odb: teach read_blob_entry to use size_t

There is mixed use of size_t and unsigned long to deal with sizes in the
codebase. Recall that Windows defines unsigned long as 32 bits even on
64-bit platforms, meaning that converting size_t to unsigned long narrows
the range. This mostly doesn't cause a problem since Git rarely deals
with files larger than 2^32 bytes.

But adjunct systems such as Git LFS, which use smudge/clean filters to
keep huge files out of the repository, may have huge file contents passed
through some of the functions in entry.c and convert.c. On Windows, this
results in a truncated file being written to the workdir. I traced this to
one specific use of unsigned long in write_entry (and a similar instance
in write_pc_item_to_fd for parallel checkout). That appeared to be for
the call to read_blob_entry, which expects a pointer to unsigned long.

By altering the signature of read_blob_entry to expect a size_t,
write_entry can be switched to use size_t internally (which all of its
callers and most of its callees already used). To avoid touching dozens of
additional files, read_blob_entry uses a local unsigned long to call a
chain of functions which aren't prepared to accept size_t.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Matt Cooper <vtbassmatt@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

entry.c

@@ -82,11 +82,13 @@ static int create_file(const char *path, unsigned int mode)
 	return open(path, O_WRONLY | O_CREAT | O_EXCL, mode);
 }
 
-void *read_blob_entry(const struct cache_entry *ce, unsigned long *size)
+void *read_blob_entry(const struct cache_entry *ce, size_t *size)
 {
 	enum object_type type;
-	void *blob_data = read_object_file(&ce->oid, &type, size);
+	unsigned long ul;
+	void *blob_data = read_object_file(&ce->oid, &type, &ul);
 
+	*size = ul;
 	if (blob_data) {
 		if (type == OBJ_BLOB)
 			return blob_data;
@@ -270,7 +272,7 @@ static int write_entry(struct cache_entry *ce, char *path, struct conv_attrs *ca
 	int fd, ret, fstat_done = 0;
 	char *new_blob;
 	struct strbuf buf = STRBUF_INIT;
-	unsigned long size;
+	size_t size;
 	ssize_t wrote;
 	size_t newsize = 0;
 	struct stat st;