Convert "struct cache_entry *" to "const ..." wherever possible

I attempted to make index_state->cache[] a "const struct cache_entry **"
to find out how existing entries in index are modified and where. The
question I have is what do we do if we really need to keep track of on-disk
changes in the index. The result is

 - diff-lib.c: setting CE_UPTODATE

 - name-hash.c: setting CE_HASHED

 - preload-index.c, read-cache.c, unpack-trees.c and
   builtin/update-index: obvious

 - entry.c: write_entry() may refresh the checked out entry via
   fill_stat_cache_info(). This causes "non-const struct cache_entry
   *" in builtin/apply.c, builtin/checkout-index.c and
   builtin/checkout.c

 - builtin/ls-files.c: --with-tree changes stagemask and may set
   CE_UPDATE

Of these, write_entry() and its call sites are probably most
interesting because it modifies on-disk info. But this is stat info
and can be retrieved via refresh, at least for porcelain
commands. Other just uses ce_flags for local purposes.

So, keeping track of "dirty" entries is just a matter of setting a
flag in index modification functions exposed by read-cache.c. Except
unpack-trees, the rest of the code base does not do anything funny
behind read-cache's back.

The actual patch is less valueable than the summary above. But if
anyone wants to re-identify the above sites. Applying this patch, then
this:

    diff --git a/cache.h b/cache.h
    index 430d021..1692891 100644
    --- a/cache.h
    +++ b/cache.h
    @@ -267,7 +267,7 @@ static inline unsigned int canon_mode(unsigned int mode)
     #define cache_entry_size(len) (offsetof(struct cache_entry,name) + (len) + 1)

     struct index_state {
    -	struct cache_entry **cache;
    +	const struct cache_entry **cache;
     	unsigned int version;
     	unsigned int cache_nr, cache_alloc, cache_changed;
     	struct string_list *resolve_undo;

will help quickly identify them without bogus warnings.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/builtin/apply.c b/builtin/apply.c
index 0e9b631db..023bb3aad 100644
--- a/builtin/apply.c
+++ b/builtin/apply.c
@@ -2999,7 +2999,7 @@ static int read_blob_object(struct strbuf *buf, const unsigned char *sha1, unsig
 	return 0;
 }
 
-static int read_file_or_gitlink(struct cache_entry *ce, struct strbuf *buf)
+static int read_file_or_gitlink(const struct cache_entry *ce, struct strbuf *buf)
 {
 	if (!ce)
 		return 0;
@@ -3117,7 +3117,7 @@ static struct patch *previous_patch(struct patch *patch, int *gone)
 	return previous;
 }
 
-static int verify_index_match(struct cache_entry *ce, struct stat *st)
+static int verify_index_match(const struct cache_entry *ce, struct stat *st)
 {
 	if (S_ISGITLINK(ce->ce_mode)) {
 		if (!S_ISDIR(st->st_mode))
@@ -3130,7 +3130,7 @@ static int verify_index_match(struct cache_entry *ce, struct stat *st)
 #define SUBMODULE_PATCH_WITHOUT_INDEX 1
 
 static int load_patch_target(struct strbuf *buf,
-			     struct cache_entry *ce,
+			     const struct cache_entry *ce,
 			     struct stat *st,
 			     const char *name,
 			     unsigned expected_mode)
@@ -3160,7 +3160,8 @@ static int load_patch_target(struct strbuf *buf,
  * we read from the result of a previous diff.
  */
 static int load_preimage(struct image *image,
-			 struct patch *patch, struct stat *st, struct cache_entry *ce)
+			 struct patch *patch, struct stat *st,
+			 const struct cache_entry *ce)
 {
 	struct strbuf buf = STRBUF_INIT;
 	size_t len;
@@ -3273,7 +3274,7 @@ static int load_current(struct image *image, struct patch *patch)
 }
 
 static int try_threeway(struct image *image, struct patch *patch,
-			struct stat *st, struct cache_entry *ce)
+			struct stat *st, const struct cache_entry *ce)
 {
 	unsigned char pre_sha1[20], post_sha1[20], our_sha1[20];
 	struct strbuf buf = STRBUF_INIT;
@@ -3343,7 +3344,7 @@ static int try_threeway(struct image *image, struct patch *patch,
 	return 0;
 }
 
-static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *ce)
+static int apply_data(struct patch *patch, struct stat *st, const struct cache_entry *ce)
 {
 	struct image image;