline style nit on test grouping alongside approval.

Objections, suggestions, and questions: none recorded.

Support:

• darosior (member) (Concept ACK, 2026-10-30): (no reason given) [not substantive]
• polespinasa (member) (ACK, 2026-10-30): (no reason given) [not substantive]
• maflcko (member) (2026-10-30): lgtm [not substantive]
• pablomartin4btc (member) (ACK, 2026-10-30): (no reason given) [not substantive]

Participants:

• darosior (member, support, 1 statements 2026-10-30 to 2026-10-30): Concept ACK
• maflcko (member, support, 2 statements 2026-10-30 to 2026-10-30): approved with lgtm and a test formatting nit
• pablomartin4btc (member, support, 1 statements 2026-10-30 to 2026-10-30): ACKed commit 40647a2
• polespinasa (member, support, 1 statements 2026-10-30 to 2026-10-30): ACKed commit 40647a2

5 lines under test/bench/ci.

• src/policy/feerate.cpp +6/-0
• src/test/amount_tests.cpp +5/-0
• src/policy/feerate.h +1/-1

The short text the ranking pass compares across PRs.

Prevents an unhandled division-by-zero crash in CFeeRate when calling GetFeePerK() on a zero-sized rate. An empty fee rate now returns zero instead of dividing by zero in EvaluateFeeDown(1000), matching the behavior of GetFee(). This also fixes crashes when using operator+= on empty fee rates and adds unit test coverage. Reported by Jacob F Cudney.

P2 · bug fix

Prevents a node crash caused by division by zero when CFeeRate::GetFeePerK() or operator+= is invoked on a zero-sized fee rate. While it resolves an unhandled arithmetic fault, there is no evidence of an external remote denial-of-service vector elevating it to category-critical.

Membership: Matches the 'TX fees and policy' label and directly modifies policy fee rate calculation in src/policy/feerate.{cpp,h}.

Factors: security/stability 2, bug 2, performance 0, user value 0, leverage 0