[PATCH v2 2/3] repo-settings: use index.version=4 by default

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

If a repo is large, it likely has many paths in its working directory.
This means the index could be compressed using version 4. Set this as
a default when core.featureAdoptionRate is at least three.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/config/core.txt  |  3 +++
 Documentation/config/index.txt |  2 ++
 read-cache.c                   | 12 +++++++-----
 repo-settings.c                |  6 ++++++
 repo-settings.h                |  1 +
 5 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index 6a9f707815..d16503a9d7 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -620,3 +620,6 @@ not modify the user-facing output of porcelain commands.
 +
 * `gc.writeCommitGraph=true` eneables writing commit-graph files during
 `git gc`.
++
+* `index.version=4` uses prefix-compression to reduce the size of the
+.git/index file.
diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt
index f181503041..98a88c30be 100644
--- a/Documentation/config/index.txt
+++ b/Documentation/config/index.txt
@@ -24,3 +24,5 @@ index.threads::
 index.version::
 	Specify the version with which new index files should be
 	initialized.  This does not affect existing repositories.
+	If `core.featureAdoptionRate` is at least three, then the
+	default value is 4.
diff --git a/read-cache.c b/read-cache.c
index 22e7b9944e..7fab8ff748 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -25,6 +25,7 @@
 #include "fsmonitor.h"
 #include "thread-utils.h"
 #include "progress.h"
+#include "repo-settings.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -1599,16 +1600,17 @@ struct cache_entry *refresh_cache_entry(struct index_state *istate,
 
 #define INDEX_FORMAT_DEFAULT 3
 
-static unsigned int get_index_format_default(void)
+static unsigned int get_index_format_default(struct repository *r)
 {
 	char *envversion = getenv("GIT_INDEX_VERSION");
 	char *endp;
-	int value;
 	unsigned int version = INDEX_FORMAT_DEFAULT;
 
 	if (!envversion) {
-		if (!git_config_get_int("index.version", &value))
-			version = value;
+		prepare_repo_settings(r);
+
+		if (r->settings->index_version >= 0)
+			version = r->settings->index_version;
 		if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
 			warning(_("index.version set, but the value is invalid.\n"
 				  "Using version %i"), INDEX_FORMAT_DEFAULT);
@@ -2765,7 +2767,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	}
 
 	if (!istate->version) {
-		istate->version = get_index_format_default();
+		istate->version = get_index_format_default(the_repository);
 		if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
 			init_split_index(istate);
 	}
diff --git a/repo-settings.c b/repo-settings.c
index f7fc2a1959..5753153a84 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -14,6 +14,7 @@ static int git_repo_config(const char *key, const char *value, void *cb)
 		if (rate >= 3) {
 			UPDATE_DEFAULT(rs->core_commit_graph, 1);
 			UPDATE_DEFAULT(rs->gc_write_commit_graph, 1);
+			UPDATE_DEFAULT(rs->index_version, 4);
 		}
 		return 0;
 	}
@@ -25,6 +26,10 @@ static int git_repo_config(const char *key, const char *value, void *cb)
 		rs->gc_write_commit_graph = git_config_bool(key, value);
 		return 0;
 	}
+	if (!strcmp(key, "index.version")) {
+		rs->index_version = git_config_int(key, value);
+		return 0;
+	}
 
 	return 1;
 }
@@ -39,6 +44,7 @@ void prepare_repo_settings(struct repository *r)
 	/* Defaults */
 	r->settings->core_commit_graph = -1;
 	r->settings->gc_write_commit_graph = -1;
+	r->settings->index_version = -1;
 
 	repo_config(r, git_repo_config, r->settings);
 }
diff --git a/repo-settings.h b/repo-settings.h
index 11d08648e1..9b8104042e 100644
--- a/repo-settings.h
+++ b/repo-settings.h
@@ -4,6 +4,7 @@
 struct repo_settings {
 	char core_commit_graph;
 	char gc_write_commit_graph;
+	int index_version;
 };
 
 struct repository;
-- 
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