Spend a hashcash stamp only when the request succeeds (closes #158)

A session request refused with 400 or 409, or a +H channel message
refused for membership, a ban or +m, used to spend its stamp, so a
retry with the same stamp was refused as already used.

hashcash.Validator.Validate is split into Check, which spends
nothing, and Claim, which records the stamp; Release undoes a claim.
Session creation checks the stamp first, claims it just before
CreateSession and releases it if CreateSession fails.
SendChannelMessage checks the stamp after the membership, ban and +m
checks.

Model: opus-5-5

internal/handlers/api_test.go

@@ -29,6 +29,7 @@ import (
 	"golang.org/x/crypto/bcrypt"
 	"modernc.org/sqlite"
 	"sneak.berlin/go/neoirc/internal/broker"
+	api "sneak.berlin/go/neoirc/internal/cli/api"
 	"sneak.berlin/go/neoirc/internal/config"
 	"sneak.berlin/go/neoirc/internal/db"
 	"sneak.berlin/go/neoirc/internal/globals"
@@ -4073,6 +4074,75 @@ func TestChannelHashcashMissingBitsArg(t *testing.T) {
 	}
 }
 
+// postSessionWithStamp posts a session request for nick
+// carrying stamp and returns the response status.
+func postSessionWithStamp(
+	t *testing.T,
+	tserver *testServer,
+	nick, stamp string,
+) int {
+	t.Helper()
+
+	body, err := json.Marshal(map[string]string{
+		nickKey: nick, "pow_token": stamp,
+	})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	resp, err := doRequest(
+		t, http.MethodPost, tserver.url(apiSession),
+		bytes.NewReader(body),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_ = resp.Body.Close()
+
+	return resp.StatusCode
+}
+
+// TestRefusedSessionLeavesStampUnspent checks that a
+// session request refused with 400 or 409 does not spend
+// its hashcash stamp, so the same stamp then creates a
+// session, after which it is spent.
+func TestRefusedSessionLeavesStampUnspent(t *testing.T) {
+	const bits = 2
+
+	tserver := newTestServerWith(t, bits, "", "")
+
+	// A server with no SERVER_NAME takes stamps for "neoirc".
+	status := postSessionWithStamp(
+		t, tserver, "taken", api.MintHashcash(bits, "neoirc"),
+	)
+	if status != http.StatusCreated {
+		t.Fatalf("create taken: want 201, got %d", status)
+	}
+
+	stamp := api.MintHashcash(bits, "neoirc")
+
+	steps := []struct {
+		nick string
+		want int
+	}{
+		{"bad nick", http.StatusBadRequest},
+		{"taken", http.StatusConflict},
+		{"fresh", http.StatusCreated},
+		{"another", http.StatusPaymentRequired},
+	}
+
+	for _, step := range steps {
+		status = postSessionWithStamp(t, tserver, step.nick, stamp)
+		if status != step.want {
+			t.Fatalf(
+				"nick %q: want %d, got %d",
+				step.nick, step.want, status,
+			)
+		}
+	}
+}
+
 func TestNamesShowsHostmask(t *testing.T) {
 	tserver := newTestServer(t)