lnrpc/invoicesrpc: add blinded path policy buffer

This commit adds a helper function that will be used to adjust a hops
policy values by certain given increase and decrease multipliers. This
will be used in blinded paths to give policy values some buffer to avoid
easy probing of blinded paths.

lnrpc/invoicesrpc/addinvoice_test.go

@@ -902,6 +902,159 @@ func TestPopulateHopHints(t *testing.T) {
 	}
 }
 
+// TestApplyBlindedPathPolicyBuffer tests blinded policy adjustments.
+func TestApplyBlindedPathPolicyBuffer(t *testing.T) {
+	tests := []struct {
+		name          string
+		policyIn      *blindedHopPolicy
+		expectedOut   *blindedHopPolicy
+		incMultiplier float64
+		decMultiplier float64
+		expectedError string
+	}{
+		{
+			name:          "invalid increase multiplier",
+			incMultiplier: 0,
+			expectedError: "blinded path policy increase " +
+				"multiplier must be greater than or equal to 1",
+		},
+		{
+			name:          "decrease multiplier too small",
+			incMultiplier: 1,
+			decMultiplier: -1,
+			expectedError: "blinded path policy decrease " +
+				"multiplier must be in the range [0;1]",
+		},
+		{
+			name:          "decrease multiplier too big",
+			incMultiplier: 1,
+			decMultiplier: 2,
+			expectedError: "blinded path policy decrease " +
+				"multiplier must be in the range [0;1]",
+		},
+		{
+			name:          "no change",
+			incMultiplier: 1,
+			decMultiplier: 1,
+			policyIn: &blindedHopPolicy{
+				cltvExpiryDelta: 1,
+				minHTLCMsat:     2,
+				maxHTLCMsat:     3,
+				baseFee:         4,
+				feeRate:         5,
+			},
+			expectedOut: &blindedHopPolicy{
+				cltvExpiryDelta: 1,
+				minHTLCMsat:     2,
+				maxHTLCMsat:     3,
+				baseFee:         4,
+				feeRate:         5,
+			},
+		},
+		{
+			name: "buffer up by 100% and down by and down " +
+				"by 50%",
+			incMultiplier: 2,
+			decMultiplier: 0.5,
+			policyIn: &blindedHopPolicy{
+				cltvExpiryDelta: 10,
+				minHTLCMsat:     20,
+				maxHTLCMsat:     300,
+				baseFee:         40,
+				feeRate:         50,
+			},
+			expectedOut: &blindedHopPolicy{
+				cltvExpiryDelta: 20,
+				minHTLCMsat:     40,
+				maxHTLCMsat:     150,
+				baseFee:         80,
+				feeRate:         100,
+			},
+		},
+		{
+			name: "new HTLC minimum larger than OG " +
+				"maximum",
+			incMultiplier: 2,
+			decMultiplier: 1,
+			policyIn: &blindedHopPolicy{
+				cltvExpiryDelta: 10,
+				minHTLCMsat:     20,
+				maxHTLCMsat:     30,
+				baseFee:         40,
+				feeRate:         50,
+			},
+			expectedOut: &blindedHopPolicy{
+				cltvExpiryDelta: 20,
+				minHTLCMsat:     20,
+				maxHTLCMsat:     30,
+				baseFee:         80,
+				feeRate:         100,
+			},
+		},
+		{
+			name: "new HTLC maximum smaller than OG " +
+				"minimum",
+			incMultiplier: 1,
+			decMultiplier: 0.5,
+			policyIn: &blindedHopPolicy{
+				cltvExpiryDelta: 10,
+				minHTLCMsat:     20,
+				maxHTLCMsat:     30,
+				baseFee:         40,
+				feeRate:         50,
+			},
+			expectedOut: &blindedHopPolicy{
+				cltvExpiryDelta: 10,
+				minHTLCMsat:     20,
+				maxHTLCMsat:     30,
+				baseFee:         40,
+				feeRate:         50,
+			},
+		},
+		{
+			name: "new HTLC minimum and maximums are not " +
+				"compatible",
+			incMultiplier: 2,
+			decMultiplier: 0.5,
+			policyIn: &blindedHopPolicy{
+				cltvExpiryDelta: 10,
+				minHTLCMsat:     30,
+				maxHTLCMsat:     100,
+				baseFee:         40,
+				feeRate:         50,
+			},
+			expectedOut: &blindedHopPolicy{
+				cltvExpiryDelta: 20,
+				minHTLCMsat:     30,
+				maxHTLCMsat:     100,
+				baseFee:         80,
+				feeRate:         100,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			bufferedPolicy, err := addPolicyBuffer(
+				test.policyIn, test.incMultiplier,
+				test.decMultiplier,
+			)
+			if test.expectedError != "" {
+				require.ErrorContains(
+					t, err, test.expectedError,
+				)
+
+				return
+			}
+
+			require.Equal(t, test.expectedOut, bufferedPolicy)
+		})
+	}
+}
+
 // TestBlindedPathAccumulatedPolicyCalc tests the logic for calculating the
 // accumulated routing policies of a blinded route against an example mentioned
 // in the spec document: