url: Add option CURLOPT_HAPPY_EYEBALLS_TIMEOUT

- Add new option CURLOPT_HAPPY_EYEBALLS_TIMEOUT to set libcurl's happy
  eyeball timeout value.

- Add new optval macro CURL_HET_DEFAULT to represent the default happy
  eyeballs timeout value (currently 200 ms).

- Add new tool option --happy-eyeballs-timeout-ms to expose
  CURLOPT_HAPPY_EYEBALLS_TIMEOUT. The -ms suffix is used because the
  other -timeout options in the tool expect seconds not milliseconds.

Closes https://github.com/curl/curl/pull/2260

lib/connect.c

@@ -783,7 +783,8 @@ CURLcode Curl_is_connected(struct connectdata *conn,
 
       /* should we try another protocol family? */
       if(i == 0 && conn->tempaddr[1] == NULL &&
-         Curl_timediff(now, conn->connecttime) >= HAPPY_EYEBALLS_TIMEOUT) {
+         (Curl_timediff(now, conn->connecttime) >=
+          data->set.happy_eyeballs_timeout)) {
         trynextip(conn, sockindex, 1);
       }
     }
@@ -1206,7 +1207,8 @@ CURLcode Curl_connecthost(struct connectdata *conn,  /* context */
   }
 
   data->info.numconnects++; /* to track the number of connections made */
-  Curl_expire(conn->data, HAPPY_EYEBALLS_TIMEOUT, EXPIRE_HAPPY_EYEBALLS);
+  Curl_expire(conn->data, data->set.happy_eyeballs_timeout,
+              EXPIRE_HAPPY_EYEBALLS);
 
   return CURLE_OK;
 }

lib/connect.h

@@ -41,8 +41,6 @@ timediff_t Curl_timeleft(struct Curl_easy *data,
                          bool duringconnect);
 
 #define DEFAULT_CONNECT_TIMEOUT 300000 /* milliseconds == five minutes */
-#define HAPPY_EYEBALLS_TIMEOUT     200 /* milliseconds to wait between
-                                          IPv4/IPv6 connection attempts */
 
 /*
  * Used to extract socket and connectdata struct for the most recent

lib/setopt.c

@@ -2533,6 +2533,12 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option,
   case CURLOPT_SSH_COMPRESSION:
     data->set.ssh_compression = (0 != va_arg(param, long))?TRUE:FALSE;
     break;
+  case CURLOPT_HAPPY_EYEBALLS_TIMEOUT:
+    arg = va_arg(param, long);
+    if(arg < 0)
+      return CURLE_BAD_FUNCTION_ARGUMENT;
+    data->set.happy_eyeballs_timeout = arg;
+    break;
   default:
     /* unknown tag and its companion, just ignore: */
     result = CURLE_UNKNOWN_OPTION;

lib/url.c

@@ -527,6 +527,7 @@ CURLcode Curl_init_userdefined(struct Curl_easy *data)
   set->expect_100_timeout = 1000L; /* Wait for a second by default. */
   set->sep_headers = TRUE; /* separated header lists by default */
   set->buffer_size = READBUFFER_SIZE;
+  set->happy_eyeballs_timeout = CURL_HET_DEFAULT;
 
   Curl_http2_init_userset(set);
   return result;

lib/urldata.h

@@ -1520,6 +1520,7 @@ struct UserDefined {
   long timeout;         /* in milliseconds, 0 means no timeout */
   long connecttimeout;  /* in milliseconds, 0 means no timeout */
   long accepttimeout;   /* in milliseconds, 0 means no timeout */
+  long happy_eyeballs_timeout; /* in milliseconds, 0 is a valid value */
   long server_response_timeout; /* in milliseconds, 0 means no timeout */
   long tftp_blksize;    /* in bytes, 0 means use default */
   bool tftp_no_options; /* do not send TFTP options requests */