Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 Documentation/config.txt |  4 ++++
 cache.h                  |  1 +
 config.c                 | 12 ++++++++++++
 environment.c            |  1 +
 read-cache.c             | 43 +++++++++++++++++++++++++++++++++++++++++++
 submodule.c              |  1 +
 6 files changed, 62 insertions(+)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index d8b6cc9..ccbe00b 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt

@@ -617,6 +617,10 @@ relatively high IO latencies.  With this set to 'true', Git will do the
 index comparison to the filesystem data in parallel, allowing
 overlapping IO's.

+core.useReadCacheDaemon::
+       Use `git read-cache--daemon` to speed up index reading. See
+       linkgit:git-read-cache--daemon for more information.
+
 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

diff --git a/cache.h b/cache.h
index d0ff11c..fb29c7e 100644
--- a/cache.h
+++ b/cache.h

@@ -603,6 +603,7 @@ extern size_t packed_git_limit;
 extern size_t delta_base_cache_limit;
 extern unsigned long big_file_threshold;
 extern unsigned long pack_size_limit_cfg;
+extern int use_read_cache_daemon;

 /*
  * Do replace refs need to be checked this run?  This variable is

diff --git a/config.c b/config.c
index a30cb5c..5c832ad 100644
--- a/config.c
+++ b/config.c

@@ -874,6 +874,18 @@ static int git_default_core_config(const char *var, const char *value)
                return 0;
        }

+#ifdef HAVE_SHM
+       /*
+        * Currently git-read-cache--daemon is only built when
+        * HAVE_SHM is set. Ignore user settings if HAVE_SHM is not
+        * defined.
+        */
+       if (!strcmp(var, "core.usereadcachedaemon")) {
+               use_read_cache_daemon = git_config_bool(var, value);
+               return 0;
+       }
+#endif
+
        /* Add other config variables here and to Documentation/config.txt. */
        return 0;
 }

diff --git a/environment.c b/environment.c
index 5c4815d..b76a414 100644
--- a/environment.c
+++ b/environment.c

@@ -63,6 +63,7 @@ int merge_log_config = -1;
 int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 struct startup_info *startup_info;
 unsigned long pack_size_limit_cfg;
+int use_read_cache_daemon;

 /*
  * The character that begins a commented line in user-editable file

diff --git a/read-cache.c b/read-cache.c
index a5031f3..0e46523 100644
--- a/read-cache.c
+++ b/read-cache.c

@@ -1462,6 +1462,48 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
        return ce;
 }

+static void *try_shm(void *mmap, size_t *mmap_size)
+{
+       struct strbuf sb = STRBUF_INIT;
+       size_t old_size = *mmap_size;
+       void *new_mmap;
+       struct stat st;
+       int fd;
+
+       if (old_size <= 20 || !use_read_cache_daemon)
+               return mmap;
+
+       strbuf_addf(&sb, "/git-index-%s.lock",
+                   sha1_to_hex((unsigned char *)mmap + old_size - 20));
+       fd = shm_open(sb.buf, O_RDONLY, 0777);