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, 10 insertions, 0 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 88059ff21e..0a71526246 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -5088,6 +5088,16 @@ static dict_T *create_environment(const dictitem_T *job_env, const bool clear_en
     tv_dict_add_str(env, S_LEN("TERM"), pty_term_name);
   }
 
+  // Set $NVIM (in the child process) to v:servername. #3118
+  char *nvim_addr = (char *)get_vim_var_str(VV_SEND_SERVER);
+  if (nvim_addr[0] != '\0') {
+    dictitem_T *dv = tv_dict_find(env, S_LEN("NVIM"));
+    if (dv) {
+      tv_dict_item_remove(env, dv);
+    }
+    tv_dict_add_str(env, S_LEN("NVIM"), nvim_addr);
+  }
+
   if (job_env) {
 #ifdef WIN32
     TV_DICT_ITER(job_env->di_tv.vval.v_dict, var, {