Auto merge of #5957 - lostiniceland:5717-curl-low_speed, r=alexcrichton

Provide http-config option for curl low-speed-limit/time

Fixes #5717

Provides new override options for curl in Cargo http config-section.

```
[http]
timeout = 2
low-speed-limit = 5
```

Test with the following during crate-download
`sudo tc qdisc add dev eth0 handle 1: root htb default 11 && sudo tc class add dev eth0 parent 1: classid 1:1 htb rate 8bps && sudo tc class add dev eth0 parent 1:1 classid 1:11 htb rate 8bps`

Clear with
`sudo tc qdisc del dev eth0 root`

src/cargo/ops/registry.rs

@@ -367,8 +367,7 @@ pub fn configure_http_handle(config: &Config, handle: &mut Easy) -> CargoResult<
     // connect phase as well as a "low speed" timeout so if we don't receive
     // many bytes in a large-ish period of time then we time out.
     handle.connect_timeout(Duration::new(30, 0))?;
-    handle.low_speed_limit(10 /* bytes per second */)?;
-    handle.low_speed_time(Duration::new(30, 0))?;
+    handle.low_speed_limit(http_low_speed_limit(config)?)?;
     if let Some(proxy) = http_proxy(config)? {
         handle.proxy(&proxy)?;
     }
@@ -390,6 +389,14 @@ pub fn configure_http_handle(config: &Config, handle: &mut Easy) -> CargoResult<
     Ok(())
 }
 
+/// Find an override from config for curl low-speed-limit option, otherwise use default value
+fn http_low_speed_limit(config: &Config) -> CargoResult<u32> {
+    if let Some(s) = config.get::<Option<u32>>("http.low-speed-limit")? {
+        return Ok(s);
+    }
+    Ok(10)
+}
+
 /// Find an explicit HTTP proxy if one is available.
 ///
 /// Favor cargo's `http.proxy`, then git's `http.proxy`. Proxies specified

src/doc/src/reference/config.md

@@ -95,6 +95,7 @@ proxy = "host:port" # HTTP proxy to use for HTTP requests (defaults to none)
 timeout = 60000     # Timeout for each HTTP request, in milliseconds
 cainfo = "cert.pem" # Path to Certificate Authority (CA) bundle (optional)
 check-revoke = true # Indicates whether SSL certs are checked for revocation
+low-speed-limit = 0 # Lower threshold for bytes/sec (10 = default, 0 = disabled)
 
 [build]
 jobs = 1                  # number of parallel jobs, defaults to # of CPUs