Re: [PATCH 7/8] combine-diff: Fast changed-to-all-parents paths scanning

Kirill Smelkov [off-list ref] writes:

Doesn't desc point into and walks over the data we read from the
tree object directly?

We try to keep (tree|commit)->buffer intact and that is done pretty
deliberately.  While you are walking a tree or parsing a commit,
somebody else, perhaps called indirectly by a helper function you
call, may call lookup_object() for the same object, get the copy
that has already been read and start using it.  This kind of change
will introduce bugs that are hard to debug unless it is done very
carefully (e.g. starting from making tree.buffer into a const pointer
and propagating constness throughout the system), which might not be
worth the pain.

On Wed, Feb 05, 2014 at 09:36:55AM -0800, Junio C Hamano wrote:

Kirill Smelkov [off-list ref] writes:

quoted

Only, before I clean things up, I'd like to ask - would the following
patch be accepted

---- 8< ---

diff --git a/tree-walk.c b/tree-walk.c
index 79dba1d..4dc86c7 100644
--- a/tree-walk.c
+++ b/tree-walk.c

@@ -37,7 +37,7 @@ static void decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned
 
 	/* Initialize the descriptor entry */
 	desc->entry.path = path;
-	desc->entry.mode = mode;
+	desc->entry.mode = canon_mode(mode);
 	desc->entry.sha1 = (const unsigned char *)(path + len);
 }
 

diff --git a/tree-walk.h b/tree-walk.h
index ae04b64..ae7fb3a 100644
--- a/tree-walk.h
+++ b/tree-walk.h

@@ -16,7 +16,7 @@ struct tree_desc {
 static inline const unsigned char *tree_entry_extract(struct tree_desc *desc, const char **pathp, unsigned int *modep)
 {
 	*pathp = desc->entry.path;
-	*modep = canon_mode(desc->entry.mode);
+	*modep = desc->entry.mode;
 	return desc->entry.sha1;
 }
---- 8< ---
 

?

Doesn't desc point into and walks over the data we read from the
tree object directly?

We try to keep (tree|commit)->buffer intact and that is done pretty
deliberately.  While you are walking a tree or parsing a commit,
somebody else, perhaps called indirectly by a helper function you
call, may call lookup_object() for the same object, get the copy
that has already been read and start using it.  This kind of change
will introduce bugs that are hard to debug unless it is done very
carefully (e.g. starting from making tree.buffer into a const pointer
and propagating constness throughout the system), which might not be
worth the pain.

I agree object data should be immutable for good. The only thing I'm talking
about here is mode, which is parsed from a tree buffer and is stored in
separate field:

        ---- 8< ---- tree-walk.h
        struct name_entry {
                const unsigned char *sha1;
                const char *path;
                unsigned int mode;              <---
        };

        struct tree_desc {
                const void *buffer;
                struct name_entry entry;
                unsigned int size;
        };

        ---- 8< ---- tree-walk.c
        const char *get_mode(const char *str, unsigned int *modep)
        { ... }     /* parses symbolic mode from tree buffer to uint */

        void decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned long size)
        {
                const char *path;
                unsigned int mode, len;

                ...
                path = get_mode(buf, &mode);

                /* Initialize the descriptor entry */
                desc->entry.path = path;
                desc->entry.mode = mode;        <---
                desc->entry.sha1 = (const unsigned char *)(path + len);


so we are not talking about modifying object buffers here. All I'm
asking is about canonicalizing _already_ parsed and copied mode on the
fly.

Does that change anything?


Sorry, if I'm maybe misunderstanding something, and thanks,
Kirill