read-cache.c: read prefix-compressed names in index on-disk version v4

Because the entries are sorted by path, adjacent entries in the index tend
to share the leading components of them, and it makes sense to only store
the differences in later entries.  In the v4 on-disk format of the index,
each on-disk cache entry stores the number of bytes to be stripped from
the end of the previous name, and the bytes to append to the result, to
come up with its name.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -12,6 +12,8 @@
 #include "commit.h"
 #include "blob.h"
 #include "resolve-undo.h"
+#include "strbuf.h"
+#include "varint.h"
 
 static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int really);
 
@@ -1236,6 +1238,7 @@ struct ondisk_cache_entry_extended {
 	char name[FLEX_ARRAY]; /* more */
 };
 
+/* These are only used for v3 or lower */
 #define align_flex_name(STRUCT,len) ((offsetof(struct STRUCT,name) + (len) + 8) & ~7)
 #define ondisk_cache_entry_size(len) align_flex_name(ondisk_cache_entry,len)
 #define ondisk_cache_entry_extended_size(len) align_flex_name(ondisk_cache_entry_extended,len)
@@ -1252,7 +1255,7 @@ static int verify_hdr(struct cache_header *hdr, unsigned long size)
 	if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
 		return error("bad signature");
 	hdr_version = ntohl(hdr->hdr_version);
-	if (hdr_version < 2 || 3 < hdr_version)
+	if (hdr_version < 2 || 4 < hdr_version)
 		return error("bad index version %d", hdr_version);
 	git_SHA1_Init(&c);
 	git_SHA1_Update(&c, hdr, size - 20);
@@ -1331,8 +1334,30 @@ static struct cache_entry *cache_entry_from_ondisk(struct ondisk_cache_entry *on
 	return ce;
 }
 
+/*
+ * Adjacent cache entries tend to share the leading paths, so it makes
+ * sense to only store the differences in later entries.  In the v4
+ * on-disk format of the index, each on-disk cache entry stores the
+ * number of bytes to be stripped from the end of the previous name,
+ * and the bytes to append to the result, to come up with its name.
+ */
+static unsigned long expand_name_field(struct strbuf *name, const char *cp_)
+{
+	const unsigned char *ep, *cp = (const unsigned char *)cp_;
+	size_t len = decode_varint(&cp);
+
+	if (name->len < len)
+		die("malformed name field in the index");
+	strbuf_remove(name, name->len - len, len);
+	for (ep = cp; *ep; ep++)
+		; /* find the end */
+	strbuf_add(name, cp, ep - cp);
+	return (const char *)ep + 1 - cp_;
+}
+
 static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
-					    unsigned long *ent_size)
+					    unsigned long *ent_size,
+					    struct strbuf *previous_name)
 {
 	struct cache_entry *ce;
 	size_t len;
@@ -1357,10 +1382,22 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
 	else
 		name = ondisk->name;
 
-	if (len == CE_NAMEMASK)
-		len = strlen(name);
-	ce = cache_entry_from_ondisk(ondisk, flags, name, len);
-	*ent_size = ondisk_ce_size(ce);
+	if (!previous_name) {
+		/* v3 and earlier */
+		if (len == CE_NAMEMASK)
+			len = strlen(name);
+		ce = cache_entry_from_ondisk(ondisk, flags, name, len);
+
+		*ent_size = ondisk_ce_size(ce);
+	} else {
+		unsigned long consumed;
+		consumed = expand_name_field(previous_name, name);
+		ce = cache_entry_from_ondisk(ondisk, flags,
+					     previous_name->buf,
+					     previous_name->len);
+
+		*ent_size = (name - ((char *)ondisk)) + consumed;
+	}
 	return ce;
 }
 
@@ -1373,6 +1410,7 @@ int read_index_from(struct index_state *istate, const char *path)
 	struct cache_header *hdr;
 	void *mmap;
 	size_t mmap_size;
+	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
 
 	errno = EBUSY;
 	if (istate->initialized)
@@ -1410,18 +1448,24 @@ int read_index_from(struct index_state *istate, const char *path)
 	istate->cache = xcalloc(istate->cache_alloc, sizeof(struct cache_entry *));
 	istate->initialized = 1;
 
+	if (hdr->hdr_version == htonl(4))
+		previous_name = &previous_name_buf;
+	else
+		previous_name = NULL;
+
 	src_offset = sizeof(*hdr);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct ondisk_cache_entry *disk_ce;
 		struct cache_entry *ce;
 		unsigned long consumed;
 
 		disk_ce = (struct ondisk_cache_entry *)((char *)mmap + src_offset);
-		ce = create_from_disk(disk_ce, &consumed);
+		ce = create_from_disk(disk_ce, &consumed, previous_name);
 		set_index_entry(istate, i, ce);
 
 		src_offset += consumed;
 	}
+	strbuf_release(&previous_name_buf);
 	istate->timestamp.sec = st.st_mtime;
 	istate->timestamp.nsec = ST_MTIME_NSEC(st);