Robust gas oracles (#1222)

* Pass reqwest Client to constructors

* Add Median oracle aggregator

* DRY

* Weighted median

* Add cache layer

* Simplify lifetimes

* Add with_client constructors

* Update GasNow urls

* Add u256_from_f64_saturating

* Add polygon oracle

* Fixes

* Fix lints

* Remove dbg statements

Cargo.lock

@@ -1292,6 +1292,7 @@ dependencies = [
  "ethers-providers",
  "ethers-signers",
  "ethers-solc",
+ "futures-locks",
  "futures-util",
  "hex",
  "instant",
@@ -1588,6 +1589,17 @@ version = "0.3.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc4045962a5a5e935ee2fdedaa4e08284547402885ab326734432bed5d12966b"
 
+[[package]]
+name = "futures-locks"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3eb42d4fb72227be5778429f9ef5240a38a358925a49f05b5cf702ce7c7e558a"
+dependencies = [
+ "futures-channel",
+ "futures-task",
+ "tokio",
+]
+
 [[package]]
 name = "futures-macro"
 version = "0.3.21"

ethers-core/src/types/mod.rs

@@ -21,6 +21,9 @@ pub use address_or_bytes::AddressOrBytes;
 mod path_or_string;
 pub use path_or_string::PathOrString;
 
+mod u256;
+pub use u256::*;
+
 mod i256;
 pub use i256::{Sign, I256};
 

ethers-core/src/types/u256.rs

@@ -0,0 +1,121 @@
+use ethabi::ethereum_types::U256;
+
+/// Convert a floating point value to its nearest f64 integer.
+///
+/// It is saturating, so values $\ge 2^{256}$ will be rounded
+/// to [`U256::max_value()`] and values $< 0$ to zero. This includes
+/// positive and negative infinity.
+///
+/// TODO: Move to ethabi::ethereum_types::U256.
+/// TODO: Add [`super::I256`] version.
+///
+/// # Panics
+///
+/// Panics if `f` is NaN.
+pub fn u256_from_f64_saturating(mut f: f64) -> U256 {
+    if f.is_nan() {
+        panic!("NaN is not a valid value for U256");
+    }
+    if f < 0.5 {
+        return U256::zero()
+    }
+    if f >= 1.157_920_892_373_162e77_f64 {
+        return U256::max_value()
+    }
+    // All non-normal cases should have been handled above
+    assert!(f.is_normal());
+    // Turn nearest rounding into truncated rounding
+    f += 0.5;
+
+    // Parse IEEE-754 double into U256
+    // Sign should be zero, exponent should be >= 0.
+    let bits = f.to_bits();
+    let sign = bits >> 63;
+    assert!(sign == 0);
+    let biased_exponent = (bits >> 52) & 0x7ff;
+    assert!(biased_exponent >= 1023);
+    let exponent = biased_exponent - 1023;
+    let fraction = bits & 0xfffffffffffff;
+    let mantissa = 0x10000000000000 | fraction;
+    if exponent > 255 {
+        U256::max_value()
+    } else if exponent < 52 {
+        // Truncate mantissa
+        U256([mantissa, 0, 0, 0]) >> (52 - exponent)
+    } else {
+        U256([mantissa, 0, 0, 0]) << (exponent - 52)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::f64;
+
+    #[test]
+    fn test_small_integers() {
+        for i in 0..=255 {
+            let f = i as f64;
+            let u = u256_from_f64_saturating(f);
+            assert_eq!(u, U256::from(i));
+        }
+    }
+
+    #[test]
+    fn test_small_integers_round_down() {
+        for i in 0..=255 {
+            let f = (i as f64) + 0.499;
+            let u = u256_from_f64_saturating(f);
+            assert_eq!(u, U256::from(i));
+        }
+    }
+
+    #[test]
+    fn test_small_integers_round_up() {
+        for i in 0..=255 {
+            let f = (i as f64) - 0.5;
+            let u = u256_from_f64_saturating(f);
+            assert_eq!(u, U256::from(i));
+        }
+    }
+
+    #[test]
+    fn test_infinities() {
+        assert_eq!(u256_from_f64_saturating(f64::INFINITY), U256::max_value());
+        assert_eq!(u256_from_f64_saturating(f64::NEG_INFINITY), U256::zero());
+    }
+
+    #[test]
+    fn test_saturating() {
+        assert_eq!(u256_from_f64_saturating(-1.0), U256::zero());
+        assert_eq!(u256_from_f64_saturating(1e90_f64), U256::max_value());
+    }
+
+    #[test]
+    fn test_large() {
+        // Check with e.g. `python3 -c 'print(int(1.0e36))'`
+        assert_eq!(
+            u256_from_f64_saturating(1.0e36_f64),
+            U256::from_dec_str("1000000000000000042420637374017961984").unwrap()
+        );
+        assert_eq!(
+            u256_from_f64_saturating(f64::consts::PI * 2.0e60_f64),
+            U256::from_dec_str("6283185307179586084560863929317662625677330590403879287914496")
+                .unwrap()
+        );
+        assert_eq!(
+            u256_from_f64_saturating(5.78960446186581e76_f64),
+            U256::from_dec_str(
+                "57896044618658097711785492504343953926634992332820282019728792003956564819968"
+            )
+            .unwrap()
+        );
+        assert_eq!(
+            u256_from_f64_saturating(1.157920892373161e77_f64),
+            U256::from_dec_str(
+                "115792089237316105435040506505232477503392813560534822796089932171514352762880"
+            )
+            .unwrap()
+        );
+    }
+}

ethers-middleware/Cargo.toml

@@ -24,6 +24,7 @@ async-trait = { version = "0.1.50", default-features = false }
 serde = { version = "1.0.124", default-features = false, features = ["derive"] }
 thiserror = { version = "1.0.31", default-features = false }
 futures-util = { version = "^0.3" }
+futures-locks = { version = "0.7" }
 tracing = { version = "0.1.34", default-features = false }
 tracing-futures = { version = "0.2.5", default-features = false }
 

ethers-middleware/src/gas_oracle/blocknative.rs

@@ -1,12 +1,9 @@
 use crate::gas_oracle::{GasCategory, GasOracle, GasOracleError, GWEI_TO_WEI};
 use async_trait::async_trait;
 use ethers_core::types::U256;
-use reqwest::{
+use reqwest::{header::AUTHORIZATION, Client};
-    header::{HeaderMap, HeaderValue, AUTHORIZATION},
-    Client, ClientBuilder,
-};
 use serde::Deserialize;
-use std::{collections::HashMap, convert::TryInto, iter::FromIterator};
+use std::{collections::HashMap, convert::TryInto};
 use url::Url;
 
 const BLOCKNATIVE_GAS_PRICE_ENDPOINT: &str = "https://api.blocknative.com/gasprices/blockprices";
@@ -26,6 +23,7 @@ fn gas_category_to_confidence(gas_category: &GasCategory) -> u64 {
 pub struct BlockNative {
     client: Client,
     url: Url,
+    api_key: String,
     gas_category: GasCategory,
 }
 
@@ -84,13 +82,16 @@ pub struct BaseFeeEstimate {
 }
 
 impl BlockNative {
-    /// Creates a new [BlockNative](https://www.blocknative.com/gas-estimator) gas oracle
+    /// Creates a new [BlockNative](https://www.blocknative.com/gas-estimator) gas oracle.
-    pub fn new(api_key: &str) -> Self {
+    pub fn new(api_key: String) -> Self {
-        let header_value = HeaderValue::from_str(api_key).unwrap();
+        Self::with_client(Client::new(), api_key)
-        let headers = HeaderMap::from_iter([(AUTHORIZATION, header_value)]);
+    }
-        let client = ClientBuilder::new().default_headers(headers).build().unwrap();
+
+    /// Same as [`Self::new`] but with a custom [`Client`].
+    pub fn with_client(client: Client, api_key: String) -> Self {
         Self {
             client,
+            api_key,
             url: BLOCKNATIVE_GAS_PRICE_ENDPOINT.try_into().unwrap(),
             gas_category: GasCategory::Standard,
         }
@@ -105,7 +106,15 @@ impl BlockNative {
 
     /// Perform request to Blocknative, decode response
     pub async fn request(&self) -> Result<BlockNativeGasResponse, GasOracleError> {
-        Ok(self.client.get(self.url.as_ref()).send().await?.error_for_status()?.json().await?)
+        self.client
+            .get(self.url.as_ref())
+            .header(AUTHORIZATION, &self.api_key)
+            .send()
+            .await?
+            .error_for_status()?
+            .json()
+            .await
+            .map_err(GasOracleError::HttpClientError)
     }
 }
 

ethers-middleware/src/gas_oracle/cache.rs

@@ -0,0 +1,70 @@
+use crate::gas_oracle::{GasOracle, GasOracleError};
+use async_trait::async_trait;
+use ethers_core::types::U256;
+use futures_locks::RwLock;
+use std::{
+    fmt::Debug,
+    future::Future,
+    time::{Duration, Instant},
+};
+
+#[derive(Debug)]
+pub struct Cache<T: GasOracle> {
+    inner: T,
+    validity: Duration,
+    fee: Cached<U256>,
+    eip1559: Cached<(U256, U256)>,
+}
+
+#[derive(Default, Debug)]
+struct Cached<T: Clone>(RwLock<Option<(Instant, T)>>);
+
+impl<T: Clone> Cached<T> {
+    async fn get<F, E, Fut>(&self, validity: Duration, fetch: F) -> Result<T, E>
+    where
+        F: FnOnce() -> Fut,
+        Fut: Future<Output = Result<T, E>>,
+    {
+        // Try with a read lock
+        {
+            let lock = self.0.read().await;
+            if let Some((last_fetch, value)) = lock.as_ref() {
+                if Instant::now().duration_since(*last_fetch) < validity {
+                    return Ok(value.clone())
+                }
+            }
+        }
+        // Acquire a write lock
+        {
+            let mut lock = self.0.write().await;
+            // Check again, a concurrent thread may have raced us to the write.
+            if let Some((last_fetch, value)) = lock.as_ref() {
+                if Instant::now().duration_since(*last_fetch) < validity {
+                    return Ok(value.clone())
+                }
+            }
+            // Set a fresh value
+            let value = fetch().await?;
+            *lock = Some((Instant::now(), value.clone()));
+            Ok(value)
+        }
+    }
+}
+
+impl<T: GasOracle> Cache<T> {
+    pub fn new(validity: Duration, inner: T) -> Self {
+        Self { inner, validity, fee: Cached::default(), eip1559: Cached::default() }
+    }
+}
+
+#[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
+#[cfg_attr(not(target_arch = "wasm32"), async_trait)]
+impl<T: GasOracle> GasOracle for Cache<T> {
+    async fn fetch(&self) -> Result<U256, GasOracleError> {
+        self.fee.get(self.validity, || self.inner.fetch()).await
+    }
+
+    async fn estimate_eip1559_fees(&self) -> Result<(U256, U256), GasOracleError> {
+        self.eip1559.get(self.validity, || self.inner.estimate_eip1559_fees()).await
+    }
+}

ethers-middleware/src/gas_oracle/eth_gas_station.rs

@@ -61,15 +61,17 @@ pub struct EthGasStationResponse {
 
 impl EthGasStation {
     /// Creates a new [EthGasStation](https://docs.ethgasstation.info/) gas oracle
-    pub fn new(api_key: Option<&'static str>) -> Self {
+    pub fn new(api_key: Option<&str>) -> Self {
-        let url = match api_key {
+        Self::with_client(Client::new(), api_key)
-            Some(key) => format!("{}?api-key={}", ETH_GAS_STATION_URL_PREFIX, key),
+    }
-            None => ETH_GAS_STATION_URL_PREFIX.to_string(),
-        };
 
-        let url = Url::parse(&url).expect("invalid url");
+    /// Same as [`Self::new`] but with a custom [`Client`].
-
+    pub fn with_client(client: Client, api_key: Option<&str>) -> Self {
-        EthGasStation { client: Client::new(), url, gas_category: GasCategory::Standard }
+        let mut url = Url::parse(ETH_GAS_STATION_URL_PREFIX).expect("invalid url");
+        if let Some(key) = api_key {
+            url.query_pairs_mut().append_pair("api-key", key);
+        }
+        EthGasStation { client, url, gas_category: GasCategory::Standard }
     }
 
     /// Sets the gas price category to be used when fetching the gas price.
@@ -84,6 +86,12 @@ impl EthGasStation {
     }
 }
 
+impl Default for EthGasStation {
+    fn default() -> Self {
+        Self::new(None)
+    }
+}
+
 #[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
 #[cfg_attr(not(target_arch = "wasm32"), async_trait)]
 impl GasOracle for EthGasStation {

ethers-middleware/src/gas_oracle/etherchain.rs

@@ -18,12 +18,6 @@ pub struct Etherchain {
     gas_category: GasCategory,
 }
 
-impl Default for Etherchain {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 #[derive(Clone, Debug, Deserialize, PartialEq, PartialOrd)]
 #[serde(rename_all = "camelCase")]
 pub struct EtherchainResponse {
@@ -38,9 +32,14 @@ pub struct EtherchainResponse {
 impl Etherchain {
     /// Creates a new [Etherchain](https://etherchain.org/tools/gasPriceOracle) gas price oracle.
     pub fn new() -> Self {
+        Self::with_client(Client::new())
+    }
+
+    /// Same as [`Self::new`] but with a custom [`Client`].
+    pub fn with_client(client: Client) -> Self {
         let url = Url::parse(ETHERCHAIN_URL).expect("invalid url");
 
-        Etherchain { client: Client::new(), url, gas_category: GasCategory::Standard }
+        Etherchain { client, url, gas_category: GasCategory::Standard }
     }
 
     /// Sets the gas price category to be used when fetching the gas price.
@@ -55,6 +54,12 @@ impl Etherchain {
     }
 }
 
+impl Default for Etherchain {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 #[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
 #[cfg_attr(not(target_arch = "wasm32"), async_trait)]
 impl GasOracle for Etherchain {

ethers-middleware/src/gas_oracle/gas_now.rs

@@ -7,9 +7,9 @@ use url::Url;
 
 use crate::gas_oracle::{GasCategory, GasOracle, GasOracleError};
 
-const GAS_NOW_URL: &str = "https://www.gasnow.org/api/v3/gas/price";
+const GAS_NOW_URL: &str = "https://etherchain.org/api/gasnow";
 
-/// A client over HTTP for the [GasNow](https://www.gasnow.org/api/v1/gas/price) gas tracker API
+/// A client over HTTP for the [Etherchain GasNow](https://etherchain.org/tools/gasnow) gas tracker API
 /// that implements the `GasOracle` trait
 #[derive(Clone, Debug)]
 pub struct GasNow {
@@ -18,12 +18,6 @@ pub struct GasNow {
     gas_category: GasCategory,
 }
 
-impl Default for GasNow {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 #[derive(Deserialize)]
 struct GasNowResponseWrapper {
     data: GasNowResponse,
@@ -38,11 +32,16 @@ pub struct GasNowResponse {
 }
 
 impl GasNow {
-    /// Creates a new [GasNow](https://gasnow.org) gas price oracle.
+    /// Creates a new [Etherchain GasNow](https://etherchain.org/tools/gasnow) gas price oracle.
     pub fn new() -> Self {
+        Self::with_client(Client::new())
+    }
+
+    /// Same as [`Self::new`] but with a custom [`Client`].
+    pub fn with_client(client: Client) -> Self {
         let url = Url::parse(GAS_NOW_URL).expect("invalid url");
 
-        Self { client: Client::new(), url, gas_category: GasCategory::Standard }
+        Self { url, gas_category: GasCategory::Standard }
     }
 
     /// Sets the gas price category to be used when fetching the gas price.
@@ -63,6 +62,12 @@ impl GasNow {
     }
 }
 
+impl Default for GasNow {
+    fn default() -> Self {
+        Self::new(Client::new())
+    }
+}
+
 #[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
 #[cfg_attr(not(target_arch = "wasm32"), async_trait)]
 impl GasOracle for GasNow {

ethers-middleware/src/gas_oracle/median.rs

@@ -0,0 +1,125 @@
+use crate::gas_oracle::{GasOracle, GasOracleError};
+use async_trait::async_trait;
+use ethers_core::types::U256;
+use futures_util::future::join_all;
+use std::{fmt::Debug, future::Future};
+use tracing::warn;
+
+#[derive(Default, Debug)]
+pub struct Median {
+    oracles: Vec<(f32, Box<dyn GasOracle>)>,
+}
+
+/// Computes the median gas price from a selection of oracles.
+///
+/// Don't forget to set a timeout on the source oracles. By default
+/// the reqwest based oracles will never time out.
+impl Median {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn add<T: 'static + GasOracle>(&mut self, oracle: T) {
+        self.add_weighted(1.0, oracle)
+    }
+
+    pub fn add_weighted<T: 'static + GasOracle>(&mut self, weight: f32, oracle: T) {
+        assert!(weight > 0.0);
+        self.oracles.push((weight, Box::new(oracle)));
+    }
+
+    pub async fn query_all<'a, Fn, Fut, O>(
+        &'a self,
+        mut f: Fn,
+    ) -> Result<Vec<(f32, O)>, GasOracleError>
+    where
+        Fn: FnMut(&'a dyn GasOracle) -> Fut,
+        Fut: Future<Output = Result<O, GasOracleError>>,
+    {
+        // Process the oracles in parallel
+        let futures = self.oracles.iter().map(|(_, oracle)| f(oracle.as_ref()));
+        let results = join_all(futures).await;
+
+        // Filter out any errors
+        let values =
+            self.oracles.iter().zip(results).filter_map(
+                |((weight, oracle), result)| match result {
+                    Ok(value) => Some((*weight, value)),
+                    Err(err) => {
+                        warn!("Failed to fetch gas price from {:?}: {}", oracle, err);
+                        None
+                    }
+                },
+            );
+        let values = values.collect::<Vec<_>>();
+        if values.is_empty() {
+            return Err(GasOracleError::NoValues)
+        }
+        Ok(values)
+    }
+}
+
+#[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
+#[cfg_attr(not(target_arch = "wasm32"), async_trait)]
+impl GasOracle for Median {
+    async fn fetch(&self) -> Result<U256, GasOracleError> {
+        let mut values = self.query_all(|oracle| oracle.fetch()).await?;
+        // `query_all` guarantees `values` is not empty
+        Ok(*weighted_fractile_by_key(0.5, &mut values, |fee| fee).unwrap())
+    }
+
+    async fn estimate_eip1559_fees(&self) -> Result<(U256, U256), GasOracleError> {
+        let mut values = self.query_all(|oracle| oracle.estimate_eip1559_fees()).await?;
+        // `query_all` guarantees `values` is not empty
+        Ok((
+            weighted_fractile_by_key(0.5, &mut values, |(max_fee, _)| max_fee).unwrap().0,
+            weighted_fractile_by_key(0.5, &mut values, |(_, priority_fee)| priority_fee).unwrap().1,
+        ))
+    }
+}
+
+/// Weighted fractile by key
+///
+/// Sort the values in place by key and return the weighted fractile value such
+/// that `fractile` fraction of the values by weight are less than or equal to
+/// the value.
+///
+/// Returns None if the values are empty.
+///
+/// Note: it doesn't handle NaNs or other special float values.
+///
+/// See <https://en.wikipedia.org/wiki/Percentile#The_weighted_percentile_method>
+///
+/// # Panics
+///
+/// Panics if [`fractile`] is not in the range $[0, 1]$.
+fn weighted_fractile_by_key<'a, T, F, K>(
+    fractile: f32,
+    values: &'a mut [(f32, T)],
+    mut key: F,
+) -> Option<&'a T>
+where
+    F: for<'b> FnMut(&'b T) -> &'b K,
+    K: Ord,
+{
+    assert!((0.0..=1.0).contains(&fractile));
+    if values.is_empty() {
+        return None
+    }
+    let weight_rank = fractile * values.iter().map(|(weight, _)| *weight).sum::<f32>();
+    values.sort_unstable_by(|a, b| key(&a.1).cmp(key(&b.1)));
+    let mut cumulative_weight = 0.0_f32;
+    for (weight, value) in values.iter() {
+        cumulative_weight += *weight;
+        if cumulative_weight >= weight_rank {
+            return Some(value)
+        }
+    }
+    // By the last element, cumulative_weight == weight_rank and we should have
+    // returned already. Assume there is a slight rounding error causing
+    // cumulative_weight to be slightly less than expected. In this case the last
+    // element is appropriate. (This is not exactly right, since the last
+    // elements may have zero weight.)
+    // `values` is not empty.
+    Some(&values.last().unwrap().1)
+}

ethers-middleware/src/gas_oracle/mod.rs

@@ -13,6 +13,15 @@ pub use etherscan::Etherscan;
 mod middleware;
 pub use middleware::{GasOracleMiddleware, MiddlewareError};
 
+mod median;
+pub use median::Median;
+
+mod cache;
+pub use cache::Cache;
+
+mod polygon;
+pub use polygon::Polygon;
+
 use ethers_core::types::U256;
 
 use async_trait::async_trait;
@@ -58,6 +67,12 @@ pub enum GasOracleError {
 
     #[error("EIP-1559 gas estimation not supported")]
     Eip1559EstimationNotSupported,
+
+    #[error("None of the oracles returned a value")]
+    NoValues,
+
+    #[error("Chain is not supported by the oracle")]
+    UnsupportedChain,
 }
 
 /// `GasOracle` is a trait that an underlying gas oracle needs to implement.

ethers-middleware/src/gas_oracle/polygon.rs

@@ -0,0 +1,91 @@
+use crate::gas_oracle::{GasCategory, GasOracle, GasOracleError};
+use async_trait::async_trait;
+use ethers_core::types::{u256_from_f64_saturating, Chain, U256};
+use reqwest::Client;
+use serde::Deserialize;
+use url::Url;
+
+const GAS_PRICE_ENDPOINT: &str = "https://gasstation-mainnet.matic.network/v2";
+const MUMBAI_GAS_PRICE_ENDPOINT: &str = "https://gasstation-mumbai.matic.today/v2";
+
+/// The [Polygon](https://docs.polygon.technology/docs/develop/tools/polygon-gas-station/) gas station API
+/// Queries over HTTP and implements the `GasOracle` trait
+#[derive(Clone, Debug)]
+pub struct Polygon {
+    client: Client,
+    url: Url,
+    gas_category: GasCategory,
+}
+
+/// The response from the Polygon gas station API.
+/// Gas prices are in Gwei.
+#[derive(Debug, Deserialize, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct Response {
+    estimated_base_fee: f64,
+    safe_low: GasEstimate,
+    standard: GasEstimate,
+    fast: GasEstimate,
+}
+
+#[derive(Clone, Copy, Debug, Deserialize, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct GasEstimate {
+    max_priority_fee: f64,
+    max_fee: f64,
+}
+
+impl Polygon {
+    pub fn new(chain: Chain) -> Result<Self, GasOracleError> {
+        Self::with_client(Client::new(), chain)
+    }
+
+    pub fn with_client(client: Client, chain: Chain) -> Result<Self, GasOracleError> {
+        // TODO: Sniff chain from chain id.
+        let url = match chain {
+            Chain::Polygon => Url::parse(GAS_PRICE_ENDPOINT).unwrap(),
+            Chain::PolygonMumbai => Url::parse(MUMBAI_GAS_PRICE_ENDPOINT).unwrap(),
+            _ => return Err(GasOracleError::UnsupportedChain),
+        };
+        Ok(Self { client, url, gas_category: GasCategory::Standard })
+    }
+
+    /// Sets the gas price category to be used when fetching the gas price.
+    #[must_use]
+    pub fn category(mut self, gas_category: GasCategory) -> Self {
+        self.gas_category = gas_category;
+        self
+    }
+
+    /// Perform request to Blocknative, decode response
+    pub async fn request(&self) -> Result<(f64, GasEstimate), GasOracleError> {
+        let response: Response =
+            self.client.get(self.url.as_ref()).send().await?.error_for_status()?.json().await?;
+        let estimate = match self.gas_category {
+            GasCategory::SafeLow => response.safe_low,
+            GasCategory::Standard => response.standard,
+            GasCategory::Fast => response.fast,
+            GasCategory::Fastest => response.fast,
+        };
+        Ok((response.estimated_base_fee, estimate))
+    }
+}
+
+#[cfg_attr(target_arch = "wasm32", async_trait(?Send))]
+#[cfg_attr(not(target_arch = "wasm32"), async_trait)]
+impl GasOracle for Polygon {
+    async fn fetch(&self) -> Result<U256, GasOracleError> {
+        let (base_fee, estimate) = self.request().await?;
+        let fee = base_fee + estimate.max_priority_fee;
+        Ok(from_gwei(fee))
+    }
+
+    async fn estimate_eip1559_fees(&self) -> Result<(U256, U256), GasOracleError> {
+        let (_, estimate) = self.request().await?;
+        Ok((from_gwei(estimate.max_fee), from_gwei(estimate.max_priority_fee)))
+    }
+}
+
+fn from_gwei(gwei: f64) -> U256 {
+    u256_from_f64_saturating(gwei * 1.0e18_f64)
+}

ethers-middleware/tests/gas_oracle.rs

@@ -57,7 +57,7 @@ async fn using_gas_oracle() {
 #[tokio::test]
 async fn eth_gas_station() {
     // initialize and fetch gas estimates from EthGasStation
-    let eth_gas_station_oracle = EthGasStation::new(None);
+    let eth_gas_station_oracle = EthGasStation::default();
     let data = eth_gas_station_oracle.fetch().await;
     assert!(data.is_ok());
 }
@@ -83,7 +83,7 @@ async fn etherscan() {
 #[tokio::test]
 async fn etherchain() {
     // initialize and fetch gas estimates from Etherchain
-    let etherchain_oracle = Etherchain::new().category(GasCategory::Fast);
+    let etherchain_oracle = Etherchain::default().category(GasCategory::Fast);
     let data = etherchain_oracle.fetch().await;
     assert!(data.is_ok());
 }