[gzliudan]

Summary

Ports the three upstream fixes that log/handler.go never carried. With this, the file is identical to go-ethereum's log/handler.go at 409aaefac (checked with diff against geth origin/master, zero lines).

1. log: return TerminalHandler write errors from Handle ethereum/go-ethereum#35055 (ab20d50db) -- Handle discarded the error of the underlying writer, so a failing TerminalHandler silently dropped records. Propagate it to the caller instead, matching go-ethereum. In-tree this reaches only callers holding the handler directly: GlogHandler.Handle returns it onwards, while log.Logger.Write has no error return and drops it (upstream included), so records logged through the Logger facade behave exactly as before.
2. log: share mutex in log handles with same writer and other fields ethereum/go-ethereum#35074 (eea624274) -- mu was a sync.Mutex value, so every handler derived by WithAttrs created its own mutex over the same writer. Make it a *sync.Mutex and hand the parent's mutex down, so a handler family serializes its writes.
3. log: clone terminal handler attrs ethereum/go-ethereum#35141 (409aaefac) -- WithAttrs appended to h.attrs directly, so when the parent's attribute slice had spare capacity two sibling handlers wrote into the same backing array and overwrote each other's attributes. Clone it first.

This is a follow-up to #2825: the same attribute-aliasing class fixed there inside internal/testlog also existed in the production TerminalHandler. It is kept in a separate PR because #2825 is test-only and already reviewed.

Evidence

Mutation checks, one fix reverted at a time with the tests unchanged:

• error dropped (h.wr.Write(buf); return nil) -- TestTerminalHandlerReportsWriteErrors fails with Handle error is <nil>, want write refused.
• WithAttrs handing down &sync.Mutex{} -- TestTerminalHandlerSharesMutex fails with sub-handler does not share the parent's mutex, and TestTerminalHandlerDerivedHandlerBlocksOnParentMutex fails with sub-handler wrote without waiting for the parent's mutex.
• append(h.attrs, attrs...) without the clone -- TestTerminalHandlerKeepsSiblingAttributes fails with first child's attribute was overwritten: child="second".

The mutex check is deterministic: the test holds the parent's mutex, so a derived handler with its own mutex writes through at once instead of blocking.

Validation

• make all exits 0.
• gofmt / goimports clean; go vet ./log/... passes.
• go test -count=1 ./log/... passes (four new tests plus the existing ones); go test -race ./log/ passes.
• golangci-lint 2.15.0 --new-from-rev=f85916e0c3 ./log/... (current dev-upgrade tip) -- 0 issues.
• Diff against go-ethereum origin/master (8817f99f9): log/handler.go identical.

Merge Danger

Door: two-way -- one production file with three independent hunks (error return, mutex pointer, clone) plus one additive test file. Reverting any hunk restores the previous behaviour byte for byte.

Blast Radius: TerminalHandler is only built through NewTerminalHandler / NewTerminalHandlerWithLevel (cmd/bootnode, cmd/abigen, cmd/puppeth, internal/debug/flags.go, p2p/simulations/adapters, p2p/simulations/examples (the ping-pong example program), plus tests); there are no struct literals outside the package, so every instance has an initialized mu. Behavioural deltas: sub-handlers now share the parent's mutex, and Handle returns the writer error instead of nil. No API or signature change.

No file overlap with #2825 (internal/testlog/*), so the two can be merged in any order.

Ref: go-ethereum ethereum#35055 (ab20d50db), ethereum#35074 (eea624274), ethereum#35141 (409aaefac)

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