feat(server): set $NVIM, unset $NVIM_LISTEN_ADDRESS #11009

PROBLEM
------------------------------------------------------------------------
$NVIM_LISTEN_ADDRESS has conflicting purposes as both a parameter ("the
current process should listen on this address") and a descriptor ("the
current process is a child of this address").

This contradiction means the presence of NVIM_LISTEN_ADDRESS is
ambiguous, so child Nvim always tries to listen on its _parent's_
socket. This is the cause of lots of  "Failed to start server" spam in
our test/CI logs:

    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-4480-0
    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-2168-0

SOLUTION
------------------------------------------------------------------------

1. Set $NVIM to the parent v:servername, *only* in child processes.
   - Now the correct way to detect a "parent" Nvim is to check for $NVIM.
2. Do NOT set $NVIM_LISTEN_ADDRESS in child processes.
3. On startup if $NVIM_LISTEN_ADDRESS exists, unset it immediately after
   server init.
4. Open a channel to parent automatically, expose it as v:parent.

Fixes #3118
Fixes #6764
Fixes #9336
Ref https://github.com/neovim/neovim/pull/8247#issuecomment-380275696
Ref #8696

1 files changed, 2 insertions, 2 deletions

diff --git a/test/functional/provider/perl_spec.lua b/test/functional/provider/perl_spec.lua
index 125674660b..aff5e36e24 100644
--- a/test/functional/provider/perl_spec.lua
+++ b/test/functional/provider/perl_spec.lua
@@ -83,7 +83,7 @@ describe('perl provider', function()
       use Neovim::Ext;
       use Neovim::Ext::MsgPack::RPC;
 
-      my $session = Neovim::Ext::MsgPack::RPC::socket_session($ENV{NVIM_LISTEN_ADDRESS});
+      my $session = Neovim::Ext::MsgPack::RPC::socket_session($ENV{NVIM});
       my $nvim = Neovim::Ext::from_session($session);
       $nvim->command('let g:job_out = "hello"');
       1;
@@ -116,7 +116,7 @@ describe('perl provider', function()
       use Neovim::Ext;
       use Neovim::Ext::MsgPack::RPC;
 
-      my $session = Neovim::Ext::MsgPack::RPC::socket_session($ENV{NVIM_LISTEN_ADDRESS});
+      my $session = Neovim::Ext::MsgPack::RPC::socket_session($ENV{NVIM});
       my $nvim = Neovim::Ext::from_session($session);
       my $plugin = TestPlugin->new($nvim);
       $plugin->test_command();