lsp: set working directory based on module directory

Set the working directory of gopls based on the value of GOMOD. When in
GOPATH mode, the value of Vim's working directory will be used.

Fixes fatih#2263

autoload/go/lsp.vim

@@ -7,8 +7,7 @@ scriptencoding utf-8
 let s:lspfactory = {}
 
 function! s:lspfactory.get() dict abort
-  if !has_key(self, 'current')
-    " TODO(bc): check that the lsp is still running.
+  if !has_key(self, 'current') || empty(self.current)
     let self.current = s:newlsp()
   endif
 
@@ -150,7 +149,17 @@ function! s:newlsp() abort
     if !self.last_request_id
       " TODO(bc): run a server per module and one per GOPATH? (may need to
       " keep track of servers by rootUri).
-      let l:msg = self.newMessage(go#lsp#message#Initialize(getcwd()))
+      let l:wd = go#util#ModuleRoot()
+      if l:wd == -1
+        call go#util#EchoError('could not determine appropriate working directory for gopls')
+        return
+      endif
+
+      if l:wd == ''
+        let l:wd = getcwd()
+      endif
+
+      let l:msg = self.newMessage(go#lsp#message#Initialize(l:wd))
 
       let l:state = s:newHandlerState('')
       let l:state.handleResult = funcref('self.handleInitializeResult', [], l:self)

autoload/go/util.vim

@@ -137,11 +137,33 @@ function! go#util#gomod() abort
   return substitute(s:exec(['go', 'env', 'GOMOD'])[0], '\n', '', 'g')
 endfunction
 
-
 function! go#util#osarch() abort
   return go#util#env("goos") . '_' . go#util#env("goarch")
 endfunction
 
+" go#util#ModuleRoot returns the root directory of the module of the current
+" buffer.
+function! go#util#ModuleRoot() abort
+  let [l:out, l:err] = go#util#ExecInDir(['go', 'env', 'GOMOD'])
+  if l:err != 0
+    return -1
+  endif
+
+  let l:module = split(l:out, '\n', 1)[0]
+
+  " When run with `GO111MODULE=on and not in a module directory, the module will be reported as /dev/null.
+  let l:fakeModule = '/dev/null'
+  if go#util#IsWin()
+    let l:fakeModule = 'NUL'
+  endif
+
+  if l:fakeModule == l:module
+    return expand('%:p:h')
+  endif
+
+  return fnamemodify(l:module, ':p:h')
+endfunction
+
 " Run a shell command.
 "
 " It will temporary set the shell to /bin/sh for Unix-like systems if possible,