[PATCH v2] unpack-trees: do not abort when overwriting an existing file with the same content

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Changes: do not lose worktree's executable permission.

 t/t1011-read-tree-sparse-checkout.sh |  3 ++-
 t/t2021-checkout-overwrite.sh        | 18 ++++++++++++++++++
 unpack-trees.c                       | 31 +++++++++++++++++++++++++++++++
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
index 5c0053a..38f9899 100755
--- a/t/t1011-read-tree-sparse-checkout.sh
+++ b/t/t1011-read-tree-sparse-checkout.sh
@@ -238,7 +238,8 @@ test_expect_success 'print errors when failed to update worktree' '
 	echo sub >.git/info/sparse-checkout &&
 	git checkout -f init &&
 	mkdir sub &&
-	touch sub/added sub/addedtoo &&
+	echo modified >sub/added &&
+	echo modified >sub/addedtoo &&
 	test_must_fail git checkout top 2>actual &&
 	cat >expected <<\EOF &&
 error: The following untracked working tree files would be overwritten by checkout:
diff --git a/t/t2021-checkout-overwrite.sh b/t/t2021-checkout-overwrite.sh
index 5da63e9..bb1696d 100755
--- a/t/t2021-checkout-overwrite.sh
+++ b/t/t2021-checkout-overwrite.sh
@@ -47,4 +47,22 @@ test_expect_success SYMLINKS 'checkout commit with dir must not remove untracked
 	test -h a/b
 '
 
+test_expect_success 'do not abort on overwriting an existing file with the same content' '
+	echo abc >bar &&
+	git add bar &&
+	git commit -m "new file" &&
+	git reset HEAD^ &&
+	git checkout HEAD@{1}
+'
+
+test_expect_success POSIXPERM 'do abort on an existing file, same content but different permission' '
+	git checkout -f HEAD^ &&
+	echo abc >bar &&
+	git add bar &&
+	git commit -m "new file" &&
+	git reset HEAD^ &&
+	chmod a+x bar &&
+	test_must_fail git checkout HEAD@{1}
+'
+
 test_done
diff --git a/unpack-trees.c b/unpack-trees.c
index 0e1a196..ea204ae 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -9,6 +9,8 @@
 #include "refs.h"
 #include "attr.h"
 
+#define SAME_CONTENT_SIZE_LIMIT (1024 * 1024)
+
 /*
  * Error messages expected by scripts out of plumbing commands such as
  * read-tree.  Non-scripted Porcelain is not required to use these messages
@@ -1363,6 +1365,7 @@ static int check_ok_to_remove(const char *name, int len, int dtype,
 			      struct unpack_trees_options *o)
 {
 	struct cache_entry *result;
+	unsigned long ce_size;
 
 	/*
 	 * It may be that the 'lstat()' succeeded even though
@@ -1405,6 +1408,34 @@ static int check_ok_to_remove(const char *name, int len, int dtype,
 			return 0;
 	}
 
+	/*
+	 * If it has the same content that we are going to overwrite,
+	 * there's no point in complaining. We still overwrite it in
+	 * the end though.
+	 */
+	if (ce &&
+	    S_ISREG(st->st_mode) && S_ISREG(ce->ce_mode) &&
+	    (!trust_executable_bit ||
+	     (0100 & (ce->ce_mode ^ st->st_mode)) == 0) &&
+	    st->st_size < SAME_CONTENT_SIZE_LIMIT &&
+	    sha1_object_info(ce->sha1, &ce_size) == OBJ_BLOB &&
+	    ce_size == st->st_size) {
+		void *buffer = NULL;
+		unsigned long size;
+		enum object_type type;
+		struct strbuf sb = STRBUF_INIT;
+		int matched =
+			strbuf_read_file(&sb, ce->name, ce_size) == ce_size &&
+			(buffer = read_sha1_file(ce->sha1, &type, &size)) != NULL &&
+			type == OBJ_BLOB &&
+			size == ce_size &&
+			!memcmp(buffer, sb.buf, size);
+		free(buffer);
+		strbuf_release(&sb);
+		if (matched)
+			return 0;
+	}
+
 	return o->gently ? -1 :
 		add_rejected_path(o, error_type, name);
 }
-- 
1.8.0.rc2.23.g1fb49df