Update setup_cosmos_modules.example

* Only update cosmos-modules.conf if a change is needed
* Do an atomic replace of the file if updating it
* Add a note at the top mentioning that it is tool-generated
* Make pylint and black happy

Tested in ubuntu 18.04.

docs/setup_cosmos_modules.example

@@ -1,14 +1,81 @@
 #!/usr/bin/env python3
+""" Write out a puppet cosmos-modules.conf """
+
+import hashlib
+import os
+import os.path
+import sys
 
 try:
     from configobj import ConfigObj
 
-    os_info = ConfigObj("/etc/os-release")
+    OS_INFO = ConfigObj("/etc/os-release")
 except (IOError, ModuleNotFoundError):
-    os_info = None
+    OS_INFO = None
 
 
+def get_file_hash(modulesfile):
+    """
+    Based on https://github.com/python/cpython/pull/31930: should use
+    hashlib.file_digest() but it is only available in python 3.11
+    """
+    try:
+        with open(modulesfile, "rb") as fileobj:
+            digestobj = hashlib.sha256()
+            _bufsize = 2**18
+            buf = bytearray(_bufsize)  # Reusable buffer to reduce allocations.
+            view = memoryview(buf)
+            while True:
+                size = fileobj.readinto(buf)
+                if size == 0:
+                    break  # EOF
+                digestobj.update(view[:size])
+    except FileNotFoundError:
+        return ""
+
+    return digestobj.hexdigest()
+
+
+def get_list_hash(file_lines):
+    """Get hash of list contents"""
+
+    file_lines_hash = hashlib.sha256()
+    for line in file_lines:
+        file_lines_hash.update(line)
+
+    return file_lines_hash.hexdigest()
+
+
+def create_file_content(modules):
+    """
+    Write out the expected file contents to a list so we can check the
+    expected checksum before writing anything
+    """
+    file_lines = []
+    file_lines.append(
+        "# Generated by {}\n".format(  # pylint: disable=consider-using-f-string
+            os.path.basename(sys.argv[0])
+        ).encode("utf-8")
+    )
+    for key in modules:
+        file_lines.append(
+            "{0:11} {1} {2} {3}\n".format(  # pylint: disable=consider-using-f-string
+                key,
+                modules[key]["repo"],
+                modules[key]["upgrade"],
+                modules[key]["tag"],
+            ).encode("utf-8")
+        )
+
+    return file_lines
+
+
+def main():
+    """Starting point of the program"""
+
     modulesfile: str = "/etc/puppet/cosmos-modules.conf"
+    modulesfile_tmp: str = modulesfile + ".tmp"
+
     modules: dict = {
         "concat": {
             "repo": "https://github.com/SUNET/puppetlabs-concat.git",
@@ -118,17 +185,32 @@ modules: dict = {
     }
 
     # When/if we want we can do stuff to modules here
-if os_info:
+    if OS_INFO:
-    if os_info["VERSION_CODENAME"] == "bullseye":
+        if OS_INFO["VERSION_CODENAME"] == "bullseye":
             pass
 
-with open(modulesfile, "w") as fh:
+    # Build list of expected file content
-    for key in modules:
+    file_lines = create_file_content(modules)
-        fh.write(
+
-            "{0:11} {1} {2} {3}\n".format(
+    # Get hash of the list
-                key,
+    list_hash = get_list_hash(file_lines)
-                modules[key]["repo"],
+
-                modules[key]["upgrade"],
+    # Get hash of the existing file on disk
-                modules[key]["tag"],
+    file_hash = get_file_hash(modulesfile)
-            )
+
-        )
+    # Update the file if necessary
+    if list_hash != file_hash:
+        # Since we are reading the file with 'rb' when computing our hash use 'wb' when
+        # writing so we dont end up creating a file that does not match the
+        # expected hash
+        with open(modulesfile_tmp, "wb") as fileobj:
+            for line in file_lines:
+                fileobj.write(line)
+
+        # Rename it in place so the update is atomic for anything else trying to
+        # read the file
+        os.rename(modulesfile_tmp, modulesfile)
+
+
+if __name__ == "__main__":
+    main()