diff_tform: fix rename detection with rewrite/delete pair

A rewritten file can either be classified as a modification of its
contents or of a delete of the complete file followed by an addition of
the new content. This distinction becomes important when we want to
detect renames for rewrites. Given a scenario where a file "a" has been
deleted and another file "b" has been renamed to "a", this should be
detected as a deletion of "a" followed by a rename of "a" -> "b". Thus,
splitting of the original rewrite into a delete/add pair is important
here.

This splitting is represented by a flag we can set at the current delta.
While the flag is already being set in case we want to break rewrites,
we do not do so in case where the `GIT_DIFF_FIND_RENAMES_FROM_REWRITES`
flag is set. This can trigger an assert when we try to match the source
and target deltas.

Fix the issue by setting the `GIT_DIFF_FLAG__TO_SPLIT` flag at the delta
when it is a rename target and `GIT_DIFF_FIND_RENAMES_FROM_REWRITES` is
set.

2 files changed, 216 insertions, 1 deletions

diff --git a/src/diff_tform.c b/src/diff_tform.c
index c0461a3a3..bc664dd05 100644
--- a/src/diff_tform.c
+++ b/src/diff_tform.c
@@ -686,8 +686,10 @@ static bool is_rename_target(
 			break;
 		}
 		if (FLAG_SET(opts, GIT_DIFF_FIND_RENAMES_FROM_REWRITES) &&
-			delta->similarity < opts->rename_from_rewrite_threshold)
+			delta->similarity < opts->rename_from_rewrite_threshold) {
+			delta->flags |= GIT_DIFF_FLAG__TO_SPLIT;
 			break;
+		}
 
 		return false;
 
diff --git a/tests/diff/rename.c b/tests/diff/rename.c
index 3625d6884..ddc1d5d78 100644
--- a/tests/diff/rename.c
+++ b/tests/diff/rename.c
@@ -1765,3 +1765,216 @@ void test_diff_rename__identical(void)
 	git_tree_free(new_tree);
 }
 
+void test_diff_rename__rewrite_and_delete(void)
+{
+	const char *old_sha = RENAME_MODIFICATION_COMMIT;
+	const char *new_sha = REWRITE_DELETE_COMMIT;
+	git_tree *old_tree, *new_tree;
+	git_diff *diff;
+	git_diff_find_options find_opts = GIT_DIFF_FIND_OPTIONS_INIT;
+	git_buf diff_buf = GIT_BUF_INIT;
+	const char *expected =
+		"diff --git a/ikeepsix.txt b/ikeepsix.txt\n"
+		"deleted file mode 100644\n"
+		"index eaf4a3e..0000000\n"
+		"--- a/ikeepsix.txt\n"
+		"+++ /dev/null\n"
+		"@@ -1,27 +0,0 @@\n"
+		"-I Keep Six Honest Serving-Men\n"
+		"-=============================\n"
+		"-\n"
+		"-She sends'em abroad on her own affairs,\n"
+		"- From the second she opens her eyes—\n"
+		"-One million Hows, two million Wheres,\n"
+		"-And seven million Whys!\n"
+		"-\n"
+		"-I let them rest from nine till five,\n"
+		"- For I am busy then,\n"
+		"-As well as breakfast, lunch, and tea,\n"
+		"- For they are hungry men.\n"
+		"-But different folk have different views;\n"
+		"-I know a person small—\n"
+		"-She keeps ten million serving-men,\n"
+		"-Who get no rest at all!\n"
+		"-\n"
+		"-  -- Rudyard Kipling\n"
+		"-\n"
+		"-I KEEP six honest serving-men\n"
+		"- (They taught me all I knew);\n"
+		"-Their names are What and Why and When\n"
+		"- And How and Where and Who.\n"
+		"-I send them over land and sea,\n"
+		"- I send them east and west;\n"
+		"-But after they have worked for me,\n"
+		"- I give them all a rest.\n"
+		"diff --git a/songof7cities.txt b/songof7cities.txt\n"
+		"index 4210ffd..95ceb12 100644\n"
+		"--- a/songof7cities.txt\n"
+		"+++ b/songof7cities.txt\n"
+		"@@ -1,45 +1,45 @@\n"
+		"-The Song of Seven Cities\n"
+		"+THE SONG OF SEVEN CITIES\n"
+		" ------------------------\n"
+		" \n"
+		"-I WAS Lord of Cities very sumptuously builded.\n"
+		"-Seven roaring Cities paid me tribute from afar.\n"
+		"-Ivory their outposts were--the guardrooms of them gilded,\n"
+		"-And garrisoned with Amazons invincible in war.\n"
+		"-\n"
+		"-All the world went softly when it walked before my Cities--\n"
+		"-Neither King nor Army vexed my peoples at their toil,\n"
+		"-Never horse nor chariot irked or overbore my Cities,\n"
+		"-Never Mob nor Ruler questioned whence they drew their spoil.\n"
+		"-\n"
+		"-Banded, mailed and arrogant from sunrise unto sunset;\n"
+		"-Singing while they sacked it, they possessed the land at large.\n"
+		"-Yet when men would rob them, they resisted, they made onset\n"
+		"-And pierced the smoke of battle with a thousand-sabred charge.\n"
+		"-\n"
+		"-So they warred and trafficked only yesterday, my Cities.\n"
+		"-To-day there is no mark or mound of where my Cities stood.\n"
+		"-For the River rose at midnight and it washed away my Cities.\n"
+		"-They are evened with Atlantis and the towns before the Flood.\n"
+		"-\n"
+		"-Rain on rain-gorged channels raised the water-levels round them,\n"
+		"-Freshet backed on freshet swelled and swept their world from sight,\n"
+		"-Till the emboldened floods linked arms and, flashing forward, drowned them--\n"
+		"-Drowned my Seven Cities and their peoples in one night!\n"
+		"-\n"
+		"-Low among the alders lie their derelict foundations,\n"
+		"-The beams wherein they trusted and the plinths whereon they built--\n"
+		"-My rulers and their treasure and their unborn populations,\n"
+		"-Dead, destroyed, aborted, and defiled with mud and silt!\n"
+		"-\n"
+		"-The Daughters of the Palace whom they cherished in my Cities,\n"
+		"-My silver-tongued Princesses, and the promise of their May--\n"
+		"-Their bridegrooms of the June-tide--all have perished in my Cities,\n"
+		"-With the harsh envenomed virgins that can neither love nor play.\n"
+		"-\n"
+		"-I was Lord of Cities--I will build anew my Cities,\n"
+		"-Seven, set on rocks, above the wrath of any flood.\n"
+		"-Nor will I rest from search till I have filled anew my Cities\n"
+		"-With peoples undefeated of the dark, enduring blood.\n"
+		"+I WAS LORD OF CITIES VERY SUMPTUOUSLY BUILDED.\n"
+		"+SEVEN ROARING CITIES PAID ME TRIBUTE FROM AFAR.\n"
+		"+IVORY THEIR OUTPOSTS WERE--THE GUARDROOMS OF THEM GILDED,\n"
+		"+AND GARRISONED WITH AMAZONS INVINCIBLE IN WAR.\n"
+		"+\n"
+		"+ALL THE WORLD WENT SOFTLY WHEN IT WALKED BEFORE MY CITIES--\n"
+		"+NEITHER KING NOR ARMY VEXED MY PEOPLES AT THEIR TOIL,\n"
+		"+NEVER HORSE NOR CHARIOT IRKED OR OVERBORE MY CITIES,\n"
+		"+NEVER MOB NOR RULER QUESTIONED WHENCE THEY DREW THEIR SPOIL.\n"
+		"+\n"
+		"+BANDED, MAILED AND ARROGANT FROM SUNRISE UNTO SUNSET;\n"
+		"+SINGING WHILE THEY SACKED IT, THEY POSSESSED THE LAND AT LARGE.\n"
+		"+YET WHEN MEN WOULD ROB THEM, THEY RESISTED, THEY MADE ONSET\n"
+		"+AND PIERCED THE SMOKE OF BATTLE WITH A THOUSAND-SABRED CHARGE.\n"
+		"+\n"
+		"+SO THEY WARRED AND TRAFFICKED ONLY YESTERDAY, MY CITIES.\n"
+		"+TO-DAY THERE IS NO MARK OR MOUND OF WHERE MY CITIES STOOD.\n"
+		"+FOR THE RIVER ROSE AT MIDNIGHT AND IT WASHED AWAY MY CITIES.\n"
+		"+THEY ARE EVENED WITH ATLANTIS AND THE TOWNS BEFORE THE FLOOD.\n"
+		"+\n"
+		"+RAIN ON RAIN-GORGED CHANNELS RAISED THE WATER-LEVELS ROUND THEM,\n"
+		"+FRESHET BACKED ON FRESHET SWELLED AND SWEPT THEIR WORLD FROM SIGHT,\n"
+		"+TILL THE EMBOLDENED FLOODS LINKED ARMS AND, FLASHING FORWARD, DROWNED THEM--\n"
+		"+DROWNED MY SEVEN CITIES AND THEIR PEOPLES IN ONE NIGHT!\n"
+		"+\n"
+		"+LOW AMONG THE ALDERS LIE THEIR DERELICT FOUNDATIONS,\n"
+		"+THE BEAMS WHEREIN THEY TRUSTED AND THE PLINTHS WHEREON THEY BUILT--\n"
+		"+MY RULERS AND THEIR TREASURE AND THEIR UNBORN POPULATIONS,\n"
+		"+DEAD, DESTROYED, ABORTED, AND DEFILED WITH MUD AND SILT!\n"
+		"+\n"
+		"+THE DAUGHTERS OF THE PALACE WHOM THEY CHERISHED IN MY CITIES,\n"
+		"+MY SILVER-TONGUED PRINCESSES, AND THE PROMISE OF THEIR MAY--\n"
+		"+THEIR BRIDEGROOMS OF THE JUNE-TIDE--ALL HAVE PERISHED IN MY CITIES,\n"
+		"+WITH THE HARSH ENVENOMED VIRGINS THAT CAN NEITHER LOVE NOR PLAY.\n"
+		"+\n"
+		"+I WAS LORD OF CITIES--I WILL BUILD ANEW MY CITIES,\n"
+		"+SEVEN, SET ON ROCKS, ABOVE THE WRATH OF ANY FLOOD.\n"
+		"+NOR WILL I REST FROM SEARCH TILL I HAVE FILLED ANEW MY CITIES\n"
+		"+WITH PEOPLES UNDEFEATED OF THE DARK, ENDURING BLOOD.\n"
+		" \n"
+		" To the sound of trumpets shall their seed restore my Cities\n"
+		" Wealthy and well-weaponed, that once more may I behold\n";
+
+	old_tree = resolve_commit_oid_to_tree(g_repo, old_sha);
+	new_tree = resolve_commit_oid_to_tree(g_repo, new_sha);
+
+	find_opts.flags = GIT_DIFF_FIND_RENAMES_FROM_REWRITES;
+
+	cl_git_pass(git_diff_tree_to_tree(&diff, g_repo, old_tree, new_tree, NULL));
+	cl_git_pass(git_diff_find_similar(diff, &find_opts));
+
+	cl_git_pass(git_diff_to_buf(&diff_buf, diff, GIT_DIFF_FORMAT_PATCH));
+
+	cl_assert_equal_s(expected, diff_buf.ptr);
+
+	git_buf_free(&diff_buf);
+	git_diff_free(diff);
+	git_tree_free(old_tree);
+	git_tree_free(new_tree);
+}
+
+void test_diff_rename__delete_and_rename(void)
+{
+	const char *old_sha = RENAME_MODIFICATION_COMMIT;
+	const char *new_sha = DELETE_RENAME_COMMIT;
+	git_tree *old_tree, *new_tree;
+	git_diff *diff;
+	git_diff_find_options find_opts = GIT_DIFF_FIND_OPTIONS_INIT;
+	git_buf diff_buf = GIT_BUF_INIT;
+	const char *expected =
+		"diff --git a/sixserving.txt b/sixserving.txt\n"
+		"deleted file mode 100644\n"
+		"index f90d4fc..0000000\n"
+		"--- a/sixserving.txt\n"
+		"+++ /dev/null\n"
+		"@@ -1,25 +0,0 @@\n"
+		"-I  KEEP  six  honest  serving-men\n"
+		"-  (They  taught  me  all  I  knew);\n"
+		"-Their  names  are  What  and  Why  and  When\n"
+		"-  And  How  and  Where  and  Who.\n"
+		"-I  send  them  over  land  and  sea,\n"
+		"-  I  send  them  east  and  west;\n"
+		"-But  after  they  have  worked  for  me,\n"
+		"-  I  give  them  all  a  rest.\n"
+		"-\n"
+		"-I  let  them  rest  from  nine  till  five,\n"
+		"-  For  I  am  busy  then,\n"
+		"-As  well  as  breakfast,  lunch,  and  tea,\n"
+		"-  For  they  are  hungry  men.\n"
+		"-But  different  folk  have  different  views;\n"
+		"-I  know  a  person  small—\n"
+		"-She  keeps  ten  million  serving-men,\n"
+		"-Who  get  no  rest  at  all!\n"
+		"-\n"
+		"-She  sends'em  abroad  on  her  own  affairs,\n"
+		"-  From  the  second  she  opens  her  eyes—\n"
+		"-One  million  Hows,  two  million  Wheres,\n"
+		"-And  seven  million  Whys!\n"
+		"-\n"
+		"-    --  Rudyard  Kipling\n"
+		"-\n"
+		"diff --git a/songof7cities.txt b/sixserving.txt\n"
+		"similarity index 100%\n"
+		"rename from songof7cities.txt\n"
+		"rename to sixserving.txt\n";
+
+	old_tree = resolve_commit_oid_to_tree(g_repo, old_sha);
+	new_tree = resolve_commit_oid_to_tree(g_repo, new_sha);
+
+	find_opts.flags = GIT_DIFF_FIND_RENAMES_FROM_REWRITES;
+
+	cl_git_pass(git_diff_tree_to_tree(&diff, g_repo, old_tree, new_tree, NULL));
+	cl_git_pass(git_diff_find_similar(diff, &find_opts));
+
+	cl_git_pass(git_diff_to_buf(&diff_buf, diff, GIT_DIFF_FORMAT_PATCH));
+
+	cl_assert_equal_s(expected, diff_buf.ptr);
+
+	git_buf_free(&diff_buf);
+	git_diff_free(diff);
+	git_tree_free(old_tree);
+	git_tree_free(new_tree);
+}