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[PATCH v1.1] Script to generate ChangeLog-like output


The utility of a ChangeLog file has been discussed in various mailing
list threads and GNU Tools Cauldrons in the past years and the general
consensus is that while the file may have been very useful in the past
when revision control did not exist or was not as powerful as it is
today, it's current utility is fast diminishing.  Further, the
ChangeLog format gets in the way of modernisation of processes since
it almost always results in rewriting of a commit, thus preventing use
of any code review tools to automatically manage patches in the glibc
project.

There is consensus in the glibc community that documentation of why a
change was done (i.e. a detailed description in a git commit) is more
useful than what changed (i.e. a ChangeLog entry) since the latter can
be deduced from the patch.  The GNU community would however like to
keep the option of ascertaining what changed through a ChangeLog-like
output and as a compromise, it was proposed that a script be developed
that generates this output.

The script below is the result of these discussions.  This script
takes two git revisions references as input and generates the git log
between those revisions in a form that resembles a ChangeLog.  Its
capabilities and limitations are listed in a comment in the script.
On a high level it is capable of parsing C code and telling what
changed at the top level, but not within constructs such as functions.

For input other than C, the script only identifies if a file has been
added, removed, modified, permissions changed, etc. but cannot
understand the change in content.  The design of the script however is
pluggable, so it should be possible to develop additional parsers to
process other types of files.

I have tested it with a number of commits in the glibc log and also
fixed a couple of errors that were reported earlier.

Transition:

Once this script is in place, it should be possible for us to stop
maintaining the ChangeLog file and rely on the ChangeLog script to
give an output that serves a similar purpose.  Given that the majority
of our code is in C, we have adequate coverage with just the C parser.
In any case the readability of ChangeLog entries for other formats
(makefiles for example) is just too convoluted and is perhaps not even
worth the effort.

I propose that we stop ChangeLog file maintenance once 2.30 opens for
development in February and focus on making the ChangeLog script more
accurate if we encounter bugs.  I will also take another swipe at
patchwork to try and automate things in it now that the ChangeLog is
gone.

If there is agreement, then as part of 2.29 release management I will
update the wiki to reflect the change in our patch submission process
and also mention the Changelog script there for those who need it.  I
believe Joseph is working with RMS to change the wording in the GNU
Coding Standards to make ChangeLog management optional.

Looking forward, once 2.30 is released in August we will be in a good
position to decide if patchwork is useful or if we want to consider
other alternative patch review processes and tools.

ChangeLog:

	* scripts/gen-changed-entities.py: New script.
---

Changes from V1:

- Added support for multiple authors using the co-authored-by tag
- Added support for annotating trivial changes using the
  copyright-paperwork-exempt tag.

 scripts/gen-changed-entities.py | 817 ++++++++++++++++++++++++++++++++
 1 file changed, 817 insertions(+)
 create mode 100755 scripts/gen-changed-entities.py

diff --git a/scripts/gen-changed-entities.py b/scripts/gen-changed-entities.py
new file mode 100755
index 0000000000..59ebd47c69
--- /dev/null
+++ b/scripts/gen-changed-entities.py
@@ -0,0 +1,817 @@
+#!/usr/bin/python3
+# Copyright (C) 2018 Free Software Foundation, Inc.
+# This file is part of the GNU C Library.
+#
+# The GNU C Library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# The GNU C Library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with the GNU C Library; if not, see
+# <http://www.gnu.org/licenses/>.
+''' Generate a ChangeLog style output based on the git log.
+
+This script takes two revisions as input and generates a ChangeLog style output
+for all revisions between the two revisions.  This output is intended to be an
+approximation and not the exact ChangeLog.
+
+At a high level, the script enumerates all C source files (*.c and *.h) and
+builds a tree of top level objects within macro conditionals.  The top level
+objects the script currently attempts to identify are:
+
+    - Include statements
+    - Macro definitions and undefs
+    - Declarations and definitions of variables and functions
+    - Composite types
+
+The script attempts to identify quirks typically used in glibc sources such as
+the symbol hack macro calls that don't use a semicolon and tries to adjust for
+them.
+
+Known Limitations:
+
+    - Does not identify changes in or to comments.  Comments are simply stripped
+      out.
+    - Weird nesting of macro conditionals may break things.  Attempts have been
+      made to try and maintain state across macro conditional scopes, but
+      there's still scope to fool the script.
+    - Does not identify changes within functions.
+'''
+import subprocess
+import sys
+import os
+import re
+from enum import Enum
+
+# General Utility functions.
+def eprint(*args, **kwargs):
+    ''' Print to stderr.
+    '''
+    print(*args, file=sys.stderr, **kwargs)
+
+
+debug = False
+def debug_print(*args, **kwargs):
+    ''' Convenience function to print diagnostic information in the program.
+    '''
+    if debug:
+        eprint(*args, **kwargs)
+
+
+def usage(name):
+    ''' Print program usage.
+    '''
+    eprint("usage: %s <from-ref> <to-ref>" % name)
+    sys.exit(os.EX_USAGE)
+
+
+def decode(string):
+    ''' Attempt to decode a string.
+
+    Decode a string read from the source file.  The multiple attempts are needed
+    due to the presence of the page break characters and some tests in locales.
+    '''
+    codecs = ['utf8', 'latin1', 'cp1252']
+
+    for i in codecs:
+        try:
+            return string.decode(i)
+        except UnicodeDecodeError:
+            pass
+
+    eprint('Failed to decode: %s' % string)
+
+
+#------------------------------------------------------------------------------
+# C Parser.
+#------------------------------------------------------------------------------
+
+
+def new_block(name, type, contents, parent):
+    '''  Create a new code block with the parent as PARENT.
+
+    The code block is a basic structure around which the tree representation of
+    the source code is built.  It has the following attributes:
+
+    - type: Any one of the following types in BLOCK_TYPE.
+    - name: A name to refer it by in the ChangeLog
+    - contents: The contents of the block.  For a block of types file or
+      macro_cond, this would be a list of blocks that it nests.  For other types
+      it is a list with a single string specifying its contents.
+    - parent: This is the parent of the current block, useful in setting up
+      #elif or #else blocks in the tree.
+    - matched: A flag to specify if the block in a tree has found a match in the
+      other tree to which it is being compared.
+    '''
+    block = {}
+    block['matched'] = False
+    block['name'] = name
+    block['type'] = type
+    block['contents'] = contents
+    block['parent'] = parent
+    if parent:
+        parent['contents'].append(block)
+
+    return block
+
+
+class block_type(Enum):
+    ''' Type of code block.
+    '''
+    file = 1
+    macro_cond = 2
+    macro_def = 3
+    macro_undef = 4
+    macro_include = 5
+    macro_info = 6
+    decl = 7
+    func = 8
+    composite = 9
+    macrocall = 10
+    fndecl = 11
+    assign = 12
+
+
+# A dictionary describing what each action (add, modify, delete) show up as in
+# the ChangeLog output.
+actions = {0:{'new': 'New', 'mod': 'Modified', 'del': 'Remove'},
+           block_type.file:{'new': 'New file', 'mod': 'Modified file',
+                            'del': 'Remove file'},
+           block_type.macro_cond:{'new': 'New', 'mod': 'Modified',
+                                  'del': 'Remove'},
+           block_type.macro_def:{'new': 'New macro', 'mod': 'Modified macro',
+                                 'del': 'Remove macro'},
+           block_type.macro_undef:{'new': 'Undefine', 'mod': 'Modified',
+                                   'del': 'Remove'},
+           block_type.macro_include:{'new': 'Include file', 'mod': 'Modified',
+                                     'del': 'Remove include'},
+           block_type.macro_info:{'new': 'New preprocessor message',
+                                  'mod': 'Modified', 'del': 'Remove'},
+           block_type.decl:{'new': 'New', 'mod': 'Modified', 'del': 'Remove'},
+           block_type.func:{'new': 'New function', 'mod': 'Modified function',
+                 'del': 'Remove function'},
+           block_type.composite:{'new': 'New', 'mod': 'Modified',
+                                 'del': 'Remove'},
+           block_type.macrocall:{'new': 'New', 'mod': 'Modified',
+                                 'del': 'Remove'},
+           block_type.fndecl:{'new': 'New function', 'mod': 'Modified',
+                              'del': 'Remove'},
+           block_type.assign:{'new': 'New', 'mod': 'Modified', 'del': 'Remove'}}
+
+# Regular expressions.
+
+# The __attribute__ are written in a bunch of different ways in glibc.
+ATTRIBUTE = \
+        r'((_*(attribute|ATTRIBUTE)_*(\s*\(\([^)]+\)\)|\w+))|weak_function)';
+
+# Function regex
+FUNC_RE = re.compile(ATTRIBUTE + r'*\s*(\w+)\s*\([^(][^{]+\)\s*{')
+
+# The macrocall_re peeks into the next line to ensure that it doesn't eat up
+# a FUNC by accident.  The func_re regex is also quite crude and only
+# intends to ensure that the function name gets picked up correctly.
+MACROCALL_RE = re.compile(r'(\w+)\s*\((?!void).(\s*,\s*[\w\.]+)*\)$')
+
+# Composite types such as structs and unions.
+COMPOSITE_RE = re.compile(r'(struct|union|enum)\s*(\w*)\s*{')
+
+# Static assignments.
+ASSIGN_RE = re.compile(r'(\w+)\s*(\[[^\]]*\])*\s*([^\s]*attribute[\s\w()]+)?\s*=')
+
+# Function Declarations.
+FNDECL_RE = re.compile(r'(\w+)\s*\([^;]+\)\s*' + ATTRIBUTE + '*;')
+
+# Function pointer typedefs.
+TYPEDEF_FN_RE = re.compile(r'\(\*(\w+)\)\s*\([^)]+\);')
+
+# Simple decls.
+DECL_RE = re.compile(r'(\w+)(\[\w+\])?\s*' + ATTRIBUTE + '?;')
+
+
+def remove_comments(op):
+    ''' Remove comments.
+
+    Return OP by removing all comments from it.
+    '''
+    debug_print('REMOVE COMMENTS')
+
+    sep='\n'
+    opstr = sep.join(op)
+    opstr = re.sub(r'/\*.*?\*/', r'', opstr, flags=re.MULTILINE | re.DOTALL)
+    opstr = re.sub(r'\\\n', r' ', opstr, flags=re.MULTILINE | re.DOTALL)
+    new_op = list(filter(None, opstr.split(sep)))
+
+    return new_op
+
+
+# Parse macros.
+def parse_preprocessor(op, loc, code, start = '', else_start = ''):
+    ''' Parse a preprocessor directive.
+
+    In case a preprocessor condition (i.e. if/elif/else), create a new code
+    block to nest code into and in other cases, identify and add entities suchas
+    include files, defines, etc.
+
+    - NAME is the name of the directive
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+    - CODE is the block to which we add this function
+
+    - Returns: The next location to be read in the array.
+    '''
+    cur = op[loc]
+    loc = loc + 1
+    endblock = False
+
+    debug_print('PARSE_MACRO: %s' % cur)
+
+    # Remove the # and strip spaces again.
+    cur = cur[1:]
+
+    # Include file.
+    if cur.find('include') == 0:
+        m = re.search(r'include\s*["<]?([^">]+)[">]?', cur)
+        new_block(m.group(1), block_type.macro_include, [cur], code)
+
+    # Macro definition.
+    if cur.find('define') == 0:
+        m = re.search(r'define\s+([a-zA-Z0-9_]+)', cur)
+        new_block(m.group(1), block_type.macro_def, [cur], code)
+
+    # Macro undef.
+    if cur.find('undef') == 0:
+        m = re.search(r'undef\s+([a-zA-Z0-9_]+)', cur)
+        new_block(m.group(1), block_type.macro_undef, [cur], code)
+
+    # #error and #warning macros.
+    if cur.find('error') == 0 or cur.find('warning') == 0:
+        m = re.search(r'(error|warning)\s+"?(.*)"?', cur)
+        if m:
+            name = m.group(2)
+        else:
+            name = '<blank>'
+        new_block(name, block_type.macro_info, [cur], code)
+
+    # Start of an #if or #ifdef block.
+    elif cur.find('if') == 0:
+        rem = re.sub(r'ifndef', r'!', cur).strip()
+        rem = re.sub(r'(ifdef|defined|if)', r'', rem).strip()
+        ifdef = new_block(rem, block_type.macro_cond, [], code)
+        loc = c_parse(op, loc, ifdef, start)
+
+    # End the previous #if/#elif and begin a new block.
+    elif cur.find('elif') == 0 and code['parent']:
+        rem = re.sub(r'(elif|defined)', r'', cur).strip()
+        # The #else and #elif blocks should go into the current block's parent.
+        ifdef = new_block(rem, block_type.macro_cond, [], code['parent'])
+        loc = c_parse(op, loc, ifdef, else_start)
+        endblock = True
+
+    # End the previous #if/#elif and begin a new block.
+    elif cur.find('else') == 0 and code['parent']:
+        name = '!(' + code['name'] + ')'
+        ifdef = new_block(name, block_type.macro_cond, [], code['parent'])
+        loc = c_parse(op, loc, ifdef, else_start)
+        endblock = True
+
+    elif cur.find('endif') == 0 and code['parent']:
+        endblock = True
+
+    return (loc, endblock)
+
+
+# Given the start of a scope CUR, lap up all code up to the end of scope
+# indicated by the closing brace.
+def fast_forward_scope(cur, op, loc):
+    ''' Consume lines in a code block.
+
+    Consume all lines of a block of code such as a composite type declaration or
+    a function declaration.
+
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+
+    - Returns: The next location to be read in the array as well as the updated
+      value of CUR, which will now have the body of the function or composite
+      type.
+    '''
+    nesting = cur.count('{') - cur.count('}')
+    while nesting > 0 and loc < len(op):
+        cur = cur + ' ' + op[loc]
+
+        nesting = nesting + op[loc].count('{')
+        nesting = nesting - op[loc].count('}')
+        loc = loc + 1
+
+    return (cur, loc)
+
+
+# Different types of declarations.
+def parse_decl(name, cur, op, loc, code, blocktype):
+    ''' Parse a top level declaration.
+
+    All types of declarations except function declarations.
+
+    - NAME is the name of the declarated entity
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+    - CODE is the block to which we add this function
+
+    - Returns: The next location to be read in the array.
+    '''
+    debug_print('FOUND DECL: %s' % name)
+    new_block(name, blocktype, [cur], code)
+
+    return loc
+
+
+# Assignments.
+def parse_assign(name, cur, op, loc, code, blocktype):
+    ''' Parse an assignment statement.
+
+    This includes array assignments.
+
+    - NAME is the name of the assigned entity
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+    - CODE is the block to which we add this
+
+    - Returns: The next location to be read in the array.
+    '''
+    debug_print('FOUND ASSIGN: %s' % name)
+    # Lap up everything up to semicolon.
+    while ';' not in cur and loc < len(op):
+        cur = op[loc]
+        loc = loc + 1
+
+    new_block(name, blocktype, [cur], code)
+
+    return loc
+
+
+def parse_composite(name, cur, op, loc, code, blocktype):
+    ''' Parse a composite type.
+
+    Match declaration of a composite type such as a sruct or a union..
+
+    - NAME is the name of the composite type
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+    - CODE is the block to which we add this
+
+    - Returns: The next location to be read in the array.
+    '''
+    if not name:
+        name = '<anonymous>'
+
+    # Lap up all of the struct definition.
+    (cur, loc) = fast_forward_scope(cur, op, loc)
+
+    new_block(name, blocktype, [cur], code)
+
+    return loc
+
+
+def parse_func(name, cur, op, loc, code, blocktype):
+    ''' Parse a function.
+
+    Match a function definition.
+
+    - NAME is the name of the function
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - LOC is the first unread location in CUR
+    - CODE is the block to which we add this
+
+    - Returns: The next location to be read in the array.
+    '''
+    debug_print('FOUND FUNC: %s' % name)
+
+    # Consume everything up to the ending brace of the function.
+    (cur, loc) = fast_forward_scope(cur, op, loc)
+
+    new_block(name, blocktype, [cur], code)
+
+    return loc
+
+
+def parse_macrocall(name, cur, op, loc, code, blocktype):
+    ''' Parse a macro call.
+
+    Match a symbol hack macro calls that get added without semicolons.
+
+    - NAME is the name of the macro call
+    - CUR is the string array for the file
+    - CUR is the string to consume this expression from
+    - OP is the string array for the file
+    - CODE is the block to which we add this
+
+    - Returns: The next location to be read in the array.
+    '''
+    debug_print('FOUND MACROCALL: %s' % name)
+
+    new_block(name, blocktype, [cur], code)
+
+    return loc
+
+
+# Regular expressions and token consumption functions for expressions we
+# encounter in C.
+c_expr_parsers = [
+        {'regex' : COMPOSITE_RE, 'func' : parse_composite, 'name' : 2,
+            'type' : block_type.composite},
+        {'regex' : ASSIGN_RE, 'func' : parse_assign, 'name' : 1,
+            'type' : block_type.assign},
+        {'regex' : TYPEDEF_FN_RE, 'func' : parse_decl, 'name' : 1,
+            'type' : block_type.decl},
+        {'regex' : FNDECL_RE, 'func' : parse_decl, 'name' : 1,
+            'type' : block_type.fndecl},
+        {'regex' : FUNC_RE, 'func' : parse_func, 'name' : 5,
+            'type' : block_type.func},
+        {'regex' : MACROCALL_RE, 'func' : parse_macrocall, 'name' : 1,
+            'type' : block_type.macrocall},
+        {'regex' : DECL_RE, 'func' : parse_decl, 'name' : 1,
+            'type' : block_type.decl}]
+
+
+def parse_c_expr(cur, op, loc, code):
+    ''' Parse a C expression.
+
+    CUR is the string to be parsed, which continues to grow until a match is
+    found.  OP is the string array and LOC is the first unread location in the
+    string array.  CODE is the block in which any identified expressions should
+    be added.
+    '''
+    debug_print('PARSING: %s' % cur)
+
+    for p in c_expr_parsers:
+        found = re.search(p['regex'], cur)
+        if found:
+            return '', p['func'](found.group(p['name']), cur, op, loc, code,
+                                    p['type'])
+
+    return cur, loc
+
+
+def c_parse(op, loc, code, start = ''):
+    '''
+    Parse the file line by line.  The function assumes a mostly GNU coding
+    standard compliant input so it might barf with anything that is eligible for
+    the Obfuscated C code contest.
+
+    The basic idea of the parser is to identify macro conditional scopes and
+    definitions, includes, etc. and then parse the remaining C code in the
+    context of those macro scopes.  The parser does not try to understand the
+    semantics of the code or even validate its syntax.  It only records high
+    level symbols in the source and makes a tree structure to indicate the
+    declaration/definition of those symbols and their scope in the macro
+    definitions.
+
+    LOC is the first unparsed line.
+    '''
+    cur = start
+    endblock = False
+
+    while loc < len(op):
+        nextline = op[loc]
+
+        # Macros.
+        if nextline[0] == '#':
+            (loc, endblock) = parse_preprocessor(op, loc, code, cur, start)
+            if endblock and not cur:
+                return loc
+        # Rest of C Code.
+        else:
+            cur = cur + ' ' + nextline
+            cur, loc = parse_c_expr(cur, op, loc + 1, code)
+
+    return loc
+
+
+def c_parse_output(op):
+    ''' File parser.
+
+    Parse the input array of lines OP and generate a tree structure to
+    represent the file.  This tree structure is then used for comparison between
+    the old and new file.
+    '''
+    tree = new_block('', block_type.file, [], None)
+    op = remove_comments(op)
+    op = [re.sub(r'#\s+', '#', x) for x in op]
+    op = c_parse(op, 0, tree)
+
+    return tree
+
+
+def print_change(tree, action, prologue = ''):
+    ''' Print the nature of the differences found in the tree compared to the
+    other tree.  TREE is the tree that changed, action is what the change was
+    (Added, Removed, Modified) and prologue specifies the macro scope the change
+    is in.  The function calls itself recursively for all macro condition tree
+    nodes.
+    '''
+
+    if tree['type'] != block_type.macro_cond:
+        print('\t%s(%s): %s.' % (prologue, tree['name'], action))
+        return
+
+    prologue = '%s[%s]' % (prologue, tree['name'])
+    for t in tree['contents']:
+        if t['type'] == block_type.macro_cond:
+            print_change(t, action, prologue)
+        else:
+            print('\t%s(%s): %s.' % (prologue, t['name'], action))
+
+
+def c_compare_trees(left, right, prologue = ''):
+    ''' Compare two trees and print the difference.
+
+    This routine is the entry point to compare two trees and print out their
+    differences.  LEFT and RIGHT will always have the same name and type,
+    starting with block_type.file and '' at the top level.
+    '''
+
+    if left['type'] == block_type.macro_cond or left['type'] == block_type.file:
+
+        if left['type'] == block_type.macro_cond:
+            prologue = '%s[%s]' % (prologue, left['name'])
+
+        # Make sure that everything in the left tree exists in the right tree.
+        for cl in left['contents']:
+            found = False
+            for cr in right['contents']:
+                if not cl['matched'] and not cr['matched'] and \
+                        cl['name'] == cr['name'] and cl['type'] == cr['type']:
+                    cl['matched'] = cr['matched'] = True
+                    c_compare_trees(cl, cr, prologue)
+                    found = True
+                    break
+            if not found:
+                print_change(cl, actions[cl['type']]['del'], prologue)
+
+        # ... and vice versa.  This time we only need to look at unmatched
+        # contents.
+        for cr in right['contents']:
+            if not cr['matched']:
+                print_change(cr, actions[cr['type']]['new'], prologue)
+    else:
+        if left['contents'] != right['contents']:
+            print_change(left, actions[left['type']]['mod'], prologue)
+
+
+def c_dump_tree(tree, indent):
+    ''' Print the entire tree.
+    '''
+    if not debug:
+        return
+
+    if tree['type'] == block_type.macro_cond or tree['type'] == block_type.file:
+        print('%sScope: %s' % (' ' * indent, tree['name']))
+        for c in tree['contents']:
+            c_dump_tree(c, indent + 4)
+        print('%sEndScope: %s' % (' ' * indent, tree['name']))
+    else:
+        if tree['type'] == block_type.func:
+            print('%sFUNC: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.composite:
+            print('%sCOMPOSITE: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.assign:
+            print('%sASSIGN: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.fndecl:
+            print('%sFNDECL: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.decl:
+            print('%sDECL: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.macrocall:
+            print('%sMACROCALL: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.macro_def:
+            print('%sDEFINE: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.macro_include:
+            print('%sINCLUDE: %s' % (' ' * indent, tree['name']))
+        elif tree['type'] == block_type.macro_undef:
+            print('%sUNDEF: %s' % (' ' * indent, tree['name']))
+        else:
+            print('%sMACRO LEAF: %s' % (' ' * indent, tree['name']))
+
+
+def c_compare(oldfile, newfile):
+    ''' Entry point for the C backend.
+
+    Parse the two files into trees and compare them.  Print the result of the
+    comparison in the ChangeLog-like format.
+    '''
+    left = c_parse_output(oldfile)
+    right = c_parse_output(newfile)
+
+    c_compare_trees(left, right)
+
+    debug_print('LEFT TREE')
+    debug_print('-' * 80)
+    c_dump_tree(left, 0)
+    debug_print('RIGHT TREE')
+    debug_print('-' * 80)
+    c_dump_tree(right, 0)
+
+
+#------------------------------------------------------------------------------
+
+
+# Register backends for specific file extensions.
+BACKENDS = \
+        {'.c': {'compare': c_compare},
+         '.h': {'compare': c_compare}}
+
+
+def get_backend(filename):
+    ''' Get an appropriate backend for FILENAME.
+    '''
+    name, ext = os.path.splitext(filename)
+
+    if not ext in BACKENDS.keys():
+        return None
+
+    return BACKENDS[ext]
+
+
+def exec_git_cmd(args):
+    ''' Execute a git command and return its result as a list of strings.
+    '''
+    args.insert(0, 'git')
+    debug_print(args)
+    proc = subprocess.Popen(args, stdout=subprocess.PIPE)
+
+    # Clean up the output by removing trailing spaces, newlines and dropping
+    # blank lines.
+    op = [decode(x[:-1]).strip() for x in proc.stdout]
+    op = [re.sub(r'\s+', ' ', x) for x in op]
+    op = [x for x in op if x]
+    return op
+
+
+def analyze_diff(oldfile, newfile, filename):
+    ''' Parse the output of the old and new files and print the difference.
+
+    For input files OLDFILE and NEWFILE with name FILENAME, generate reduced
+    trees for them and compare them.  We limit our comparison to only C source
+    files.
+    '''
+    backend = get_backend(filename)
+
+    if not backend:
+        return
+
+    backend['compare'](exec_git_cmd(['show', oldfile]),
+                       exec_git_cmd(['show', newfile]))
+
+
+def list_changes(commit):
+    ''' List changes in a single commit.
+
+    For the input commit id COMMIT, identify the files that have changed and the
+    nature of their changes.  Print commit information in the ChangeLog format,
+    calling into helper functions as necessary.
+    '''
+
+    op = exec_git_cmd(['show', '--date=short', '--raw', commit])
+    authors = []
+    date = ''
+    merge = False
+    copyright_exempt=''
+
+    for l in op:
+        if l.lower().find('copyright-paperwork-exempt:') == 0 \
+                and 'yes' in l.lower():
+            copyright_exempt=' (tiny change)'
+        elif l.lower().find('co-authored-by:') == 0 or \
+                l.find('Author:') == 0:
+            author = l.split(':')[1]
+            author = re.sub(r'(.*)\s*(<.*)', r'\1  \2', author)
+            authors.append(author)
+        elif l.find('Date:') == 0:
+            date=l[5:].strip()
+        elif l.find('Merge:') == 0:
+            merge = True
+
+    # Find raw commit information for all non-ChangeLog files.
+    op = [x[1:] for x in op if len(x) > 0 and re.match(r'^:[0-9]+', x) \
+            and 'ChangeLog' not in x]
+
+    # It was only the ChangeLog, ignore.
+    if len(op) == 0:
+        return
+
+    print('%s  %s' % (date, authors[0]))
+
+    if (len(authors) > 1):
+        authors = authors[1:]
+        for author in authors:
+            print('            %s' % author)
+
+    print()
+
+    if merge:
+       print('\t MERGE COMMIT: %s\n' % commit)
+       return
+
+    print('\tCOMMIT%s: %s' % (copyright_exempt, commit))
+
+    # Each of these lines has a space separated format like so:
+    # :<OLD MODE> <NEW MODE> <OLD REF> <NEW REF> <OPERATION> <FILE1> <FILE2>
+    #
+    # where OPERATION can be one of the following:
+    # A: File added
+    # D: File removed
+    # M[0-9]{3}: File modified
+    # R[0-9]{3}: File renamed, with the 3 digit number following it indicating
+    # what percentage of the file is intact.
+    # C[0-9]{3}: File copied.  Same semantics as R.
+    # T: The permission bits of the file changed
+    # U: Unmerged.  We should not encounter this, so we ignore it/
+    # X, or anything else: Most likely a bug.  Report it.
+    #
+    # FILE2 is set only when OPERATION is R or C, to indicate the new file name.
+    #
+    # Also note that merge commits have a different format here, with three
+    # entries each for the modes and refs, but we don't bother with it for now.
+    #
+    # For more details: https://git-scm.com/docs/diff-format
+    for f in op:
+        data = f.split()
+        if data[4] == 'A':
+            print('\t* %s: New file.' % data[5])
+        elif data[4] == 'D':
+            print('\t* %s: Delete file.' % data[5])
+        elif data[4] == 'T':
+            print('\t* %s: Changed file permission bits from %s to %s' % \
+                    (data[5], data[0], data[1]))
+        elif data[4][0] == 'M':
+            print('\t* %s: Modified.' % data[5])
+            analyze_diff(data[2], data[3], data[5])
+        elif data[4][0] == 'R' or data[4][0] == 'C':
+            change = int(data[4][1:])
+            print('\t* %s: Move to...' % data[5])
+            print('\t* %s: ... here.' % data[6])
+            if change < 100:
+                analyze_diff(data[2], data[3], data[6])
+        # We should never encounter this, so ignore for now.
+        elif data[4] == 'U':
+            pass
+        else:
+            eprint('%s: Unknown line format %s' % (commit, data[4]))
+            sys.exit(42)
+
+    print('')
+
+
+def list_commits(revs):
+    ''' List commit IDs between the two revs in the REVS list.
+    '''
+    ref = revs[0] + '..' + revs[1]
+    return exec_git_cmd(['log', '--pretty=%H', ref])
+
+
+def main(revs):
+    ''' ChangeLog Generator Entry Point.
+    '''
+    commits = list_commits(revs)
+    for commit in commits:
+        list_changes(commit)
+
+
+def backend_file_test(f):
+    ''' Parser debugger Entry Point.
+    '''
+    backend = get_backend(f)
+
+    if not backend:
+        debug_print('%s: No backend for this file type, cannot debug' % f)
+        return
+
+    with open(f) as srcfile:
+        op = srcfile.readlines()
+        op = [x[:-1] for x in op]
+        tree = backend['parse_output'](op)
+        backend['dump_tree'](tree, 0)
+
+
+# Program Entry point.  If -d is specified, the second argument is assumed to be
+# a file and only the backend for the file is run in verbose mode.
+if __name__ == '__main__':
+    if len(sys.argv) != 3:
+        usage(sys.argv[0])
+
+    if sys.argv[1] == '-d':
+        debug = True
+        backend_file_test(sys.argv[2])
+    else:
+        main(sys.argv[1:])
-- 
2.19.2


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