Run ctags even if previous run produced erros, closes #125

If a ctags-compatible program returns an error status code on a parsing
failure then it would never be run again on the same file, even if later
invocations would have returned normally (for example after fixing an
error in the current file). Therefore we need to execute the program on
updates as usual, but do not repeat error messages if the previous run
already did.

autoload/tagbar.vim

@@ -1225,6 +1225,7 @@ function! s:BaseTag.initFoldState() abort dict
     let fileinfo = self.fileinfo
 
     if s:known_files.has(fileinfo.fpath) &&
+     \ has_key(fileinfo, '_tagfolds_old') &&
      \ has_key(fileinfo._tagfolds_old[self.fields.kind], self.fullpath)
         " The file has been updated and the tag was there before, so copy its
         " old fold state
@@ -1877,13 +1878,16 @@ function! s:ProcessFile(fname, ftype) abort
 
     " If the file has only been updated preserve the fold states, otherwise
     " create a new entry
-    if s:known_files.has(a:fname)
+    if s:known_files.has(a:fname) && !empty(s:known_files.get(a:fname))
         let fileinfo = s:known_files.get(a:fname)
         call fileinfo.reset()
     else
         let fileinfo = s:FileInfo.New(a:fname, a:ftype)
     endif
 
+    " Use a temporary files for ctags processing instead of the original one.
+    " This allows using Tagbar for files accessed with netrw, and also doesn't
+    " slow down Tagbar for files that sit on slow network drives.
     let tempfile = tempname()
     let ext = fnamemodify(fileinfo.fpath, ':e')
     if ext != ''
@@ -1893,7 +1897,7 @@ function! s:ProcessFile(fname, ftype) abort
     call writefile(getbufline(fileinfo.bufnr, 1, '$'), tempfile)
     let fileinfo.mtime = getftime(tempfile)
 
-    let ctags_output = s:ExecuteCtagsOnFile(tempfile, a:ftype)
+    let ctags_output = s:ExecuteCtagsOnFile(tempfile, a:fname, a:ftype)
 
     call delete(tempfile)
 
@@ -1992,7 +1996,7 @@ function! s:ProcessFile(fname, ftype) abort
 endfunction
 
 " s:ExecuteCtagsOnFile() {{{2
-function! s:ExecuteCtagsOnFile(fname, ftype) abort
+function! s:ExecuteCtagsOnFile(fname, realfname, ftype) abort
     call s:LogDebugMessage('ExecuteCtagsOnFile called [' . a:fname . ']')
 
     let typeinfo = s:known_types[a:ftype]
@@ -2041,15 +2045,18 @@ function! s:ExecuteCtagsOnFile(fname, ftype) abort
     let ctags_output = s:ExecuteCtags(ctags_cmd)
 
     if v:shell_error || ctags_output =~ 'Warning: cannot open source file'
-        echoerr 'Tagbar: Could not execute ctags for ' . a:fname . '!'
-        echomsg 'Executed command: "' . ctags_cmd . '"'
-        if !empty(ctags_output)
-            call s:LogDebugMessage('Command output:')
-            call s:LogDebugMessage(ctags_output)
-            echomsg 'Command output:'
-            for line in split(ctags_output, '\n')
-                echomsg line
-            endfor
+        if !s:known_files.has(a:realfname) ||
+         \ !empty(s:known_files.get(a:realfname))
+            echoerr 'Tagbar: Could not execute ctags for ' . a:fname . '!'
+            echomsg 'Executed command: "' . ctags_cmd . '"'
+            if !empty(ctags_output)
+                call s:LogDebugMessage('Command output:')
+                call s:LogDebugMessage(ctags_output)
+                echomsg 'Command output:'
+                for line in split(ctags_output, '\n')
+                    echomsg line
+                endfor
+            endif
         endif
         return -1
     endif
@@ -3076,18 +3083,16 @@ function! s:AutoUpdate(fname, force) abort
     let updated = 0
 
     " Process the file if it's unknown or the information is outdated.
-    " Also test for entries that exist but are empty, which will be the case
-    " if there was an error during the ctags execution.
     " Testing the mtime of the file is necessary in case it got changed
     " outside of Vim, for example by checking out a different version from a
     " VCS.
-    if s:known_files.has(a:fname) && !empty(s:known_files.get(a:fname))
+    if s:known_files.has(a:fname)
         let curfile = s:known_files.get(a:fname)
         " if a:force || getbufvar(curfile.bufnr, '&modified') ||
-        if a:force ||
+        if a:force || empty(curfile) ||
          \ (filereadable(a:fname) && getftime(a:fname) > curfile.mtime)
             call s:LogDebugMessage('File data outdated, updating' .
-                                 \ ' [' .  a:fname . ']')
+                                 \ ' [' . a:fname . ']')
             call s:ProcessFile(a:fname, sftype)
             let updated = 1
         else