Auto merge of #3112 - micbou:omnifunc-start-column, r=micbou

[READY] Move cursor to start column before calling again omnifunc

Vim internally moves the cursor to the start column before calling the omnifunc a second time. This can be confirmed by starting Vim with the following vimrc:
```vim
set nocompatible

function! Omnifunc(findstart, base)
        echom col('.')
	if a:findstart
		if getline('.') == 'foo.ba'
			return 4
		endif
	endif
	return [ 'bar', 'baz']
endfunction

set omnifunc=Omnifunc
```
typing `foo.ba`, and hitting `<C-X><C-O>`. This outputs `7` and `5` on the command line which means that the cursor at column 7 on the first call of the omnifunc is moved to column 5 on the second call.

Since some omnifuncs depend on this to compute the list of candidates (e.g. the one defined by [the LanguageClient-neovim plugin](https://github.com/autozimu/LanguageClient-neovim)), we should reproduce that behavior in YCM.

Fixes https://github.com/Valloric/YouCompleteMe/issues/3099.

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python/ycm/omni_completer.py

@@ -79,17 +79,18 @@ class OmniCompleter( Completer ):
       return []
 
     try:
-      return_value = vimsupport.GetIntValue( self._omnifunc + '(1,"")' )
-      if return_value < 0:
-        # FIXME: Technically, if the return is -1 we should raise an error
+      start_column = vimsupport.GetIntValue( self._omnifunc + '(1,"")' )
+      if start_column < 0:
+        # FIXME: Technically, if the returned value is -1 we should raise an
+        # error.
         return []
 
       # Use the start column calculated by the omnifunc, rather than our own
       # interpretation. This is important for certain languages where our
       # identifier detection is either incorrect or not compatible with the
       # behaviour of the omnifunc. Note: do this before calling the omnifunc
-      # because it affects the value returned by 'query'
-      request_data[ 'start_column' ] = return_value + 1
+      # because it affects the value returned by 'query'.
+      request_data[ 'start_column' ] = start_column + 1
 
       # Calling directly the omnifunc may move the cursor position. This is the
       # case with the default Vim omnifunc for C-family languages
@@ -99,6 +100,12 @@ class OmniCompleter( Completer ):
       # So, we restore the cursor position after calling the omnifunc.
       line, column = vimsupport.CurrentLineAndColumn()
 
+      # Vim internally moves the cursor to the start column before calling again
+      # the omnifunc. Some omnifuncs like the one defined by the
+      # LanguageClient-neovim plugin depend on this behavior to compute the list
+      # of candidates.
+      vimsupport.SetCurrentLineAndColumn( line, start_column )
+
       omnifunc_call = [ self._omnifunc,
                         "(0,'",
                         vimsupport.EscapeForVim( request_data[ 'query' ] ),

python/ycm/tests/omni_completer_test.py

@@ -649,6 +649,41 @@ def OmniCompleter_GetCompletions_RestoreCursorPositionAfterOmnifuncCall_test(
     )
 
 
+@YouCompleteMeInstance( { 'g:ycm_cache_omnifunc': 1,
+                          'g:ycm_semantic_triggers': TRIGGERS } )
+def OmniCompleter_GetCompletions_MoveCursorPositionAtStartColumn_test( ycm ):
+  # This omnifunc relies on the cursor being moved at the start column when
+  # called the second time like LanguageClient#complete from the
+  # LanguageClient-neovim plugin.
+  def Omnifunc( findstart, base ):
+    if findstart:
+      return 5
+    if vimsupport.CurrentColumn() == 5:
+      return [ 'length' ]
+    return []
+
+  current_buffer = VimBuffer( 'buffer',
+                              contents = [ 'String test',
+                                           '',
+                                           'test.le' ],
+                              filetype = FILETYPE,
+                              omnifunc = Omnifunc )
+
+  with MockVimBuffers( [ current_buffer ], [ current_buffer ], ( 3, 7 ) ):
+    ycm.SendCompletionRequest()
+    assert_that(
+      vimsupport.CurrentLineAndColumn(),
+      contains( 2, 7 )
+    )
+    assert_that(
+      ycm.GetCompletionResponse(),
+      has_entries( {
+        'completions': ToBytesOnPY2( [ 'length' ] ),
+        'completion_start_column': 6
+      } )
+    )
+
+
 @YouCompleteMeInstance( { 'g:ycm_cache_omnifunc': 0,
                           'g:ycm_semantic_triggers': TRIGGERS } )
 def OmniCompleter_GetCompletions_NoCache_NoSemanticTrigger_test( ycm ):