fix: disable multiplexing on macOS for bad curl

In certain versions of libcurl when proxy is in use with HTTP/2
multiplexing, connections will continue stacking up. This was
fixed in libcurl 8.0.0 in curl/curl@821f6e2

However, a custom built Cargo can still link against old system libcurl.
Multiplexing needs to be disabled when those versions are detected.

We only take care of macOS now, since it is one of the major platforms.
For other OS we encourage not to stick to older versions of libcurl.

src/cargo/util/config/mod.rs

@@ -1717,8 +1717,19 @@ impl Config {
     }
 
     pub fn http_config(&self) -> CargoResult<&CargoHttpConfig> {
-        self.http_config
-            .try_borrow_with(|| self.get::<CargoHttpConfig>("http"))
+        self.http_config.try_borrow_with(|| {
+            #[allow(unused_mut)]
+            let mut http = self.get::<CargoHttpConfig>("http")?;
+            #[cfg(target_os = "macos")]
+            {
+                // We only take care of macOS now, since it is one of the major
+                // platforms. For other OS we encourage not to stick to older
+                // versions of libcurl.
+                let curl_v = curl::Version::get();
+                disables_multiplexing_for_bad_curl(curl_v.version(), &mut http, self);
+            }
+            Ok(http)
+        })
     }
 
     pub fn future_incompat_config(&self) -> CargoResult<&CargoFutureIncompatConfig> {
@@ -2750,3 +2761,76 @@ impl Tool {
         }
     }
 }
+
+/// Disable HTTP/2 multiplexing for some broken versions of libcurl.
+///
+/// In certain versions of libcurl when proxy is in use with HTTP/2
+/// multiplexing, connections will continue stacking up. This was
+/// fixed in libcurl 8.0.0 in curl/curl@821f6e2a89de8aec1c7da3c0f381b92b2b801efc
+///
+/// However, a custom built Cargo can still link against old system libcurl.
+/// Multiplexing needs to be disabled when those versions are detected.
+#[allow(dead_code)]
+fn disables_multiplexing_for_bad_curl(
+    curl_version: &str,
+    http: &mut CargoHttpConfig,
+    config: &Config,
+) {
+    use crate::util::network;
+
+    if network::proxy::http_proxy_exists(http, config) && http.multiplexing.is_none() {
+        let bad_curl_versions = ["7.87.0", "7.88.0", "7.88.1"];
+        if bad_curl_versions
+            .iter()
+            .any(|v| curl_version.starts_with(v))
+        {
+            http.multiplexing = Some(false);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[cfg(target_os = "macos")]
+    #[test]
+    fn disables_multiplexing() {
+        use super::disables_multiplexing_for_bad_curl;
+        use super::CargoHttpConfig;
+        use super::Config;
+        use super::Shell;
+
+        let mut config = Config::new(Shell::new(), "".into(), "".into());
+        config.set_search_stop_path(std::path::PathBuf::new());
+        config.set_env(Default::default());
+
+        let mut http = CargoHttpConfig::default();
+        http.proxy = Some("127.0.0.1:3128".into());
+        disables_multiplexing_for_bad_curl("7.88.1", &mut http, &config);
+        assert_eq!(http.multiplexing, Some(false));
+
+        let cases = [
+            (None, None, "7.87.0", None),
+            (None, None, "7.88.0", None),
+            (None, None, "7.88.1", None),
+            (None, None, "8.0.0", None),
+            (Some("".into()), None, "7.87.0", Some(false)),
+            (Some("".into()), None, "7.88.0", Some(false)),
+            (Some("".into()), None, "7.88.1", Some(false)),
+            (Some("".into()), None, "8.0.0", None),
+            (Some("".into()), Some(false), "7.87.0", Some(false)),
+            (Some("".into()), Some(false), "7.88.0", Some(false)),
+            (Some("".into()), Some(false), "7.88.1", Some(false)),
+            (Some("".into()), Some(false), "8.0.0", Some(false)),
+        ];
+
+        for (proxy, multiplexing, curl_v, result) in cases {
+            let mut http = CargoHttpConfig {
+                multiplexing,
+                proxy,
+                ..Default::default()
+            };
+            disables_multiplexing_for_bad_curl(curl_v, &mut http, &config);
+            assert_eq!(http.multiplexing, result);
+        }
+    }
+}