add infrastructure for read-only file system level caches

Add a macro to mark code sections that only read from the file system,
along with a config option and documentation.

This facilitates implementation of relatively simple file system level
caches without the need to synchronize with the file system.

Enable read-only sections for 'git status' and preload_index.

Signed-off-by: Karsten Blees <blees@dcon.de>

Documentation/config.txt

@@ -631,6 +631,12 @@ relatively high IO latencies.  When enabled, Git will do the
 index comparison to the filesystem data in parallel, allowing
 overlapping IO's.  Defaults to true.
 
+core.fscache::
+	Enable additional caching of file system data for some operations.
++
+Git for Windows uses this to bulk-read and cache lstat data of entire
+directories (instead of doing lstat file by file).
+
 core.createObject::
 	You can set this to 'link', in which case a hardlink followed by
 	a delete of the source are used to make sure that object creation

builtin/commit.c

@@ -1368,6 +1368,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 		       PATHSPEC_PREFER_FULL,
 		       prefix, argv);
 
+	enable_fscache(1);
 	read_cache_preload(&s.pathspec);
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, &s.pathspec, NULL, NULL);
 

cache.h

@@ -648,6 +648,8 @@ enum hide_dotfiles_type {
 };
 extern enum hide_dotfiles_type hide_dotfiles;
 
+extern int core_fscache;
+
 enum branch_track {
 	BRANCH_TRACK_UNSPECIFIED = -1,
 	BRANCH_TRACK_NEVER = 0,

config.c

@@ -881,6 +881,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.fscache")) {
+		core_fscache = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }

environment.c

@@ -64,6 +64,7 @@ int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 struct startup_info *startup_info;
 unsigned long pack_size_limit_cfg;
 enum hide_dotfiles_type hide_dotfiles = HIDE_DOTFILES_DOTGITONLY;
+int core_fscache;
 
 /*
  * The character that begins a commented line in user-editable file

git-compat-util.h

@@ -794,4 +794,19 @@ struct tm *git_gmtime_r(const time_t *, struct tm *);
 #define mark_as_git_dir(x) /* noop */
 #endif
 
+/*
+ * Enable/disable a read-only cache for file system data on platforms that
+ * support it.
+ *
+ * Implementing a live-cache is complicated and requires special platform
+ * support (inotify, ReadDirectoryChangesW...). enable_fscache shall be used
+ * to mark sections of git code that extensively read from the file system
+ * without modifying anything. Implementations can use this to cache e.g. stat
+ * data or even file content without the need to synchronize with the file
+ * system.
+ */
+#ifndef enable_fscache
+#define enable_fscache(x) /* noop */
+#endif
+
 #endif

preload-index.c

@@ -84,6 +84,7 @@ static void preload_index(struct index_state *index,
 	offset = 0;
 	work = DIV_ROUND_UP(index->cache_nr, threads);
 	memset(&data, 0, sizeof(data));
+	enable_fscache(1);
 	for (i = 0; i < threads; i++) {
 		struct thread_data *p = data+i;
 		p->index = index;
@@ -100,6 +101,7 @@ static void preload_index(struct index_state *index,
 		if (pthread_join(p->pthread, NULL))
 			die("unable to join threaded lstat");
 	}
+	enable_fscache(0);
 }
 #endif