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>

Author: kblees

Documentation/config.txt

@@ -909,6 +909,12 @@ 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.unsetenvvars::
 	EXPERIMENTAL, Windows-only: comma-separated list of environment
 	variables' names that need to be unset before spawning any other

builtin/commit.c

@@ -1355,6 +1355,7 @@ 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);
 

compat/mingw.c

@@ -212,6 +212,7 @@ enum hide_dotfiles_type {
 
 static enum hide_dotfiles_type hide_dotfiles = HIDE_DOTFILES_DOTGITONLY;
 static char *unset_environment_variables;
+int core_fscache;
 
 int mingw_core_config(const char *var, const char *value, void *cb)
 {
@@ -223,6 +224,11 @@ int mingw_core_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.fscache")) {
+		core_fscache = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.unsetenvvars")) {
 		free(unset_environment_variables);
 		unset_environment_variables = xstrdup(value);

compat/mingw.h

@@ -11,6 +11,8 @@ typedef _sigset_t sigset_t;
 #undef _POSIX_THREAD_SAFE_FUNCTIONS
 #endif
 
+extern int core_fscache;
+
 extern int mingw_core_config(const char *var, const char *value, void *cb);
 #define platform_core_config mingw_core_config
 

git-compat-util.h

@@ -1234,6 +1234,21 @@ static inline int is_missing_file_error(int errno_)
 	return (errno_ == ENOENT || errno_ == ENOTDIR);
 }
 
+/*
+ * 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
+
 extern int cmd_main(int, const char **);
 
 /*

preload-index.c

@@ -93,6 +93,7 @@ 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;
@@ -110,6 +111,7 @@ static void preload_index(struct index_state *index,
 			die("unable to join threaded lstat");
 	}
 	trace_performance_since(start, "preload index");
+	enable_fscache(0);
 }
 #endif