Bug in "git blame -C"
From: Linus Torvalds <torvalds@osdl.org>
Date: 2016-08-11 19:39:40
Ok, this is interesting.
Try this sequence (which is a good sequence for showign how something like
"git blame -C" _should_ work, but only ends up showing that it doesn't at
all, because of some bug ;^):
#
# create 'testing' repository
#
mkdir testing
cd testing/
git init-db
#
# copy git.c and sha1_file.c there and commit initial
# (Just to get _some_ initial state)
#
cp ~/git/git.c .
cp ~/git/sha1_file.c .
git add git.c sha1_file.c
git commit -m Initial
#
# move the prepend_to_path() function from git.c to
# sha1_file.c (I did it to just after the
# #ifndef O_NOATIME
# block of preprocessor stuff
#
em git.c sha1_file.c
git commit -a -m Movement
git log -p
and the result of that "git log -p" should show something like
commit a583b5aee68b89b7d554b8f900a95057e8ed61d9
Author: Linus Torvalds [off-list ref]
Date: Tue Nov 28 20:13:00 2006 -0800
Movement
diff --git a/git.c b/git.c
index 357330e..43c01fd 100644
--- a/git.c
+++ b/git.c
@@ -18,28 +18,6 @@
const char git_usage_string[] =
"git [--version] [--exec-path[=GIT_EXEC_PATH]] [-p|--paginate] [--bare] [--git-dir=GIT_DIR] [--help] COMMAND [AR
-static void prepend_to_path(const char *dir, int len)
-{
- const char *old_path = getenv("PATH");
- char *path;
- int path_len = len;
...
diff --git a/sha1_file.c b/sha1_file.c
index 63f416b..20168aa 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -22,6 +22,28 @@
#endif
#endif
+static void prepend_to_path(const char *dir, int len)
+{
+ const char *old_path = getenv("PATH");
+ char *path;
+ int path_len = len;
+
...
to show how that top commit moved the function. Ok, everything looks fine
so far.
For the surreal behaviour, now do
git blame -C sha1_file.c
and watch the result make no sense what-so-ever. It doesn't show the
movement at all. It shows that everything in that file came from the
original commit, even though the file obviously did change since.
Which is kind of "true", but it's still _wrong_. Yes, all the data comes
from the same (initial) commit, but it doesn't come from the same _files_
in the same commit, so the fact that we don't see the filenames and
original lines in those filenames is _broken_. The commit information is
right, but it's decided not to show all the _other_ information that is
crucial..
So this shows two problems:
- the line numbers that "git blame -C" shows are the current line numbers
only, not the line numbers it came from in the version it shows. That
makes them useless. We _know_ the current linenumbers. What we want to
know is what they were in the commit that they came from.
So right now, the line number information that "git blame -C" shows is
just the same thing we could have gotten by doing a "cat -n file".
- "git blame -C" has apparently decided that it doesn't need to show
filenames that things came from, because they all came from the same
commit, but that's not a logical thing to compare. "same commit" does
not mean "same filename", so not showing the filename makes no sense.
I tried to bisect this a bit, but I don't think pickaxe has ever gotten
this right, so I couldn't find a place where it was correct to start
bisecting at ;)