git: add function to parse message trailers

This is a pure python implementation of the message trailer parsing
algorithm in git (and libgit2). It is intended for use on finalized
commit messages only. Lines starting with comments are not ignored.

The function returns a list of pairs (name, value) where name is the
name of the trailer. Trailer values may span over multiple lines.

Link: https://git-scm.com/docs/git-interpret-trailers
Link: https://github.com/git/git/blob/master/trailer.c
Link: https://github.com/libgit2/libgit2/blob/main/src/trailer.c
Signed-off-by: Robin Jarry <robin@jarry.cc>

hubsrht/trailers.py

@@ -0,0 +1,83 @@
+import re
+from typing import List, Tuple
+
+_git_generated_prefixes = (
+    "Signed-off-by: ",
+    "(cherry picked from commit ",
+)
+
+def commit_trailers(message: str) -> List[Tuple[str, str]]:
+    """
+    Extract the trailers from a commit message. Return a list of pairs of
+    (name, value).
+
+    This borrows a large amount of logic from git core (trailer.c).
+    """
+    lines = message.strip().splitlines()
+
+    # The first paragraph is the title and cannot be trailers
+    while lines and lines[0] != '':
+        del lines[0]
+
+    recognized_prefix = False
+    only_spaces = True
+    trailer_lines = non_trailer_lines = 0
+    possible_continuation_lines = 0
+
+    # Get the start of the trailers by looking starting from the end for a
+    # blank line before a set of non-blank lines that (i) are all trailers, or
+    # (ii) contains at least one Git-generated trailer and consists of at least
+    # 25% trailers.
+    i = len(lines) - 1
+    while i >= 0:
+        line = lines[i]
+
+        if not line.strip():
+            # blank line
+            if only_spaces:
+                i -= 1
+                continue
+            if recognized_prefix and trailer_lines * 3 >= non_trailer_lines:
+                i += 1
+                break
+            if trailer_lines > 0 and non_trailer_lines == 0:
+                i += 1
+                break
+            return []
+
+        only_spaces = False
+
+        if any(line.startswith(p) for p in _git_generated_prefixes):
+            trailer_lines += 1
+            possible_continuation_lines = 0
+            recognized_prefix = True
+        elif re.search(r"^[A-Za-z\d][A-Za-z\d-]*\s*:", line):
+            trailer_lines += 1
+            possible_continuation_lines = 0
+        elif line[0] in (" ", "\t"):
+            possible_continuation_lines += 1
+        else:
+            non_trailer_lines += 1 + possible_continuation_lines
+            possible_continuation_lines = 0
+        i -= 1
+
+    # Iterate over all remaining lines and collect trailer names and values.
+    # If a line does not match a trailer and starts with a space or tab, its
+    # contents are appended to the current trailer value.
+    trailers = []
+    name = value = None
+
+    for line in lines[i:]:
+        match = re.match(r"^([A-Za-z\d][A-Za-z\d-]*)\s*:\s*(.*)$", line)
+        if match:
+            if name is not None and value is not None:
+                trailers.append((name, value))
+            name = match[1]
+            value = match[2]
+        elif name is not None and value is not None and line[0] in (" ", "\t"):
+            # continuation line
+            value += "\n" + line
+    if name is not None and value is not None:
+        trailers.append((name, value))
+
+    return trailers