CI: Fix unneeded font rebuild commits

[why]
Despite commit
  cc2b54770 generate-original-source: Remove FFTM table
we still get unneeded font rebuilds.

The reason is that the font creation time is not only encoded in FFTM but
also in HEAD.

[how]
We could simply diable timestamps also in HEAD, but that would leave us
with a strange font; strange because no one knows when it has been
created.

Instead we take the more laberous route here: Do detect changes and not
rely on the git history:
* Find out current font's creation date
* Create the font anew with that date as creation date
* If the 'real' font content is unchanged we would now have a 100%
  identical new font file; we can detect that with `git diff`
* If it is not identical, something apart from the timestamp has
  changed and we create the font again, this time with the real current
  time as timestamp and commit that file back to the repo

This only works if creation and modification time are always the same on
all font creations; we need to ensure this by always using the
SOURCE_DATE_EPOCH method, even for 'normal' font creation.

This is a bit more involved than what I would have hoped for, but there
seems to be no easy solution.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

.github/workflows/packsvgs.yml

@@ -5,6 +5,7 @@ on:
     paths:
       - 'src/svgs/*'
       - 'bin/scripts/generate-original-source.py'
+      - 'bin/scripts/optimize-original-source.sh'
   workflow_dispatch:
 
 jobs:
@@ -15,7 +16,9 @@ jobs:
       - name: Fetch dependencies
         run: |
           sudo apt update -y -q
-          sudo apt install python3-fontforge inkscape dc -y -q
+          # ttfdump comes with texlive-binaries
+          # We use ttfdump instead of showttf, because the later core dumps at the moment
+          sudo apt install python3-fontforge inkscape texlive-binaries dc -y -q
 
       - name: Simplify the SVGs
         run: |
@@ -31,14 +34,41 @@ jobs:
           committer_name: GitHub Actions
           committer_email: 41898282+github-actions[bot]@users.noreply.github.com
 
-      - name: Create Seti and original symbols font
+      - name: Dummy create Seti-and-original-symbols font and CHECK FOR CHANGES
+        id: compare_font
         run: |
           cd bin/scripts
-          ls -l ../../src/glyphs/original-source.otf
+          echo "OLD_FONT_LS=$(ls -l ../../src/glyphs/original-source.otf)" >> $GITHUB_ENV
+          # First we create a 'test' font with the same timestamp as the old to see if
+          # there are any differences (apart from the timestamp)
+          export CURRENT_FONT_DATE=$(ttfdump -t head ../../src/glyphs/original-source.otf | \
+            grep -E '[^a-z]created:.*0x' | sed 's/.*x//' | tr 'a-f' 'A-F')
+          echo "Font creation date (1904): 0x${CURRENT_FONT_DATE}"
+          # The timestamp in the HEAD table is given in 1904 seconds, we convert that
+          # to 1970 (unix) seconds.
+          export SOURCE_DATE_EPOCH=$(dc -e "16i ${CURRENT_FONT_DATE} Ai 86400 24107 * - p")
+          echo "Font creation date (1970): ${SOURCE_DATE_EPOCH}"
+          echo "Create font with previous timestamp"
           ./generate-original-source.py
+          # Check if the new font file has changed
+          CHANGE=$(git diff --quiet ../../src/glyphs/original-source.otf && echo false || echo true)
+          echo "Change detected: ${CHANGE}"
+          echo "::set-output name=changed::${CHANGE}"
+
+      - name: Create Seti-and-original-symbols font for real with current timestamp
+        if: steps.compare_font.outputs.changed == 'true'
+        run: |
+          cd bin/scripts
+          echo "Create font with current timestamp"
+          # We need to force creation and modification timestamp to be identical
+          # to make the compare_font step work next time
+          export SOURCE_DATE_EPOCH=$(date +%s)
+          ./generate-original-source.py
+          echo "${OLD_FONT_LS}"
           ls -l ../../src/glyphs/original-source.otf
 
-      - name: Commit patched fonts back to repo
+      - name: Commit created font back to repo
+        if: steps.compare_font.outputs.changed == 'true'
         uses: EndBug/add-and-commit@v9
         with:
           fetch: false