patch 9.1.0681: tests: Analyzing failed screendumps is hard

Problem:  tests: Analyzing failed screendumps is hard
Solution: Facilitate the viewing of rendered screendumps under src/
          add some documentation on how to use the viewdumps.vim
          script (Aliaksei Budavei)

With the submitted "viewdumps.vim" script, a few manual
steps in typical workflows (see below) can be automated.
The updated "README.txt" contains additional information.

============================================================

Reviewing LOCAL failed screendump tests can be arranged as
follows:

1) Run tests and generate screendumps:
------------------------------------------------------------
cd /path/to/fork/src/testdir
make
------------------------------------------------------------

2) Examine the screendumps from the "failed" directory:
------------------------------------------------------------
../vim -u NONE -S viewdumps.vim
------------------------------------------------------------

============================================================

Reviewing UPLOADED failed screendump tests can be arranged
as follows (it can be further locally scripted):

1) Fetch an artifact with failed screendumps from
"github.com/vim/vim/actions/runs/A_ID/artifacts/B_ID".

2) Extract the archived files:
------------------------------------------------------------
unzip /tmp/failed-tests.zip -d /tmp
------------------------------------------------------------

3) Set up the "dumps" directory.  Create a symlink to
"/path/to/fork/dirs/dumps" in the extracted directories so
that term_dumpdiff() can be used.  (The lookup algorithm
resolves "dumps" for every loaded filename.  So, with
"/tmp/src/testdir/failed/*.dump" files passed as script
arguments, the algorithm will make the files in
"/tmp/src/testdir/dumps" queried.)
------------------------------------------------------------
cd /path/to/fork
ln -s $(pwd)/src/testdir/dumps /tmp/src/testdir/dumps
------------------------------------------------------------

4) Examine the extracted screendumps:
------------------------------------------------------------
./src/vim -u NONE -S src/testdir/viewdumps.vim \
  /tmp/src/testdir/failed/*.dump
------------------------------------------------------------

5) Clean up:
------------------------------------------------------------
unlink /tmp/src/testdir/dumps
rm -rf /tmp/src
------------------------------------------------------------

============================================================

Reviewing SUBMITTED FOR PULL REQUEST screendump tests can be
arranged as follows (it can be further locally scripted):

1) List the fetched changeset and write the changed "dumps"
filenames to "/tmp/filelist":
------------------------------------------------------------
cd /path/to/fork
git switch prs/1234
git diff-index --relative=src/testdir/dumps/ \
  --name-only prs/1234~1 > /tmp/filelist
------------------------------------------------------------

2) Reconcile relative filepaths, and copy next-to-be-updated
"dumps" files in the "failed" directory (note the missing
new screendumps, if any):
------------------------------------------------------------
git switch master
cd src/testdir/dumps
test -d ../failed || mkdir ../failed
cp -t ../failed $(cat /tmp/filelist)
------------------------------------------------------------

3) Remember about the introduced INVERTED relation between
"dumps" and "failed", i.e. the files to be committed are in
"dumps" already and their previous versions are in "failed";
therefore, copy the missing new screendumps from "dumps" to
"failed" (otherwise these won't be shown):
------------------------------------------------------------
git switch prs/1234
cp -t ../failed foo_10.dump foo_11.dump foo_12.dump
------------------------------------------------------------

4) Examine the screendumps from the "failed" directory (new
screendumps will be shown with no difference between their
versions):
------------------------------------------------------------
cd ..
../vim -u NONE -S viewdumps.vim
------------------------------------------------------------

closes: #15515

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/viewdumps.vim b/runtime/syntax/testdir/viewdumps.vim
index 0fbfade..e1624ff 100644
--- a/runtime/syntax/testdir/viewdumps.vim
+++ b/runtime/syntax/testdir/viewdumps.vim
@@ -1,44 +1,9 @@
 vim9script
 
-# Support sourcing this script from this directory or any other directory in
-# the direct path that leads to the project's root directory.
-const failed_path: string = finddir('failed', getcwd() .. '/**', -1)
-  ->filter(((cwdpath: string) => (_: number, dirpath: string) =>
-	cwdpath =~ '\<syntax\>' || dirpath =~ '\<syntax\>')(getcwd()))
-  ->get(-1, '') .. '/'
-var error: string = null_string
-
-if failed_path == '/'
-  error = 'No such directory: "failed"'
-else
-  const failed_fnames: string = failed_path .. readdir(failed_path,
-	  (fname: string) => fname =~ '^.\+\.dump$')
-    ->join(' ' .. failed_path)
-
-  if failed_fnames =~ 'failed/$'
-    error = 'No such file: "*.dump"'
-  else
-    exec ':0argedit ' .. failed_fnames
-    buffers
-  endif
-endif
-
-def Render()
-  const failed_fname: string = bufname()
-  try
-    setlocal suffixesadd=.dump
-    const dumps_fname: string = findfile(
-			fnamemodify(failed_fname, ':p:t'),
-			fnamemodify(failed_fname, ':p:h:h') .. '/dumps')
-    if filereadable(dumps_fname)
-      term_dumpdiff(failed_fname, dumps_fname)
-    else
-      term_dumpload(failed_fname)
-    endif
-  finally
-    exec 'bwipeout ' .. failed_fname
-  endtry
-enddef
+exec 'source ' .. (((cwdpath: string) => cwdpath
+  ->strpart(0, cwdpath->strridx('/vim')))(getcwd()))
+  .. '/vim/src/testdir/commondumps.vim'
+g:Init('\<syntax\>', -1)
 
 # THE FOLLOWING SETTINGS PERTAIN TO "input/" FILES THAT ARE LIKELY TO BE
 # LOADED SIDE BY SIDE WHENEVER BATCHES OF NEW SCREENDUMPS ARE GENERATED.
@@ -56,22 +21,4 @@
 # Anticipate non-Latin-1 characters in "input/" files.
 set encoding=utf-8 termencoding=utf-8
 
-autocmd_add([{
-  replace:	true,
-  group:	'viewdumps',
-  event:	'BufRead',
-  pattern:	'*.dump',
-  cmd:		'Render()',
-}])
-
-# Unconditionally help, in case a list of filenames is passed to the command,
-# the first terminal window with its BufRead event.
-silent doautocmd viewdumps BufRead
-
-if error != null_string
-  # Instead of sleeping, fill half a window with blanks and prompt hit-enter.
-  echom error .. repeat("\x20", (winwidth(0) * (winheight(0) / 2) - strlen(error)))
-  error = null_string
-endif
-
 # vim:fdm=syntax:sw=2:ts=8:noet:nolist:nosta: