Make TCP connect handle EINTR correctly

According to the POSIX standard, if connect() is interrupted by a
signal that is caught while blocked waiting to establish a connection,
connect() shall fail and set errno to EINTR, but the connection
request shall not be aborted, and the connection shall be established
asynchronously.

When connect() is called one more time to handle EINTR error, it
can already be established asynchronously and errno would be set
to EISCONN. This case should be handled and not produce an error.

library/std/src/sys/unix/mod.rs

@@ -323,10 +323,18 @@ where
     F: FnMut() -> T,
 {
     loop {
-        match cvt(f()) {
-            Err(ref e) if e.is_interrupted() => {}
-            other => return other,
+        let t = f();
+        if t.is_minus_one() {
+            let e = crate::io::Error::last_os_error();
+            if e.is_interrupted() {
+                continue;
+            }
+            if e.raw_os_error() == Some(libc::EISCONN) {
+                return Ok(t);
+            }
+            return Err(e);
         }
+        return Ok(t);
     }
 }