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

1 files changed, 17 insertions, 0 deletions

diff --git a/docs/cmdline-opts/happy-eyeballs-timeout-ms.d b/docs/cmdline-opts/happy-eyeballs-timeout-ms.d
new file mode 100644
index 000000000..ec9a8c228
--- /dev/null
+++ b/docs/cmdline-opts/happy-eyeballs-timeout-ms.d
@@ -0,0 +1,17 @@
+Long: happy-eyeballs-timeout-ms
+Arg: <milliseconds>
+Help: How long to wait in milliseconds for IPv6 before trying IPv4
+Added: 7.59.0
+---
+Happy eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6
+addresses for dual-stack hosts, preferring IPv6 first for the number of
+milliseconds. If the IPv6 address cannot be connected to within that time then
+a connection attempt is made to the IPv4 address in parallel. The first
+connection to be established is the one that is used.
+
+The range of suggested useful values is limited. Happy Eyeballs RFC 6555 says
+"It is RECOMMENDED that connection attempts be paced 150-250 ms apart to
+balance human factors against network load." libcurl currently defaults to
+200 ms. Firefox and Chrome currently default to 300 ms.
+
+If this option is used several times, the last one will be used.