Fix #1716 - Replace tempdir() with a wrapper to preserve TMPDIR

autoload/ale/command.vim

@@ -13,7 +13,7 @@ function! s:TemporaryFilename(buffer) abort
 
     " Create a temporary filename, <temp_dir>/<original_basename>
     " The file itself will not be created by this function.
-    return tempname() . (has('win32') ? '\' : '/') . l:filename
+    return ale#util#Tempname() . (has('win32') ? '\' : '/') . l:filename
 endfunction
 
 " Given a command string, replace every...

autoload/ale/engine.vim

@@ -98,7 +98,7 @@ endfunction
 
 " Create a new temporary directory and manage it in one go.
 function! ale#engine#CreateDirectory(buffer) abort
-    let l:temporary_directory = tempname()
+    let l:temporary_directory = ale#util#Tempname()
     " Create the temporary directory for the file, unreadable by 'other'
     " users.
     call mkdir(l:temporary_directory, '', 0750)

autoload/ale/handlers/haskell.vim

@@ -2,7 +2,7 @@
 " Description: Error handling for the format GHC outputs.
 
 " Remember the directory used for temporary files for Vim.
-let s:temp_dir = fnamemodify(tempname(), ':h')
+let s:temp_dir = fnamemodify(ale#util#Tempname(), ':h')
 " Build part of a regular expression for matching ALE temporary filenames.
 let s:temp_regex_prefix =
 \   '\M'

autoload/ale/path.vim

@@ -84,7 +84,7 @@ function! ale#path#IsAbsolute(filename) abort
     return a:filename[:0] is# '/' || a:filename[1:2] is# ':\'
 endfunction
 
-let s:temp_dir = ale#path#Simplify(fnamemodify(tempname(), ':h'))
+let s:temp_dir = ale#path#Simplify(fnamemodify(ale#util#Tempname(), ':h'))
 
 " Given a filename, return 1 if the file represents some temporary file
 " created by Vim.

autoload/ale/util.vim

@@ -277,6 +277,25 @@ function! ale#util#InSandbox() abort
     return 0
 endfunction
 
+function! ale#util#Tempname() abort
+    let l:clear_tempdir = 0
+
+    if has('unix') && empty($TMPDIR)
+        let l:clear_tempdir = 1
+        let $TMPDIR = '/tmp'
+    endif
+
+    try
+        let l:name = tempname() " no-custom-checks
+    finally
+        if l:clear_tempdir
+            let $TMPDIR = ''
+        endif
+    endtry
+
+    return l:name
+endfunction
+
 " Given a single line, or a List of lines, and a single pattern, or a List
 " of patterns, return all of the matches for the lines(s) from the given
 " patterns, using matchlist().