diff: cache textconv output

Running a textconv filter can take a long time. It's
particularly bad for a large file which needs to be spooled
to disk, but even for small files, the fork+exec overhead
can add up for something like "git log -p".

This patch uses the notes-cache mechanism to keep a fast
cache of textconv output. Caches are stored in
refs/notes/textconv/$x, where $x is the userdiff driver
defined in gitattributes.

Caching is enabled only if diff.$x.cachetextconv is true.

In my test repo, on a commit with 45 jpg and avi files
changed and a textconv to show their exif tags:

  [before]
  $ time git show >/dev/null
  real    0m13.724s
  user    0m12.057s
  sys     0m1.624s

  [after, first run]
  $ git config diff.mfo.cachetextconv true
  $ time git show >/dev/null
  real    0m14.252s
  user    0m12.197s
  sys     0m1.800s

  [after, subsequent runs]
  $ time git show >/dev/null
  real    0m0.352s
  user    0m0.148s
  sys     0m0.200s

So for a slight (3.8%) cost on the first run, we achieve an
almost 40x speed up on subsequent runs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/userdiff.c b/userdiff.c
index df992490d..67003fbb2 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -1,3 +1,4 @@
+#include "cache.h"
 #include "userdiff.h"
 #include "cache.h"
 #include "attr.h"
@@ -167,6 +168,12 @@ static int parse_tristate(int *b, const char *k, const char *v)
 	return 1;
 }
 
+static int parse_bool(int *b, const char *k, const char *v)
+{
+	*b = git_config_bool(k, v);
+	return 1;
+}
+
 int userdiff_config(const char *k, const char *v)
 {
 	struct userdiff_driver *drv;
@@ -181,6 +188,8 @@ int userdiff_config(const char *k, const char *v)
 		return parse_string(&drv->external, k, v);
 	if ((drv = parse_driver(k, v, "textconv")))
 		return parse_string(&drv->textconv, k, v);
+	if ((drv = parse_driver(k, v, "cachetextconv")))
+		return parse_bool(&drv->textconv_want_cache, k, v);
 	if ((drv = parse_driver(k, v, "wordregex")))
 		return parse_string(&drv->word_regex, k, v);