Re: [PATCH v4] fast-import: do not write bad delta for replaced subtrees

[Jonathan Nieder <jrnieder@gmail.com>]

Dmitry Ivankov wrote:

> How about adding a new bit field "no_delta" instead?

Currently the layout of "struct tree_entry_ms" is:

	uint16_t mode;			two bytes
	unsigned char sha1[20]		20 bytes

which adds up to 22 bytes.  Here is "struct tree_entry":

	struct tree_entry *tree;		one machine word
	struct atom_str *name;			one machine word
	struct tree_entry_ms versions[2];	44 bytes

Although it only looks like it adds one byte per tree entry, in
practice I suspect your patch adds four.  Is that worth it?  (The
answer might be yes.  I'm not sure.)

> The patch is
> smaller this way. Also could 04000 theoretically be S_IFDIR on some
> platform?

No, these modes are part of the format of objects as written on disk
and over the wire, so when we meet a platform with S_IFDIR != 040000,
there will have to be bigger changes (to distinguish between the
platform's idea of file status and git's idea of modes).

> - switch to a separate no_delta bit in tree_entry

If it doesn't cost too much, this is a good idea.

> - when setting no_delta = 1 don't check for S_ISDIR(versions[0].mode),
>   this is a redundant check and logic duplication. Who knows, maybe some
>   day we'll want to delta a tree against blob. :)

Why?  When versions[0] is not a tree, the hack is not needed, since
versions[0].mode and versions[0].sha1 accurately describe the delta
base and are not inconsistent with anything.

Thanks, that was helpful.