merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

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+Use of index and Racy Git problem
+=================================
+
+Background
+----------
+
+The index is one of the most important data structures in Git.
+It represents a virtual working tree state by recording list of
+paths and their object names and serves as a staging area to
+write out the next tree object to be committed.  The state is
+"virtual" in the sense that it does not necessarily have to, and
+often does not, match the files in the working tree.
+
+There are cases Git needs to examine the differences between the
+virtual working tree state in the index and the files in the
+working tree.  The most obvious case is when the user asks `git
+diff` (or its low level implementation, `git diff-files`) or
+`git-ls-files --modified`.  In addition, Git internally checks
+if the files in the working tree are different from what are
+recorded in the index to avoid stomping on local changes in them
+during patch application, switching branches, and merging.
+
+In order to speed up this comparison between the files in the
+working tree and the index entries, the index entries record the
+information obtained from the filesystem via `lstat(2)` system
+call when they were last updated.  When checking if they differ,
+Git first runs `lstat(2)` on the files and compares the result
+with this information (this is what was originally done by the
+`ce_match_stat()` function, but the current code does it in
+`ce_match_stat_basic()` function).  If some of these "cached
+stat information" fields do not match, Git can tell that the
+files are modified without even looking at their contents.
+
+Note: not all members in `struct stat` obtained via `lstat(2)`
+are used for this comparison.  For example, `st_atime` obviously
+is not useful.  Currently, Git compares the file type (regular
+files vs symbolic links) and executable bits (only for regular
+files) from `st_mode` member, `st_mtime` and `st_ctime`
+timestamps, `st_uid`, `st_gid`, `st_ino`, and `st_size` members.
+With a `USE_STDEV` compile-time option, `st_dev` is also
+compared, but this is not enabled by default because this member
+is not stable on network filesystems.  With `USE_NSEC`
+compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec`
+members are also compared. On Linux, this is not enabled by default
+because in-core timestamps can have finer granularity than
+on-disk timestamps, resulting in meaningless changes when an
+inode is evicted from the inode cache.  See commit 8ce13b0
+of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
+([PATCH] Sync in core time granularity with filesystems,
+2005-01-04). This patch is included in kernel 2.6.11 and newer, but
+only fixes the issue for file systems with exactly 1 ns or 1 s
+resolution. Other file systems are still broken in current Linux
+kernels (e.g. CEPH, CIFS, NTFS, UDF), see
+https://lkml.org/lkml/2015/6/9/714
+
+Racy Git
+--------
+
+There is one slight problem with the optimization based on the
+cached stat information.  Consider this sequence:
+
+  : modify 'foo'
+  $ git update-index 'foo'
+  : modify 'foo' again, in-place, without changing its size
+
+The first `update-index` computes the object name of the
+contents of file `foo` and updates the index entry for `foo`
+along with the `struct stat` information.  If the modification
+that follows it happens very fast so that the file's `st_mtime`
+timestamp does not change, after this sequence, the cached stat
+information the index entry records still exactly match what you
+would see in the filesystem, even though the file `foo` is now
+different.
+This way, Git can incorrectly think files in the working tree
+are unmodified even though they actually are.  This is called
+the "racy Git" problem (discovered by Pasky), and the entries
+that appear clean when they may not be because of this problem
+are called "racily clean".
+
+To avoid this problem, Git does two things:
+
+. When the cached stat information says the file has not been
+  modified, and the `st_mtime` is the same as (or newer than)
+  the timestamp of the index file itself (which is the time `git
+  update-index foo` finished running in the above example), it
+  also compares the contents with the object registered in the
+  index entry to make sure they match.
+
+. When the index file is updated that contains racily clean
+  entries, cached `st_size` information is truncated to zero
+  before writing a new version of the index file.
+
+Because the index file itself is written after collecting all
+the stat information from updated paths, `st_mtime` timestamp of
+it is usually the same as or newer than any of the paths the
+index contains.  And no matter how quick the modification that
+follows `git update-index foo` finishes, the resulting
+`st_mtime` timestamp on `foo` cannot get a value earlier
+than the index file.  Therefore, index entries that can be
+racily clean are limited to the ones that have the same
+timestamp as the index file itself.
+
+The callers that want to check if an index entry matches the
+corresponding file in the working tree continue to call
+`ce_match_stat()`, but with this change, `ce_match_stat()` uses
+`ce_modified_check_fs()` to see if racily clean ones are
+actually clean after comparing the cached stat information using
+`ce_match_stat_basic()`.
+
+The problem the latter solves is this sequence:
+
+  $ git update-index 'foo'
+  : modify 'foo' in-place without changing its size
+  : wait for enough time
+  $ git update-index 'bar'
+
+Without the latter, the timestamp of the index file gets a newer
+value, and falsely clean entry `foo` would not be caught by the
+timestamp comparison check done with the former logic anymore.
+The latter makes sure that the cached stat information for `foo`
+would never match with the file in the working tree, so later
+checks by `ce_match_stat_basic()` would report that the index entry
+does not match the file and Git does not have to fall back on more
+expensive `ce_modified_check_fs()`.
+
+
+Runtime penalty
+---------------
+
+The runtime penalty of falling back to `ce_modified_check_fs()`
+from `ce_match_stat()` can be very expensive when there are many
+racily clean entries.  An obvious way to artificially create
+this situation is to give the same timestamp to all the files in
+the working tree in a large project, run `git update-index` on
+them, and give the same timestamp to the index file:
+
+  $ date >.datestamp
+  $ git ls-files | xargs touch -r .datestamp
+  $ git ls-files | git update-index --stdin
+  $ touch -r .datestamp .git/index
+
+This will make all index entries racily clean.  The linux project, for
+example, there are over 20,000 files in the working tree.  On my
+Athlon 64 X2 3800+, after the above:
+
+  $ /usr/bin/time git diff-files
+  1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
+  0inputs+0outputs (0major+67111minor)pagefaults 0swaps
+  $ git update-index MAINTAINERS
+  $ /usr/bin/time git diff-files
+  0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
+  0inputs+0outputs (0major+935minor)pagefaults 0swaps
+
+Running `git update-index` in the middle checked the racily
+clean entries, and left the cached `st_mtime` for all the paths
+intact because they were actually clean (so this step took about
+the same amount of time as the first `git diff-files`).  After
+that, they are not racily clean anymore but are truly clean, so
+the second invocation of `git diff-files` fully took advantage
+of the cached stat information.
+
+
+Avoiding runtime penalty
+------------------------
+
+In order to avoid the above runtime penalty, post 1.4.2 Git used
+to have a code that made sure the index file
+got timestamp newer than the youngest files in the index when
+there are many young files with the same timestamp as the
+resulting index file would otherwise would have by waiting
+before finishing writing the index file out.
+
+I suspected that in practice the situation where many paths in the
+index are all racily clean was quite rare.  The only code paths
+that can record recent timestamp for large number of paths are:
+
+. Initial `git add .` of a large project.
+
+. `git checkout` of a large project from an empty index into an
+  unpopulated working tree.
+
+Note: switching branches with `git checkout` keeps the cached
+stat information of existing working tree files that are the
+same between the current branch and the new branch, which are
+all older than the resulting index file, and they will not
+become racily clean.  Only the files that are actually checked
+out can become racily clean.
+
+In a large project where raciness avoidance cost really matters,
+however, the initial computation of all object names in the
+index takes more than one second, and the index file is written
+out after all that happens.  Therefore the timestamp of the
+index file will be more than one seconds later than the
+youngest file in the working tree.  This means that in these
+cases there actually will not be any racily clean entry in
+the resulting index.
+
+Based on this discussion, the current code does not use the
+"workaround" to avoid the runtime penalty that does not exist in
+practice anymore.  This was done with commit 0fc82cff on Aug 15,
+2006.