Add tools to localetools to facilitate source file recreation

For now unused; later commits shall put them to use.
Transcriber -- base, takes care of tempfile and renaming.
SourceFileEditor -- handles copying parts before and after a common delimiter.

Task-number: QTBUG-81344
Change-Id: I28cf977d0a08825fbb873fb330da6823b88ad3ed
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>

util/locale_database/localetools.py

@@ -26,14 +26,20 @@
 ##
 #############################################################################
 """Utilities shared among the CLDR extraction tools.
+
 Functions:
   unicode2hex() -- converts unicode text to UCS-2 in hex form.
   wrap_list() -- map list to comma-separated string, 20 entries per line.
 
 Classes:
   Error -- A shared error class.
+  Transcriber -- edit a file by writing a temporary file, then renaming.
+  SourceFileEditor -- adds standard prelude and tail handling to Transcriber.
 """
 
+import os
+import tempfile
+
 class Error (StandardError):
     __upinit = StandardError.__init__
     def __init__(self, msg, *args):
@@ -63,3 +69,96 @@ def wrap_list(lst):
             head, lst = lst[:size], lst[size:]
             yield head
     return ",\n".join(", ".join(x) for x in split(lst, 20))
+
+class Transcriber (object):
+    """Helper class to facilitate rewriting source files.
+
+    This class takes care of the temporary file manipulation. Derived
+    classes need to implement transcribing of the content, with
+    whatever modifications they may want.  Members reader and writer
+    are exposed; use writer.write() to output to the new file; use
+    reader.readline() or iterate reader to read the original.
+
+    Callers should call close() on success or cleanup() on failure (to
+    clear away the temporary file).
+    """
+    def __init__(self, path, temp):
+        # Open the old file
+        self.reader = open(path)
+        # Create a temp file to write the new data into
+        temp, tempPath = tempfile.mkstemp(os.path.split(path)[1], dir = temp)
+        self.__names = path, tempPath
+        self.writer = os.fdopen(temp, "w")
+
+    def close(self):
+        self.reader.close()
+        self.writer.close()
+        self.reader = self.writer = None
+        source, temp = self.__names
+        os.remove(source)
+        os.rename(temp, source)
+
+    def cleanup(self):
+        if self.__names:
+            self.reader.close()
+            self.writer.close()
+            # Remove temp-file:
+            os.remove(self.__names[1])
+            self.__names = ()
+
+class SourceFileEditor (Transcriber):
+    """Transcriber with transcription of code around a gnerated block.
+
+    We have a common pattern of source files with a generated part
+    embedded in a context that's not touched by the regeneration
+    scripts. The generated part is, in each case, marked with a common
+    pair of start and end markers. We transcribe the old file to a new
+    temporary file; on success, we then remove the original and move
+    the new version to replace it.
+
+    This class takes care of transcribing the parts before and after
+    the generated content; on creation, an instance will copy the
+    preamble up to the start marker; its close() will skip over the
+    original's generated content and resume transcribing with the end
+    marker. Derived classes need only implement the generation of the
+    content in between.
+
+    Callers should call close() on success or cleanup() on failure (to
+    clear away the temporary file); see Transcriber.
+    """
+    __upinit = Transcriber.__init__
+    def __init__(self, path, temp):
+        """Set up the source file editor.
+
+        Requires two arguments: the path to the source file to be read
+        and, on success, replaced with a new version; and the
+        directory in which to store the temporary file during the
+        rewrite."""
+        self.__upinit(path, temp)
+        self.__copyPrelude()
+
+    __upclose = Transcriber.close
+    def close(self):
+        self.__copyTail()
+        self.__upclose()
+
+    # Implementation details:
+    GENERATED_BLOCK_START = '// GENERATED PART STARTS HERE'
+    GENERATED_BLOCK_END = '// GENERATED PART ENDS HERE'
+
+    def __copyPrelude(self):
+        # Copy over the first non-generated section to the new file
+        for line in self.reader:
+            self.writer.write(line)
+            if line.strip() == self.GENERATED_BLOCK_START:
+                break
+
+    def __copyTail(self):
+        # Skip through the old generated data in the old file
+        for line in self.reader:
+            if line.strip() == self.GENERATED_BLOCK_END:
+                self.writer.write(line)
+                break
+        # Transcribe the remainder:
+        for line in self.reader:
+            self.writer.write(line)