[PATCH v5 5/5] core.fsync: documentation and user-friendly aggregate options

["Neeraj Singh via GitGitGadget" <gitgitgadget@gmail.com>]

From: Neeraj Singh <neerajsi@microsoft.com>

This commit adds aggregate options for the core.fsync setting that are
more user-friendly. These options are specified in terms of 'levels of
safety', indicating which Git operations are considered to be sync
points for durability.

The new documentation is also included here in its entirety for ease of
review.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
---
 Documentation/config/core.txt | 36 +++++++++++++++++++++++++++++++++++
 cache.h                       | 23 +++++++++++++++++++---
 config.c                      |  6 ++++++
 3 files changed, 62 insertions(+), 3 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index 74399072843..973805e8a98 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -547,6 +547,42 @@ core.whitespace::
   is relevant for `indent-with-non-tab` and when Git fixes `tab-in-indent`
   errors. The default tab width is 8. Allowed values are 1 to 63.
 
+core.fsync::
+	A comma-separated list of parts of the repository which should be
+	hardened via the core.fsyncMethod when created or modified. You can
+	disable hardening of any component by prefixing it with a '-'. Later
+	items take precedence over earlier ones in the comma-separated list.
+	For example, `core.fsync=all,-pack-metadata` means "harden everything
+	except pack metadata." Items that are not hardened may be lost in the
+	event of an unclean system shutdown. Unless you have special
+	requirements, it is recommended that you leave this option as default
+	or pick one of `committed`, `added`, or `all`.
++
+* `none` disables fsync completely. This value must be specified alone.
+* `loose-object` hardens objects added to the repo in loose-object form.
+* `pack` hardens objects added to the repo in packfile form.
+* `pack-metadata` hardens packfile bitmaps and indexes.
+* `commit-graph` hardens the commit graph file.
+* `index` hardens the index when it is modified.
+* `objects` is an aggregate option that is equivalent to
+  `loose-object,pack`.
+* `derived-metadata` is an aggregate option that is equivalent to
+  `pack-metadata,commit-graph`.
+* `default` is an aggregate option that is equivalent to
+  `objects,derived-metadata,-loose-object`. This mode is enabled by default.
+  It has good performance, but risks losing recent work if the system shuts
+  down uncleanly, since commits, trees, and blobs in loose-object form may be
+  lost.
+* `committed` is an aggregate option that is currently equivalent to
+  `objects`. This mode sacrifices some performance to ensure that all work
+  that is committed to the repository with `git commit` or similar commands
+  is preserved.
+* `added` is an aggregate option that is currently equivalent to
+  `committed,index`. This mode sacrifices additional performance to
+  ensure that the results of commands like `git add` and similar operations
+  are preserved.
+* `all` is an aggregate option that syncs all individual components above.
+
 core.fsyncMethod::
 	A value indicating the strategy Git will use to harden repository data
 	using fsync and related primitives.
diff --git a/cache.h b/cache.h
index 3fefe8f6f60..833f0236e68 100644
--- a/cache.h
+++ b/cache.h
@@ -1007,9 +1007,26 @@ enum fsync_component {
 	FSYNC_COMPONENT_INDEX			= 1 << 4,
 };
 
-#define FSYNC_COMPONENTS_DEFAULT (FSYNC_COMPONENT_PACK | \
-				  FSYNC_COMPONENT_PACK_METADATA | \
-				  FSYNC_COMPONENT_COMMIT_GRAPH)
+#define FSYNC_COMPONENTS_OBJECTS (FSYNC_COMPONENT_LOOSE_OBJECT | \
+				  FSYNC_COMPONENT_PACK)
+
+#define FSYNC_COMPONENTS_DERIVED_METADATA (FSYNC_COMPONENT_PACK_METADATA | \
+					   FSYNC_COMPONENT_COMMIT_GRAPH)
+
+#define FSYNC_COMPONENTS_DEFAULT (FSYNC_COMPONENTS_OBJECTS | \
+				  FSYNC_COMPONENTS_DERIVED_METADATA | \
+				  ~FSYNC_COMPONENT_LOOSE_OBJECT)
+
+#define FSYNC_COMPONENTS_COMMITTED (FSYNC_COMPONENTS_OBJECTS)
+
+#define FSYNC_COMPONENTS_ADDED (FSYNC_COMPONENTS_COMMITTED | \
+				FSYNC_COMPONENT_INDEX)
+
+#define FSYNC_COMPONENTS_ALL (FSYNC_COMPONENT_LOOSE_OBJECT | \
+			      FSYNC_COMPONENT_PACK | \
+			      FSYNC_COMPONENT_PACK_METADATA | \
+			      FSYNC_COMPONENT_COMMIT_GRAPH | \
+			      FSYNC_COMPONENT_INDEX)
 
 /*
  * A bitmask indicating which components of the repo should be fsynced.
diff --git a/config.c b/config.c
index bb44f6f506d..3976ec74fd4 100644
--- a/config.c
+++ b/config.c
@@ -1332,6 +1332,12 @@ static const struct fsync_component_entry {
 	{ "pack-metadata", FSYNC_COMPONENT_PACK_METADATA },
 	{ "commit-graph", FSYNC_COMPONENT_COMMIT_GRAPH },
 	{ "index", FSYNC_COMPONENT_INDEX },
+	{ "objects", FSYNC_COMPONENTS_OBJECTS },
+	{ "derived-metadata", FSYNC_COMPONENTS_DERIVED_METADATA },
+	{ "default", FSYNC_COMPONENTS_DEFAULT },
+	{ "committed", FSYNC_COMPONENTS_COMMITTED },
+	{ "added", FSYNC_COMPONENTS_ADDED },
+	{ "all", FSYNC_COMPONENTS_ALL },
 };
 
 static enum fsync_component parse_fsync_components(const char *var, const char *string)
-- 
gitgitgadget