This bit is basically "dynamic CE_VALID". It marks entries that are
being watched by the incoming file-watcher. When an index is loaded,
file watcher is contacted and the list of updated paths is retrieved.
These paths will have CE_WATCHED cleared and lstat() will be called on
them. Those that have CE_WATCHED and not in the list will have
CE_VALID turn on to skip lstat(). The setting is temporarily, CE_VALID
is not saved to disk if CE_WATCHED is also set.
We keep the CE_WATCHED in a new extension, separated from the entries
to save some space because extended ce_flags adds 2 bytes per entry
and this flag would be present in the majority of entries. When stored
as bitmap, this extension could compress very well with ewah algorithm.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx>
---
Documentation/technical/index-format.txt | 6 +++++
cache.h | 3 +++
read-cache.c | 41 +++++++++++++++++++++++++++++++-
3 files changed, 49 insertions(+), 1 deletion(-)
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index f352a9b..24fd0ae 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -198,3 +198,9 @@ Git index format
- At most three 160-bit object names of the entry in stages from 1 to 3
(nothing is written for a missing stage).
+=== File watcher
+
+ The signature of this extension is { 'W', 'A', 'T', 'C' }.
+
+ - A bit map of all entries in the index, n-th bit of m-th byte
+ corresponds to CE_WATCHED of the <m * 8+ n>-th index entry.
diff --git a/cache.h b/cache.h
index f14d535..a0af2a5 100644
--- a/cache.h
+++ b/cache.h
@@ -169,6 +169,9 @@ struct cache_entry {
/* used to temporarily mark paths matched by pathspecs */
#define CE_MATCHED (1 << 26)
+/* set CE_VALID at runtime if the entry is guaranteed not updated */
+#define CE_WATCHED (1 << 27)
+
/*
* Extended on-disk flags
*/
diff --git a/read-cache.c b/read-cache.c
index 3b6daf1..098d3b6 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -33,6 +33,7 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
#define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
#define CACHE_EXT_TREE 0x54524545 /* "TREE" */
#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
+#define CACHE_EXT_WATCH 0x57415443 /* "WATC" */
struct index_state the_index;
@@ -1289,6 +1290,19 @@ static int verify_hdr(struct cache_header *hdr,
return 0;
}
+static void read_watch_extension(struct index_state *istate, uint8_t *data,
+ unsigned long sz)
+{
+ int i;
+ if ((istate->cache_nr + 7) / 8 != sz) {
+ error("invalid 'WATC' extension");
+ return;
+ }
+ for (i = 0; i < istate->cache_nr; i++)
+ if (data[i / 8] & (1 << (i % 8)))
+ istate->cache[i]->ce_flags |= CE_WATCHED;
+}
+
static int read_index_extension(struct index_state *istate,
const char *ext, void *data, unsigned long sz)
{
@@ -1299,6 +1313,9 @@ static int read_index_extension(struct index_state *istate,
case CACHE_EXT_RESOLVE_UNDO:
istate->resolve_undo = resolve_undo_read(data, sz);
break;
+ case CACHE_EXT_WATCH:
+ read_watch_extension(istate, data, sz);
+ break;
default:
if (*ext < 'A' || 'Z' < *ext)
return error("index uses %.4s extension, which we do not understand",
@@ -1777,7 +1794,7 @@ int write_index(struct index_state *istate, int newfd)
{
git_SHA_CTX c;
struct cache_header hdr;
- int i, err, removed, extended, hdr_version;
+ int i, err, removed, extended, hdr_version, has_watches = 0;
struct cache_entry **cache = istate->cache;
int entries = istate->cache_nr;
struct stat st;
@@ -1786,6 +1803,8 @@ int write_index(struct index_state *istate, int newfd)
for (i = removed = extended = 0; i < entries; i++) {
if (cache[i]->ce_flags & CE_REMOVE)
removed++;
+ else if (cache[i]->ce_flags & CE_WATCHED)
+ has_watches++;
/* reduce extended entries if possible */
cache[i]->ce_flags &= ~CE_EXTENDED;
@@ -1857,6 +1876,26 @@ int write_index(struct index_state *istate, int newfd)
if (err)
return -1;
}
+ if (has_watches) {
+ int id, sz = (entries - removed + 7) / 8;
+ uint8_t *data = xmalloc(sz);
+ memset(data, 0, sz);
+ for (i = 0, id = 0; i < entries && has_watches; i++) {
+ struct cache_entry *ce = cache[i];
+ if (ce->ce_flags & CE_REMOVE)
+ continue;
+ if (ce->ce_flags & CE_WATCHED) {
+ data[id / 8] |= 1 << (id % 8);
+ has_watches--;
+ }
+ id++;
+ }
+ err = write_index_ext_header(&c, newfd, CACHE_EXT_WATCH, sz) < 0
+ || ce_write(&c, newfd, data, sz) < 0;
+ free(data);
+ if (err)
+ return -1;
+ }
if (ce_flush(&c, newfd) || fstat(newfd, &st))
return -1;
--
1.8.5.2.240.g8478abd
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