Add a context wrapper for mtime preservation (#21258)

Sometimes we need to patch a file that is a dependency for some other
automatically generated file that comes in a release tarball. As a
result, make tries to regenerate the dependent file using additional
tools (e.g. help2man), which would not be needed otherwise.

In some cases, it's preferable to avoid that (e.g. see #21255). A way
to do that is to save the modification timestamps before patching and
restoring them afterwards. This PR introduces a context wrapper that
does that.

lib/spack/llnl/util/filesystem.py

@@ -63,7 +63,8 @@
     'touchp',
     'traverse_tree',
     'unset_executable_mode',
-    'working_dir'
+    'working_dir',
+    'keep_modification_time'
 ]
 
 
@@ -1819,3 +1820,24 @@ def remove_directory_contents(dir):
                 os.unlink(entry)
             else:
                 shutil.rmtree(entry)
+
+
+@contextmanager
+def keep_modification_time(*filenames):
+    """
+    Context manager to keep the modification timestamps of the input files.
+    Tolerates and has no effect on non-existent files and files that are
+    deleted by the nested code.
+
+    Parameters:
+        *filenames: one or more files that must have their modification
+            timestamps unchanged
+    """
+    mtimes = {}
+    for f in filenames:
+        if os.path.exists(f):
+            mtimes[f] = os.path.getmtime(f)
+    yield
+    for f, mtime in mtimes.items():
+        if os.path.exists(f):
+            os.utime(f, (os.path.getatime(f), mtime))