@@ -19,6 +19,7 @@
     "python_language",
     "treeherder_api",
     "library",
+    "build_bug_description",
     "lambda_capture",
     "class_passing",
     "frequency"
 
@@ -0,0 +1,155 @@
+#!/usr/bin/env python3
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import sys
+import unittest
+
+sys.path.append(".")
+sys.path.append("..")
+
+from components.scmprovider import SCMProvider, Commit
+
+GITHUB_REPO = "https://github.com/org/repo"
+
+
+def make_commit(revision, parent_revision, summary="Do a thing",
+                description="A longer description of the thing.",
+                files_modified=None):
+    # Build a fully-populated Commit without touching a real repository.
+    c = Commit("%s|2020-01-01 00:00:00 +0000|2020-01-02 00:00:00 +0000" % revision)
+    c.summary = summary
+    c.author = "Some Developer <dev@example.com>"
+    c.description = description
+    c.revision_link = "%s/commit/%s" % (GITHUB_REPO, revision)
+    c.parent_revision = parent_revision
+    c.files_modified = files_modified if files_modified is not None else ["src/thing.c"]
+    c.populated = True
+    return c
+
+
+class TestBuildBugDescription(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        # build_bug_description / _commit_block don't use the command or logging
+        # providers, so a bare provider is enough.
+        cls.scm = SCMProvider({})
+
+    def _commits(self):
+        # Ordered oldest -> newest, as build_bug_description expects.
+        return [
+            make_commit("aaaaaaa", "p0000000", summary="First commit"),
+            make_commit("bbbbbbb", "aaaaaaa", summary="Second commit"),
+            make_commit("ccccccc", "bbbbbbb", summary="Third commit"),
+        ]
+
+    # -- _commit_block ---------------------------------------------------------
+
+    def test_commit_block_verbosity_levels(self):
+        c = make_commit("abcdef0", "0fedcba", summary="Fix the widget",
+                        description="Body of the change.", files_modified=["src/w.c"])
+
+        v1 = self.scm._commit_block(c, 1)
+        self.assertIn("abcdef0 by Some Developer", v1)
+        self.assertIn(c.revision_link, v1)
+        self.assertNotIn("Authored:", v1)
+        self.assertNotIn("Fix the widget", v1)
+        self.assertNotIn("Files Modified", v1)
+
+        v2 = self.scm._commit_block(c, 2)
+        self.assertIn("Authored:", v2)
+        self.assertIn("Committed:", v2)
+        self.assertIn("Fix the widget", v2)
+        self.assertNotIn("Body of the change.", v2)
+        self.assertNotIn("Files Modified", v2)
+
+        v3 = self.scm._commit_block(c, 3)
+        self.assertIn("Fix the widget", v3)
+        self.assertIn("Body of the change.", v3)
+        self.assertIn("Files Modified", v3)
+        self.assertIn("src/w.c", v3)
+
+    def test_commit_block_requires_populated(self):
+        c = Commit("deadbee|d|d")  # not populated
+        with self.assertRaises(Exception):
+            self.scm._commit_block(c, 1)
+
+    # -- dispatch --------------------------------------------------------------
+
+    def test_dispatch_selects_builder(self):
+        commits = self._commits()
+
+        # Tiny budget makes the two behaviors clearly distinguishable: the
+        # single-comment builder elides, the chained builder splits instead.
+        single = self.scm.build_bug_description(commits, 60, GITHUB_REPO, options={})
+        self.assertEqual(len(single), 1)
+        self.assertIn("commits elided", single[0])
+
+        chained = self.scm.build_bug_description(commits, 60, GITHUB_REPO, options={"verbose-diff": True})
+        self.assertGreater(len(chained), 1)
+        self.assertNotIn("commits elided", "".join(chained))
+
+    def test_options_default_is_single_comment(self):
+        commits = self._commits()
+        # No options passed at all -> single-comment behavior.
+        result = self.scm.build_bug_description(commits, 60, GITHUB_REPO)
+        self.assertEqual(len(result), 1)
+        self.assertIn("commits elided", result[0])
+
+    # -- single-comment behavior ----------------------------------------------
+
+    def test_single_comment_full_when_it_fits(self):
+        commits = self._commits()
+        result = self.scm.build_bug_description(commits, 65534, GITHUB_REPO, options={})
+        self.assertEqual(len(result), 1)
+        self.assertNotIn("commits elided", result[0])
+        # Full detail present, newest commit first.
+        self.assertIn("Third commit", result[0])
+        self.assertIn("Files Modified", result[0])
+        self.assertLess(result[0].index("Third commit"), result[0].index("First commit"))
+
+    # -- chained behavior ------------------------------------------------------
+
+    def test_chained_single_chunk_when_it_fits(self):
+        commits = self._commits()
+        result = self.scm.build_bug_description(commits, 65534, GITHUB_REPO, options={"verbose-diff": True})
+        self.assertEqual(len(result), 1)
+        # No continuation header for a single chunk.
+        self.assertNotIn("continued in following comments", result[0])
+
+    def test_chained_splits_with_compare_url_including_oldest(self):
+        commits = self._commits()
+        result = self.scm.build_bug_description(commits, 200, GITHUB_REPO, options={"verbose-diff": True})
+        self.assertGreater(len(result), 1)
+        # The compare range starts at the parent of the oldest commit, so the
+        # oldest commit itself is included.
+        self.assertIn("All 3 commits: %s/compare/p0000000...ccccccc" % GITHUB_REPO, result[0])
+        self.assertIn("continued in following comments", result[0])
+
+    def test_chained_no_compare_url_for_unsupported_host(self):
+        commits = self._commits()
+        result = self.scm.build_bug_description(commits, 200, "https://example.com/repo", options={"verbose-diff": True})
+        self.assertGreater(len(result), 1)
+        self.assertNotIn("/compare/", "".join(result))
+
+    def test_chained_reduces_verbosity_for_oversized_commit(self):
+        # A single commit whose full detail exceeds the limit is rendered at a
+        # lower verbosity so it fits, rather than blowing past the comment size.
+        big = make_commit("abc1234", "par0000", summary="Summary line",
+                          description="D" * 500, files_modified=["src/one.c", "src/two.c"])
+        result = self.scm.build_bug_description([big], 200, GITHUB_REPO, options={"verbose-diff": True})
+        self.assertEqual(len(result), 1)
+        self.assertLessEqual(len(result[0]), 200)      # it now fits within the limit
+        self.assertIn("abc1234 by", result[0])         # still identifies the commit
+        self.assertNotIn("D" * 500, result[0])         # full description dropped
+        self.assertNotIn("Files Modified", result[0])  # file list dropped
+
+    def test_chained_empty_commit_list(self):
+        result = self.scm.build_bug_description([], 65534, GITHUB_REPO, options={"verbose-diff": True})
+        self.assertEqual(result, [""])
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)
@@ -86,6 +86,7 @@ def default_phab_submit():
         ("git log -1 --oneline", lambda: "0481f1c (HEAD -> issue-115-add-revision-to-log, origin/issue-115-add-revision-to-log) Issue #115 - Add revision of updatebot to log output"),
         ("git clone https://example.invalid .", lambda: ""),
         ("git merge-base", lambda: "_current"),
+        ("git rev-parse", lambda: "abcdef1234567890abcdef1234567890abcdef12"),
         ("git log --pretty=%H|%ai|%ci", lambda cmd: "\n".join(expected_values.git_pretty_output_func("_current" not in cmd))),
         ("git diff --name-status", lambda: GIT_DIFF_FILES_CHANGES),
         ("git log --pretty=%s", lambda: "Roll SPIRV-Tools from a61d07a72763 to 1cda495274bb (1 revision)"),