Fix #964 - Remove signs when multiple signs end up on a single line

autoload/ale/sign.vim

@@ -214,24 +214,32 @@ function! s:BuildSignMap(current_sign_list, grouped_items) abort
     let l:sign_offset = g:ale_sign_offset
 
     for [l:line, l:sign_id, l:name] in a:current_sign_list
-        let l:sign_map[l:line] = {
+        let l:sign_info = get(l:sign_map, l:line, {
-        \   'current_id': l:sign_id,
+        \   'current_id_list': [],
-        \   'current_name': l:name,
+        \   'current_name_list': [],
         \   'new_id': 0,
         \   'new_name': '',
         \   'items': [],
-        \}
+        \})
 
+        " Increment the sign offset for new signs, by the maximum sign ID.
         if l:sign_id > l:sign_offset
             let l:sign_offset = l:sign_id
         endif
+
+        " Remember the sign names and IDs in separate Lists, so they are easy
+        " to work with.
+        call add(l:sign_info.current_id_list, l:sign_id)
+        call add(l:sign_info.current_name_list, l:name)
+
+        let l:sign_map[l:line] = l:sign_info
     endfor
 
     for l:group in a:grouped_items
         let l:line = l:group[0].lnum
         let l:sign_info = get(l:sign_map, l:line, {
-        \   'current_id': 0,
+        \   'current_id_list': [],
-        \   'current_name': '',
+        \   'current_name_list': [],
         \   'new_id': 0,
         \   'new_name': '',
         \   'items': [],
@@ -240,11 +248,18 @@ function! s:BuildSignMap(current_sign_list, grouped_items) abort
         let l:sign_info.new_name = ale#sign#GetSignName(l:group)
         let l:sign_info.items = l:group
 
-        if l:sign_info.current_name isnot# l:sign_info.new_name
+        let l:index = index(
+        \   l:sign_info.current_name_list,
+        \   l:sign_info.new_name
+        \)
+
+        if l:index >= 0
+            " We have a sign with this name already, so use the same ID.
+            let l:sign_info.new_id = l:sign_info.current_id_list[l:index]
+        else
+            " This sign name replaces the previous name, so use a new ID.
             let l:sign_info.new_id = l:sign_offset + 1
             let l:sign_offset += 1
-        else
-            let l:sign_info.new_id = l:sign_info.current_id
         endif
 
         let l:sign_map[l:line] = l:sign_info
@@ -278,7 +293,7 @@ function! ale#sign#GetSignCommands(buffer, was_sign_set, sign_map) abort
                 let l:item.sign_id = l:info.new_id
             endfor
 
-            if l:info.new_id isnot l:info.current_id
+            if index(l:info.current_id_list, l:info.new_id) < 0
                 call add(l:command_list, 'sign place '
                 \   . (l:info.new_id)
                 \   . ' line=' . l:line_str
@@ -291,13 +306,15 @@ function! ale#sign#GetSignCommands(buffer, was_sign_set, sign_map) abort
 
     " Remove signs without new IDs.
     for l:info in values(a:sign_map)
-        if l:info.current_id && l:info.current_id isnot l:info.new_id
+        for l:current_id in l:info.current_id_list
+            if l:current_id isnot l:info.new_id
                 call add(l:command_list, 'sign unplace '
-            \   . (l:info.current_id)
+                \   . l:current_id
                 \   . ' buffer=' . a:buffer
                 \)
             endif
         endfor
+    endfor
 
     " Remove the dummy sign to close the sign column if we need to.
     if l:is_dummy_sign_set && !g:ale_sign_column_always

test/sign/test_sign_placement.vader

@@ -266,3 +266,14 @@ Execute(It should be possible to clear signs with empty lists):
 
 Execute(No exceptions should be thrown when setting signs for invalid buffers):
   call ale#sign#SetSigns(123456789, [{'lnum': 15, 'col': 2, 'type': 'W', 'text': 'e'}])
+
+Execute(Signs should be removed when lines have multiple sign IDs on them):
+  " We can fail to remove signs if there are multiple signs on one line,
+  " say after deleting lines in Vim, etc.
+  exec 'sign place 1000347 line=3 name=ALEErrorSign buffer=' . bufnr('')
+  exec 'sign place 1000348 line=3 name=ALEWarningSign buffer=' . bufnr('')
+  exec 'sign place 1000349 line=10 name=ALEErrorSign buffer=' . bufnr('')
+  exec 'sign place 1000350 line=10 name=ALEWarningSign buffer=' . bufnr('')
+
+  call ale#sign#SetSigns(bufnr(''), [])
+  AssertEqual [], ParseSigns()