[PATCH/RFC v1 6/6] lstat_cache(): print a warning if doing ping-pong between cache types

[Kjetil Barvik <barvik@broadpark.no>]

This is a debug patch which is only to be used while the lstat_cache()
is in the test stage, and should be removed/reverted before the final
relase.

I think it should be useful to catch these warnings, as I it could be
an indication of that the cache would not be very effective if it is
doing ping-pong by switching between different cache types too many
times.

Also, if someone is experimenting with the lstat_cache(), this patch
will maybe be useful while debugging.

If someone is able to trigger the warning, then send a mail to the GIT
mailing list, containing the first 15 lines of the warning, and a
short description of the GIT commands to trigger the warnings.

This patch is against the 'next' branch.  I hope someone is willing to
use this patch for a while, to be able to catch possible ping-pong's.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
---

Hmmm, it seems that 'git send-email' always fails for this mail, with
the following error message:

   Died at /usr/local/libexec/git-core/git-send-email line 882.

So I send this as a separate command from 'git send-email', sorry
abouth that.


 symlinks.c |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+), 0 deletions(-)

diff --git a/symlinks.c b/symlinks.c
index 81f490c..1c07ae0 100644
--- a/symlinks.c
+++ b/symlinks.c
@@ -49,6 +49,11 @@ static inline void reset_lstat_cache(int track_flags, int prefix_len_stat_func)
 	cache.prefix_len_stat_func = prefix_len_stat_func;
 }
 
+#define SWITCHES_BEFORE_WARNING 10
+static unsigned int cache_switches = 0, number_of_warnings = 0;
+static unsigned int current_cache_func = 0, last_cache_func = 0;
+static unsigned int total_calls = 0;
+
 #define FL_DIR      (1 << 0)
 #define FL_NOENT    (1 << 1)
 #define FL_SYMLINK  (1 << 2)
@@ -75,6 +80,7 @@ static int lstat_cache(int len, const char *name,
 	int match_flags, ret_flags, save_flags, max_len, ret;
 	struct stat st;
 
+	total_calls++;
 	if (cache.track_flags != track_flags ||
 	    cache.prefix_len_stat_func != prefix_len_stat_func) {
 		/*
@@ -84,6 +90,17 @@ static int lstat_cache(int len, const char *name,
 		 */
 		reset_lstat_cache(track_flags, prefix_len_stat_func);
 		match_len = last_slash = 0;
+		cache_switches++;
+		if (cache_switches > SWITCHES_BEFORE_WARNING) {
+			if (number_of_warnings < 10 || number_of_warnings % 1000 == 0)
+				printf("warning from %s:%d cache_switches:%u > %u "\
+				       "(current:%u last:%u total:%u)\n",
+				       __FILE__, __LINE__,
+				       cache_switches, SWITCHES_BEFORE_WARNING,
+				       current_cache_func, last_cache_func,
+				       total_calls);
+			number_of_warnings++;
+		}
 	} else {
 		/*
 		 * Check to see if we have a match from the cache for
@@ -211,6 +228,8 @@ void clear_lstat_cache(void)
  */
 int has_symlink_leading_path(int len, const char *name)
 {
+	last_cache_func = current_cache_func;
+	current_cache_func = 1;
 	return lstat_cache(len, name,
 			   FL_SYMLINK|FL_DIR, USE_ONLY_LSTAT) &
 		FL_SYMLINK;
@@ -222,6 +241,8 @@ int has_symlink_leading_path(int len, const char *name)
  */
 int has_symlink_or_noent_leading_path(int len, const char *name)
 {
+	last_cache_func = current_cache_func;
+	current_cache_func = 2;
 	return lstat_cache(len, name,
 			   FL_SYMLINK|FL_NOENT|FL_DIR, USE_ONLY_LSTAT) &
 		(FL_SYMLINK|FL_NOENT);
@@ -236,6 +257,8 @@ int has_symlink_or_noent_leading_path(int len, const char *name)
  */
 int has_dirs_only_path(int len, const char *name, int prefix_len)
 {
+	last_cache_func = current_cache_func;
+	current_cache_func = 3;
 	return lstat_cache(len, name,
 			   FL_DIR|FL_FULLPATH, prefix_len) &
 		FL_DIR;
-- 
1.6.1.349.g99fa5