Fix http header accept parsing problem (#3185)

## Issue Addressed

Which issue # does this PR address?
#3114 

## Proposed Changes

1. introduce `mime` package 
2. Parse `Accept` field in the header with `mime`

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

Cargo.lock

@@ -1586,6 +1586,7 @@ dependencies = [
  "futures-util",
  "libsecp256k1",
  "lighthouse_network",
+ "mime",
  "procinfo",
  "proto_array",
  "psutil",

common/eth2/Cargo.toml

@@ -26,6 +26,7 @@ futures-util = "0.3.8"
 futures = "0.3.8"
 store = { path = "../../beacon_node/store", optional = true }
 slashing_protection = { path = "../../validator_client/slashing_protection", optional = true }
+mime = "0.3.16"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 psutil = { version = "3.2.2", optional = true }

common/eth2/src/types.rs

@@ -3,7 +3,9 @@
 
 use crate::Error as ServerError;
 use lighthouse_network::{ConnectionDirection, Enr, Multiaddr, PeerConnectionStatus};
+use mime::{Mime, APPLICATION, JSON, OCTET_STREAM, STAR};
 use serde::{Deserialize, Serialize};
+use std::cmp::Reverse;
 use std::convert::TryFrom;
 use std::fmt;
 use std::str::{from_utf8, FromStr};
@@ -1008,15 +1010,37 @@ impl FromStr for Accept {
     type Err = String;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        match s {
-            "application/octet-stream" => Ok(Accept::Ssz),
-            "application/json" => Ok(Accept::Json),
-            "*/*" => Ok(Accept::Any),
-            _ => Err("accept header cannot be parsed.".to_string()),
-        }
+        let mut mimes = parse_accept(s)?;
+
+        // [q-factor weighting]: https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.2
+        // find the highest q-factor supported accept type
+        mimes.sort_by_key(|m| {
+            Reverse(m.get_param("q").map_or(1000_u16, |n| {
+                (n.as_ref().parse::<f32>().unwrap_or(0_f32) * 1000_f32) as u16
+            }))
+        });
+        mimes
+            .into_iter()
+            .find_map(|m| match (m.type_(), m.subtype()) {
+                (APPLICATION, OCTET_STREAM) => Some(Accept::Ssz),
+                (APPLICATION, JSON) => Some(Accept::Json),
+                (STAR, STAR) => Some(Accept::Any),
+                _ => None,
+            })
+            .ok_or_else(|| "accept header is not supported".to_string())
     }
 }
 
+fn parse_accept(accept: &str) -> Result<Vec<Mime>, String> {
+    accept
+        .split(',')
+        .map(|part| {
+            part.parse()
+                .map_err(|e| format!("error parsing Accept header: {}", e))
+        })
+        .collect()
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub struct LivenessRequestData {
     pub epoch: Epoch,
@@ -1045,4 +1069,23 @@ mod tests {
             }
         );
     }
+
+    #[test]
+    fn parse_accept_header_content() {
+        assert_eq!(
+            Accept::from_str("application/json; charset=utf-8").unwrap(),
+            Accept::Json
+        );
+
+        assert_eq!(
+            Accept::from_str("text/plain,application/octet-stream;q=0.3,application/json;q=0.9")
+                .unwrap(),
+            Accept::Json
+        );
+
+        assert_eq!(
+            Accept::from_str("text/plain"),
+            Err("accept header is not supported".to_string())
+        )
+    }
 }