fix: WithHTTPClient(nil) panics with a nil pointer dereference on the first request

[karlwaldman]

Closes #41

The defect

WithHTTPClient stored its argument verbatim. WithHTTPClient(nil) therefore left Client.httpClient nil and httpClient.Do(req) dereferenced it on the first request.

A nil arrives from ordinary code: a client read out of a config struct, or returned alongside an error by a helper whose error the caller did not check. The SDK already returns a typed *ConfigurationError rather than panicking for the equivalent bad input (a negative WithRetries), so panicking here was inconsistent as well as hostile.

Verified still reproducing on main (cd51e3a) before any change was written.

The fix

A nil *http.Client is ignored and the SDK keeps its default, exactly as if the option had not been supplied. One guard in WithHTTPClient, plus the doc comment stating the contract.

Explicit configuration is preserved either way round: WithHTTPClient(nil), WithTimeout(7s) and WithTimeout(7s), WithHTTPClient(nil) both leave a 7s timeout; a bare WithHTTPClient(nil) leaves DefaultTimeout.

Red

Tests written first, run against unmodified origin/main client.go (git checkout origin/main -- client.go):

--- FAIL: TestWithHTTPClientNilDoesNotPanic (0.00s)
    nil_http_client_test.go:36: WithHTTPClient(nil) left the client with a nil *http.Client
--- FAIL: TestWithHTTPClientNilKeepsDefaultTimeout (0.00s)
    nil_http_client_test.go:50: WithHTTPClient(nil) left the client with a nil *http.Client
--- FAIL: TestWithHTTPClientNilPreservesExplicitTimeout (0.00s)
    --- FAIL: TestWithHTTPClientNilPreservesExplicitTimeout/nil_then_timeout (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x104797c18]
...
github.com/OilpriceAPI/oilpriceapi-go.TestWithHTTPClientNilPreservesExplicitTimeout.WithTimeout.func3(...)
	client.go:118

Green

=== RUN   TestWithHTTPClientNilDoesNotPanic
--- PASS: TestWithHTTPClientNilDoesNotPanic (0.01s)
=== RUN   TestWithHTTPClientNilKeepsDefaultTimeout
--- PASS: TestWithHTTPClientNilKeepsDefaultTimeout (0.00s)
=== RUN   TestWithHTTPClientNilPreservesExplicitTimeout
--- PASS: TestWithHTTPClientNilPreservesExplicitTimeout (0.00s)
    --- PASS: TestWithHTTPClientNilPreservesExplicitTimeout/nil_then_timeout (0.00s)
    --- PASS: TestWithHTTPClientNilPreservesExplicitTimeout/timeout_then_nil (0.00s)
PASS
ok  	github.com/OilpriceAPI/oilpriceapi-go	0.022s

Full suite

Check	Result
go test ./...	ok github.com/OilpriceAPI/oilpriceapi-go 16.490s — 290 pass / 0 fail (285 baseline + 5 new)
go test -race ./...	ok github.com/OilpriceAPI/oilpriceapi-go 18.355s
go vet ./...	clean
gofmt -l .	clean

Interaction with #38

The #38 PR (fix/timeout-clone) also stops Client.httpClient from ever being the caller's own pointer. The two branches touch different functions and do not conflict in either merge order; this guard remains the explicit statement of the nil contract regardless of which lands first.

Scope

No version bump, no tag, no release. Public API unchanged — this only removes a panic.

🤖 Generated with Claude Code

https://claude.ai/code/session_015ao5paex73xXvuM424Libo

[coderabbitai]

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