Merge branch 'ns/core-fsyncmethod'

Replace core.fsyncObjectFiles with two new configuration variables,
core.fsync and core.fsyncMethod.

* ns/core-fsyncmethod:
  core.fsync: documentation and user-friendly aggregate options
  core.fsync: new option to harden the index
  core.fsync: add configuration parsing
  core.fsync: introduce granular fsync control infrastructure
  core.fsyncmethod: add writeout-only mode
  wrapper: make inclusion of Windows csprng header tightly scoped

1 files changed, 54 insertions, 4 deletions

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index c04f62a54a..9a3ad71e9e 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -547,13 +547,63 @@ 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 components of the repository that
+	should be hardened via the core.fsyncMethod when created or
+	modified.  You can disable hardening of any component by
+	prefixing it with a '-'.  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 empty or pick one of `committed`, `added`,
+	or `all`.
++
+When this configuration is encountered, the set of components starts with
+the platform default value, disabled components are removed, and additional
+components are added. `none` resets the state so that the platform default
+is ignored.
++
+The empty string resets the fsync configuration to the platform
+default. The default on most platforms is equivalent to
+`core.fsync=committed,-loose-object`, which has good performance,
+but risks losing recent work in the event of an unclean system shutdown.
++
+* `none` clears the set of fsynced components.
+* `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`.
+* `committed` is an aggregate option that is currently equivalent to
+  `objects`. This mode sacrifices some performance to ensure that work
+  that is committed to the repository with `git commit` or similar commands
+  is hardened.
+* `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 hardened.
+* `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.
++
+* `fsync` uses the fsync() system call or platform equivalents.
+* `writeout-only` issues pagecache writeback requests, but depending on the
+  filesystem and storage hardware, data added to the repository may not be
+  durable in the event of a system crash. This is the default mode on macOS.
+
 core.fsyncObjectFiles::
 	This boolean will enable 'fsync()' when writing object files.
+	This setting is deprecated. Use core.fsync instead.
 +
-This is a total waste of time and effort on a filesystem that orders
-data writes properly, but can be useful for filesystems that do not use
-journalling (traditional UNIX filesystems) or that only journal metadata
-and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
+This setting affects data added to the Git repository in loose-object
+form. When set to true, Git will issue an fsync or similar system call
+to flush caches so that loose-objects remain consistent in the face
+of a unclean system shutdown.
 
 core.preloadIndex::
 	Enable parallel index preload for operations like 'git diff'