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controlhttp has the responsibility of dialing a set of candidate control endpoints in a way that minimizes user facing latency. If one control endpoint is unavailable we promptly dial another, racing across the dimensions of: IPv6, IPv4, port 80, and port 443, over multiple server endpoints. In the case that the top priority endpoint was not available, the prior implementation would hang waiting for other results, so as to try to return the highest priority successful connection to the rest of the client code. This hang would take too long with a large dialplan and sufficient client to endpoint latency as to cause the server to timeout the connection due to inactivity in the intermediate state. Instead of trying to prioritize non-ideal candidate connections, the first successful connection is now used unconditionally, improving user facing latency and avoiding any delays that would encroach on the server-side timeout. The tests are converted to memnet and synctest, running on all platforms. Fixes #8442 Fixes tailscale/corp#32534 Co-authored-by: James Tucker <james@tailscale.com> Change-Id: I4eb57f046d8b40403220e40eb67a31c41adb3a38 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Signed-off-by: James Tucker <james@tailscale.com>