[v2,1/1] t6042: work around speed optimization on Windows

Commit Message

Johannes Schindelin via GitGitGadget Jan. 17, 2019, 8:29 a.m. UTC

From: Johannes Schindelin <[e-mail]>

When Git determines whether a file has changed, it looks at the mtime,
at the file size, and to detect changes even if the mtime is the same
(on Windows, the mtime granularity is 100ns, read: if two files are
written within the same 100ns time slot, they have the same mtime) and
even if the file size is the same, Git also looks at the inode/device
numbers.

This design obviously comes from a Linux background, where `lstat()`
calls were designed to be cheap.

On Windows, there is no `lstat()`. It has to be emulated. And while
obtaining the mtime and the file size is not all that expensive (you can
get both with a single `GetFileAttributesW()` call), obtaining the
equivalent of the inode and device numbers is very expensive (it
requires a call to `GetFileInformationByHandle()`, which in turn
requires a file handle, which is *a lot* more expensive than one might
imagine).

As it is very uncommon for developers to modify files within 100ns time
slots, Git for Windows chooses not to fill inode/device numbers
properly, but simply sets them to 0.

However, in t6042 the files file_v1 and file_v2 are typically written
within the same 100ns time slot, and they do not differ in file size. So
the minor modification is not picked up.

Let's work around this issue by avoiding the `git mv` calls in the
'mod6-setup: chains of rename/rename(1to2) and rename/rename(2to1)' test
case. The target files are overwritten anyway, so it is not like we
really rename those files. This fixes the issue because `git add` will
now add the files as new files (as opposed to existing, just renamed
files).

Functionally, we do not change anything because we replace two `git mv
<old> <new>` calls (where `<new>` is completely overwritten and `git
add`ed later anyway) by `git rm <old>` calls (removing other files, too,
that are also completely overwritten and `git add`ed later).

Reviewed-by: Elijah Newren <[e-mail]>
Signed-off-by: Johannes Schindelin <[e-mail]>
---
 t/t6042-merge-rename-corner-cases.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Junio C Hamano Jan. 17, 2019, 8:57 p.m. UTC | #1

"Johannes Schindelin via GitGitGadget" <[e-mail]>
writes:

> From: Johannes Schindelin <[e-mail]>
>
> When Git determines whether a file has changed, it looks at the mtime,
> at the file size, ...
> Functionally, we do not change anything because we replace two `git mv
> <old> <new>` calls (where `<new>` is completely overwritten and `git
> add`ed later anyway) by `git rm <old>` calls (removing other files, too,
> that are also completely overwritten and `git add`ed later).

Thanks, both.  Will queue.

Patch

diff --git a/t/t6042-merge-rename-corner-cases.sh b/t/t6042-merge-rename-corner-cases.sh
index 7cc34e7579..09dfa8bd92 100755
--- a/t/t6042-merge-rename-corner-cases.sh
+++ b/t/t6042-merge-rename-corner-cases.sh
@@ -1175,7 +1175,7 @@  test_expect_success 'setup nested conflicts from rename/rename(2to1)' '
 
 		# Handle the left side
 		git checkout L &&
-		git mv one three &&
+		git rm one two &&
 		mv -f file_v2 three &&
 		mv -f file_v5 two &&
 		git add two three &&
@@ -1183,7 +1183,7 @@  test_expect_success 'setup nested conflicts from rename/rename(2to1)' '
 
 		# Handle the right side
 		git checkout R &&
-		git mv two three &&
+		git rm one two &&
 		mv -f file_v3 one &&
 		mv -f file_v6 three &&
 		git add one three &&