Add script for downloading Tesseract data

Add a Python script that can run in all supported platforms, and can
download and extract the Tesseract language data from GitHub, while
also:

1. Checking that the expected hash matches.
2. Informing the user if the language data have already been downloaded.
3. Extracting only the subset of language data that Dangerzone needs

.gitignore

@@ -22,6 +22,7 @@ var/
 wheels/
 pip-wheel-metadata/
 share/python-wheels/
+share/tessdata/
 *.egg-info/
 .installed.cfg
 *.egg

install/common/download-tessdata.py

@@ -0,0 +1,80 @@
+import hashlib
+import io
+import json
+import pathlib
+import re
+import subprocess
+import sys
+import tarfile
+import urllib.request
+
+TESSDATA_RELEASES_URL = (
+    "https://api.github.com/repos/tesseract-ocr/tessdata_fast/releases/latest"
+)
+TESSDATA_ARCHIVE_URL = "https://github.com/tesseract-ocr/tessdata_fast/archive/{tessdata_version}/tessdata_fast-{tessdata_version}.tar.gz"
+TESSDATA_CHECKSUM = "d0e3bb6f3b4e75748680524a1d116f2bfb145618f8ceed55b279d15098a530f9"
+
+
+def git_root():
+    """Get the root directory of the Git repo."""
+    # FIXME: Use a Git Python binding for this.
+    # FIXME: Make this work if called outside the repo.
+    cmd = ["git", "rev-parse", "--show-toplevel"]
+    path = (
+        subprocess.run(cmd, check=True, stdout=subprocess.PIPE)
+        .stdout.decode()
+        .strip("\n")
+    )
+    return pathlib.Path(path)
+
+
+def main():
+    share_dir = git_root() / "share"
+    tessdata_dir = share_dir / "tessdata"
+
+    # Get the list of OCR languages that Dangerzone supports.
+    with open(share_dir / "ocr-languages.json") as f:
+        langs_short = sorted(json.loads(f.read()).values())
+
+    # Check if these languages have already been downloaded.
+    if tessdata_dir.exists():
+        expected_files = {f"{lang}.traineddata" for lang in langs_short}
+        files = {f.name for f in tessdata_dir.iterdir()}
+        if files == expected_files:
+            msg = "> Skipping tessdata download, language data already exists"
+            print(msg, file=sys.stderr)
+            return
+        else:
+            print(f"Found {tessdata_dir} but contents do not match", file=sys.stderr)
+            return 1
+
+    # Get latest release of Tesseract data.
+    print(f"> Getting latest tessdata release", file=sys.stderr)
+    with urllib.request.urlopen(TESSDATA_RELEASES_URL) as f:
+        resp = f.read()
+        releases = json.loads(resp)
+        tag = releases["tag_name"]
+
+    # Get latest release of Tesseract data.
+    print(f"> Downloading tessdata release {tag}", file=sys.stderr)
+    archive_url = TESSDATA_ARCHIVE_URL.format(tessdata_version=tag)
+    with urllib.request.urlopen(archive_url) as f:
+        archive = f.read()
+        digest = hashlib.sha256(archive).hexdigest()
+        if digest != TESSDATA_CHECKSUM:
+            raise RuntimeError(f"Checksum mismatch {digest} != {TESSDATA_CHECKSUM}")
+
+    # Extract the languages models from the tessdata archive.
+    print(f"> Extracting tessdata archive into {tessdata_dir}", file=sys.stderr)
+    with tarfile.open(fileobj=io.BytesIO(archive)) as t:
+        for lang in langs_short:
+            member = f"tessdata_fast-{tag}/{lang}.traineddata"
+            print(f">> Extracting {member}")
+            t.extract(member=member, path=share_dir, set_attrs=False)
+
+    tessdata_dl_dir = share_dir / f"tessdata_fast-{tag}"
+    tessdata_dl_dir.rename(tessdata_dir)
+
+
+if __name__ == "__main__":
+    sys.exit(main())