doc: extensions: doxyrunner: use file hashes for cache

Use file hashes instead of paths and modification times. This method is
fits better on systems using cache mechanisms. Note that hash is
computed using content obtained in UTF-8 text mode for portability
reasons.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/_extensions/zephyr/doxyrunner.py

@@ -39,6 +39,7 @@ Configuration options
 """
 
 import filecmp
+import hashlib
 from pathlib import Path
 import pickle
 import re
@@ -58,6 +59,21 @@ __version__ = "0.1.0"
 logger = logging.getLogger(__name__)
 
 
+def hash_file(file: Path) -> str:
+    """Compute the hash (SHA256) of a file in text mode.
+
+    Args:
+        file: File to be hashed.
+
+    Returns:
+        Hash.
+    """
+
+    with open(file, encoding="utf-8") as f:
+        sha256 = hashlib.sha256(f.read().encode("utf-8"))
+
+    return sha256.hexdigest()
+
 def get_doxygen_option(doxyfile: str, option: str) -> List[str]:
     """Obtain the value of a Doxygen option.
 
@@ -169,16 +185,16 @@ def doxygen_input_has_changed(doxyfile: str, cache_dir: Path) -> bool:
     if not file_patterns:
         raise ValueError("No FILE_PATTERNS set in Doxyfile")
 
-    # build a dict of input files <-> current modification time
-    files = dict()
+    # build a set with input files hash
+    files = set()
     for file in input_files:
         path = Path(file)
         if path.is_file():
-            files[path.as_posix()] = path.stat().st_mtime_ns
+            files.add(hash_file(path))
         else:
             for pattern in file_patterns:
                 for p_file in path.glob("**/" + pattern):
-                    files[p_file.as_posix()] = p_file.stat().st_mtime_ns
+                    files.add(hash_file(p_file))
 
     # check if any file has changed
     dirty = True