bin: Add merge-driver-ekeyword

Since the KEYWORDS=... assignment is a single line, git struggles to
handle conflicts. When rebasing a series of commits that modify the
KEYWORDS=... it's usually easier to throw them away and reapply on the
new tree than it is to manually handle conflicts during the rebase.

git allows a 'merge driver' program to handle conflicts; this program
handles conflicts in the KEYWORDS=... assignment. E.g., given an ebuild
with these keywords:

KEYWORDS="~alpha amd64 arm arm64 ~hppa ppc ppc64 x86"

One developer drops the ~alpha keyword and pushes to gentoo.git, and
another developer stabilizes hppa. Without this merge driver, git
requires the second developer to manually resolve the conflict which is
tedious and prone to mistakes when rebasing a long series of patches.
With the custom merge driver, it automatically resolves the conflict.

To use the merge driver, configure your gentoo.git as such:

gentoo.git/.git/config:

	[merge "keywords"]
		name = KEYWORDS merge driver
		driver = merge-driver-ekeyword %O %A %B %P

gentoo.git/.git/info/attributes:

	*.ebuild merge=keywords

Signed-off-by: Matt Turner <mattst88@gentoo.org>

1 files changed, 132 insertions, 0 deletions

diff --git a/bin/merge-driver-ekeyword b/bin/merge-driver-ekeyword
new file mode 100755
index 0000000..2df83fc
--- /dev/null
+++ b/bin/merge-driver-ekeyword
@@ -0,0 +1,132 @@
+#!/usr/bin/python
+#
+# Copyright 2020 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2 or later
+
+"""
+Custom git merge driver for handling conflicts in KEYWORDS assignments
+
+See https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver
+"""
+
+import difflib
+import os
+import sys
+import tempfile
+
+from typing import List, Optional, Tuple
+
+from gentoolkit.ekeyword import ekeyword
+
+
+def keyword_array(keyword_line: str) -> List[str]:
+    # Find indices of string inside the double-quotes
+    i1: int = keyword_line.find('"') + 1
+    i2: int = keyword_line.rfind('"')
+
+    # Split into array of KEYWORDS
+    return keyword_line[i1:i2].split(' ')
+
+
+def keyword_line_changes(old: str, new: str) -> List[Tuple[Optional[str],
+                                                           Optional[str]]]:
+    a: List[str] = keyword_array(old)
+    b: List[str] = keyword_array(new)
+
+    s = difflib.SequenceMatcher(a=a, b=b)
+
+    changes = []
+    for tag, i1, i2, j1, j2 in s.get_opcodes():
+        if tag == 'replace':
+            changes.append((a[i1:i2], b[j1:j2]),)
+        elif tag == 'delete':
+            changes.append((a[i1:i2], None),)
+        elif tag == 'insert':
+            changes.append((None, b[j1:j2]),)
+        else:
+            assert tag == 'equal'
+    return changes
+
+
+def keyword_changes(ebuild1: str, ebuild2: str) -> List[Tuple[Optional[str],
+                                                              Optional[str]]]:
+    with open(ebuild1) as e1, open(ebuild2) as e2:
+        lines1 = e1.readlines()
+        lines2 = e2.readlines()
+
+        diff = difflib.unified_diff(lines1, lines2, n=0)
+        assert next(diff) == '--- \n'
+        assert next(diff) == '+++ \n'
+
+        hunk: int = 0
+        old: str = ''
+        new: str = ''
+
+        for line in diff:
+            if line.startswith('@@ '):
+                if hunk > 0:
+                    break
+                hunk += 1
+            elif line.startswith('-'):
+                if old or new:
+                    break
+                old = line
+            elif line.startswith('+'):
+                if not old or new:
+                    break
+                new = line
+        else:
+            if 'KEYWORDS=' in old and 'KEYWORDS=' in new:
+                return keyword_line_changes(old, new)
+        return None
+
+
+def apply_keyword_changes(ebuild: str, pathname: str,
+                          changes: List[Tuple[Optional[str],
+                                              Optional[str]]]) -> int:
+    result: int = 0
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        # ekeyword will only modify files named *.ebuild, so make a symlink
+        ebuild_symlink: str = os.path.join(tmpdir, os.path.basename(pathname))
+        os.symlink(os.path.join(os.getcwd(), ebuild), ebuild_symlink)
+
+        for removals, additions in changes:
+            args = []
+            for rem in removals:
+                # Drop leading '~' and '-' characters and prepend '^'
+                i = 1 if rem[0] in ('~', '-') else 0
+                args.append('^' + rem[i:])
+            if additions:
+                args.extend(additions)
+            args.append(ebuild_symlink)
+
+            result = ekeyword.main(args)
+            if result != 0:
+                break
+
+    return result
+
+
+def main(argv):
+    if len(argv) != 5:
+        sys.exit(-1)
+
+    O = argv[1] # %O - filename of original
+    A = argv[2] # %A - filename of our current version
+    B = argv[3] # %B - filename of the other branch's version
+    P = argv[4] # %P - original path of the file
+
+    # Get changes from %O to %B
+    changes = keyword_changes(O, B)
+    if changes:
+        # Apply O -> B changes to A
+        result: int = apply_keyword_changes(A, P, changes)
+        sys.exit(result)
+    else:
+        result: int = os.system(f"git merge-file -L HEAD -L base -L ours {A} {O} {B}")
+        sys.exit(0 if result == 0 else -1)
+
+
+if __name__ == "__main__":
+    main(sys.argv)