replace: forbid replacing an object with one of a different type

Users replacing an object with one of a different type were not
prevented to do so, even if it was obvious, and stated in the doc,
that bad things would result from doing that.

To avoid mistakes, it is better to just forbid that though.

If -f option, which means '--force', is used, we can allow an object
to be replaced with one of a different type, as the user should know
what (s)he is doing.

If one object is replaced with one of a different type, the only way
to keep the history valid is to also replace all the other objects
that point to the replaced object. That's because:

* Annotated tags contain the type of the tagged object.

* The tree/parent lines in commits must be a tree and commits, resp.

* The object types referred to by trees are specified in the 'mode'
field:
100644 and 100755    blob
160000               commit
040000               tree
(these are the only valid modes)

* Blobs don't point at anything.

The doc will be updated in a later patch.

Acked-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/replace.c b/builtin/replace.c
index 59d31152d0068dd4a542d2c8e076c629d508500f..95736d98d24d4a4b4761bc92609e93781e0da2d5 100644 (file)
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -85,6 +85,7 @@ static int replace_object(const char *object_ref, const char *replace_ref,
                          int force)
 {
        unsigned char object[20], prev[20], repl[20];
+       enum object_type obj_type, repl_type;
        char ref[PATH_MAX];
        struct ref_lock *lock;
 
@@ -100,6 +101,15 @@ static int replace_object(const char *object_ref, const char *replace_ref,
        if (check_refname_format(ref, 0))
                die("'%s' is not a valid ref name.", ref);
 
+       obj_type = sha1_object_info(object, NULL);
+       repl_type = sha1_object_info(repl, NULL);
+       if (!force && obj_type != repl_type)
+               die("Objects must be of the same type.\n"
+                   "'%s' points to a replaced object of type '%s'\n"
+                   "while '%s' points to a replacement object of type '%s'.",
+                   object_ref, typename(obj_type),
+                   replace_ref, typename(repl_type));
+
        if (read_ref(ref, prev))
                hashclr(prev);
        else if (!force)