provider: let providers decide their status

Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.

Previously, provider loading worked as follows:

1. eval_has_provider() verified provider availability by searching for
   the provider#providername#Call function and cached this verificaion as a static
   variable for some providers
2. providers short-circuited on loading to prevent the definition of the
   Call function (with the exception of the node provider that did not)

This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.

eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.

All providers hosting a Call function were updated to respect this.

The clipboard provider now has a Reload function to reload the
provider.

6 files changed, 22 insertions, 19 deletions

diff --git a/runtime/autoload/provider/clipboard.vim b/runtime/autoload/provider/clipboard.vim
index 2b06ee8c48..e43f8fbb7a 100644
--- a/runtime/autoload/provider/clipboard.vim
+++ b/runtime/autoload/provider/clipboard.vim
@@ -48,6 +48,9 @@ endfunction
 let s:cache_enabled = 1
 let s:err = ''
 
+" eval_has_provider checks the variable to verify provider status
+let g:provider#clipboard#enabled = 0
+
 function! provider#clipboard#Error() abort
   return s:err
 endfunction
@@ -120,12 +123,11 @@ function! provider#clipboard#Executable() abort
   return ''
 endfunction
 
-if empty(provider#clipboard#Executable())
-  " provider#clipboard#Call() *must not* be defined if the provider is broken.
-  " Otherwise eval_has_provider() thinks the clipboard provider is
-  " functioning, and eval_call_provider() will happily call it.
-  finish
-endif
+" Call this to setup/reload the provider
+function! provider#clipboard#Reload()
+  " #enabled is used by eval_has_provider()
+  let g:provider#clipboard#enabled = !empty(provider#clipboard#Executable())
+endfunction
 
 function! s:clipboard.get(reg) abort
   if type(s:paste[a:reg]) == v:t_func
@@ -192,3 +194,5 @@ function! provider#clipboard#Call(method, args) abort
     let s:here = v:false
   endtry
 endfunction
+
+call provider#clipboard#Reload()
diff --git a/runtime/autoload/provider/node.vim b/runtime/autoload/provider/node.vim
index 35882849bd..6413720605 100644
--- a/runtime/autoload/provider/node.vim
+++ b/runtime/autoload/provider/node.vim
@@ -2,6 +2,7 @@ if exists('g:loaded_node_provider')
   finish
 endif
 let g:loaded_node_provider = 1
+let g:provider#node#enabled = 0
 
 function! s:is_minimum_version(version, min_major, min_minor) abort
   if empty(a:version)
@@ -143,6 +144,9 @@ let s:prog = provider#node#Detect()
 
 if empty(s:prog)
   let s:err = 'Cannot find the "neovim" node package. Try :checkhealth'
+else
+  let g:provider#node#enabled = 1
 endif
 
 call remote#host#RegisterPlugin('node-provider', 'node', [])
+
diff --git a/runtime/autoload/provider/python.vim b/runtime/autoload/provider/python.vim
index a06cbe4814..d65506a55a 100644
--- a/runtime/autoload/provider/python.vim
+++ b/runtime/autoload/provider/python.vim
@@ -10,6 +10,7 @@ endif
 let g:loaded_python_provider = 1
 
 let [s:prog, s:err] = provider#pythonx#Detect(2)
+let g:provider#python#enabled = !empty(s:prog)
 
 function! provider#python#Prog() abort
   return s:prog
@@ -19,11 +20,6 @@ function! provider#python#Error() abort
   return s:err
 endfunction
 
-if s:prog == ''
-  " Detection failed
-  finish
-endif
-
 " The Python provider plugin will run in a separate instance of the Python
 " host.
 call remote#host#RegisterClone('legacy-python-provider', 'python')
diff --git a/runtime/autoload/provider/python3.vim b/runtime/autoload/provider/python3.vim
index 242a224cb3..469611c7ce 100644
--- a/runtime/autoload/provider/python3.vim
+++ b/runtime/autoload/provider/python3.vim
@@ -10,6 +10,7 @@ endif
 let g:loaded_python3_provider = 1
 
 let [s:prog, s:err] = provider#pythonx#Detect(3)
+let g:provider#python3#enabled = !empty(s:prog)
 
 function! provider#python3#Prog() abort
   return s:prog
@@ -19,11 +20,6 @@ function! provider#python3#Error() abort
   return s:err
 endfunction
 
-if s:prog == ''
-  " Detection failed
-  finish
-endif
-
 " The Python3 provider plugin will run in a separate instance of the Python3
 " host.
 call remote#host#RegisterClone('legacy-python3-provider', 'python3')
diff --git a/runtime/autoload/provider/ruby.vim b/runtime/autoload/provider/ruby.vim
index 3b4c6c4839..df43dffa40 100644
--- a/runtime/autoload/provider/ruby.vim
+++ b/runtime/autoload/provider/ruby.vim
@@ -7,6 +7,7 @@ let g:loaded_ruby_provider = 1
 function! provider#ruby#Detect() abort
   return s:prog
 endfunction
+let g:provider#ruby#enabled = 0
 
 function! provider#ruby#Prog() abort
   return s:prog
@@ -65,6 +66,8 @@ let s:plugin_path = expand('<sfile>:p:h') . '/script_host.rb'
 
 if empty(s:prog)
   let s:err = 'Cannot find the neovim RubyGem. Try :checkhealth'
+else
+  let g:provider#ruby#enabled = 1
 endif
 
 call remote#host#RegisterClone('legacy-ruby-provider', 'ruby')
diff --git a/runtime/doc/develop.txt b/runtime/doc/develop.txt
index 843e23ee54..0f9e17e697 100644
--- a/runtime/doc/develop.txt
+++ b/runtime/doc/develop.txt
@@ -111,7 +111,7 @@ functions in eval.c:
 - eval_call_provider(name, method, arguments): calls provider#(name)#Call
   with the method and arguments.
 - eval_has_provider(name): Checks if a provider is implemented. Returns true
-  if the provider#(name)#Call function is implemented. Called by |has()|
+  if the provider#(name)#enabled variable is not 0. Called by |has()|
   (vimscript) to check if features are available.
 
 The provider#(name)#Call function implements integration with an external
@@ -119,8 +119,8 @@ system, because shell commands and |RPC| clients are easier to work with in
 vimscript.
 
 For example, the Python provider is implemented by the
-autoload/provider/python.vim script; the provider#python#Call function is only
-defined if a valid external Python host is found. That works well with the
+autoload/provider/python.vim script; the variable provider#python#enabled is only
+1 if a valid external Python host is found. That works well with the
 `has('python')` expression (normally used by Python plugins) because if the
 Python host isn't installed then the plugin will "think" it is running in
 a Vim compiled without the "+python" feature.