fsmonitor: teach git to optionally utilize a file system monitor to speed up detecting new or changed files.

When the index is read from disk, the fsmonitor index extension is used
to flag the last known potentially dirty index entries. The registered
core.fsmonitor command is called with the time the index was last
updated and returns the list of files changed since that time. This list
is used to flag any additional dirty cache entries and untracked cache
directories.

We can then use this valid state to speed up preload_index(),
ie_match_stat(), and refresh_cache_ent() as they do not need to lstat()
files to detect potential changes for those entries marked
CE_FSMONITOR_VALID.

In addition, if the untracked cache is turned on valid_cached_dir() can
skip checking directories for new or changed files as fsmonitor will
invalidate the cache only for those directories that have been
identified as having potential changes.

To keep the CE_FSMONITOR_VALID state accurate during git operations;
when git updates a cache entry to match the current state on disk,
it will now set the CE_FSMONITOR_VALID bit.

Inversely, anytime git changes a cache entry, the CE_FSMONITOR_VALID bit
is cleared and the corresponding untracked cache directory is marked
invalid.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 40 insertions, 5 deletions

diff --git a/read-cache.c b/read-cache.c
index 40da87ea7..05c0a33fd 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -19,6 +19,7 @@
 #include "varint.h"
 #include "split-index.h"
 #include "utf8.h"
+#include "fsmonitor.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -38,11 +39,12 @@
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
 #define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
+#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -62,6 +64,7 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 	free(old);
 	set_index_entry(istate, nr, ce);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 }
 
@@ -150,8 +153,10 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 	if (assume_unchanged)
 		ce->ce_flags |= CE_VALID;
 
-	if (S_ISREG(st->st_mode))
+	if (S_ISREG(st->st_mode)) {
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
+	}
 }
 
 static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
@@ -300,7 +305,7 @@ int match_stat_data_racy(const struct index_state *istate,
 	return match_stat_data(sd, st);
 }
 
-int ie_match_stat(const struct index_state *istate,
+int ie_match_stat(struct index_state *istate,
 		  const struct cache_entry *ce, struct stat *st,
 		  unsigned int options)
 {
@@ -308,7 +313,10 @@ int ie_match_stat(const struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * If it's marked as always valid in the index, it's
 	 * valid whatever the checked-out copy says.
@@ -319,6 +327,8 @@ int ie_match_stat(const struct index_state *istate,
 		return 0;
 	if (!ignore_valid && (ce->ce_flags & CE_VALID))
 		return 0;
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
+		return 0;
 
 	/*
 	 * Intent-to-add entries have not been added, so the index entry
@@ -356,7 +366,7 @@ int ie_match_stat(const struct index_state *istate,
 	return changed;
 }
 
-int ie_modified(const struct index_state *istate,
+int ie_modified(struct index_state *istate,
 		const struct cache_entry *ce,
 		struct stat *st, unsigned int options)
 {
@@ -777,6 +787,7 @@ int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
 	}
 	cache_tree_invalidate_path(istate, ce->name);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 
 	return 0;
@@ -1228,10 +1239,13 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
 	if (!refresh || ce_uptodate(ce))
 		return ce;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * CE_VALID or CE_SKIP_WORKTREE means the user promised us
 	 * that the change to the work tree does not matter and told
@@ -1245,6 +1259,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 		ce_mark_uptodate(ce);
 		return ce;
 	}
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
+		ce_mark_uptodate(ce);
+		return ce;
+	}
 
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
 		if (ignore_missing)
@@ -1282,8 +1300,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 			 * because CE_UPTODATE flag is in-core only;
 			 * we are not going to write this change out.
 			 */
-			if (!S_ISGITLINK(ce->ce_mode))
+			if (!S_ISGITLINK(ce->ce_mode)) {
 				ce_mark_uptodate(ce);
+				mark_fsmonitor_valid(ce);
+			}
 			return ce;
 		}
 	}
@@ -1391,6 +1411,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 				 */
 				ce->ce_flags &= ~CE_VALID;
 				ce->ce_flags |= CE_UPDATE_IN_BASE;
+				mark_fsmonitor_invalid(istate, ce);
 				istate->cache_changed |= CE_ENTRY_CHANGED;
 			}
 			if (quiet)
@@ -1550,6 +1571,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_UNTRACKED:
 		istate->untracked = read_untracked_extension(data, sz);
 		break;
+	case CACHE_EXT_FSMONITOR:
+		read_fsmonitor_extension(istate, data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1722,6 +1746,7 @@ static void post_read_index_from(struct index_state *istate)
 	check_ce_order(istate);
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
+	tweak_fsmonitor(istate);
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2306,6 +2331,16 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->fsmonitor_last_update) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_fsmonitor_extension(&sb, istate);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1))
 		return -1;