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git/Documentation/blame-options.txt
Barret Rhoden ae3f36dea1 blame: add the ability to ignore commits and their changes 
Commits that make formatting changes or function renames are often not
interesting when blaming a file.  A user may deem such a commit as 'not
interesting' and want to ignore and its changes it when assigning blame.

For example, say a file has the following git history / rev-list:

---O---A---X---B---C---D---Y---E---F

Commits X and Y both touch a particular line, and the other commits do
not:

X: "Take a third parameter"
-MyFunc(1, 2);
+MyFunc(1, 2, 3);

Y: "Remove camelcase"
-MyFunc(1, 2, 3);
+my_func(1, 2, 3);

git-blame will blame Y for the change.  I'd like to be able to ignore Y:
both the existence of the commit as well as any changes it made.  This
differs from -S rev-list, which specifies the list of commits to
process for the blame.  We would still process Y, but just don't let the
blame 'stick.'

This patch adds the ability for users to ignore a revision with
--ignore-rev=rev, which may be repeated.  They can specify a set of
files of full object names of revs, e.g. SHA-1 hashes, one per line.  A
single file may be specified with the blame.ignoreRevFile config option
or with --ignore-rev-file=file.  Both the config option and the command
line option may be repeated multiple times.  An empty file name "" will
clear the list of revs from previously processed files.  Config options
are processed before command line options.

For a typical use case, projects will maintain the file containing
revisions for commits that perform mass reformatting, and their users
have the option to ignore all of the commits in that file.

Additionally, a user can use the --ignore-rev option for one-off
investigation.  To go back to the example above, X was a substantive
change to the function, but not the change the user is interested in.
The user inspected X, but wanted to find the previous change to that
line - perhaps a commit that introduced that function call.

To make this work, we can't simply remove all ignored commits from the
rev-list.  We need to diff the changes introduced by Y so that we can
ignore them.  We let the blames get passed to Y, just like when
processing normally.  When Y is the target, we make sure that Y does not
*keep* any blames.  Any changes that Y is responsible for get passed to
its parent.  Note we make one pass through all of the scapegoats
(parents) to attempt to pass blame normally; we don't know if we *need*
to ignore the commit until we've checked all of the parents.

The blame_entry will get passed up the tree until we find a commit that
has a diff chunk that affects those lines.

One issue is that the ignored commit *did* make some change, and there is
no general solution to finding the line in the parent commit that
corresponds to a given line in the ignored commit.  That makes it hard
to attribute a particular line within an ignored commit's diff
correctly.

For example, the parent of an ignored commit has this, say at line 11:

commit-a 11) #include "a.h"
commit-b 12) #include "b.h"

Commit X, which we will ignore, swaps these lines:

commit-X 11) #include "b.h"
commit-X 12) #include "a.h"

We can pass that blame entry to the parent, but line 11 will be
attributed to commit A, even though "include b.h" came from commit B.
The blame mechanism will be looking at the parent's view of the file at
line number 11.

ignore_blame_entry() is set up to allow alternative algorithms for
guessing per-line blames.  Any line that is not attributed to the parent
will continue to be blamed on the ignored commit as if that commit was
not ignored.  Upcoming patches have the ability to detect these lines
and mark them in the blame output.

The existing algorithm is simple: blame each line on the corresponding
line in the parent's diff chunk.  Any lines beyond that stay with the
target.

For example, the parent of an ignored commit has this, say at line 11:

commit-a 11) void new_func_1(void *x, void *y);
commit-b 12) void new_func_2(void *x, void *y);
commit-c 13) some_line_c
commit-d 14) some_line_d

After a commit 'X', we have:

commit-X 11) void new_func_1(void *x,
commit-X 12)                 void *y);
commit-X 13) void new_func_2(void *x,
commit-X 14)                 void *y);
commit-c 15) some_line_c
commit-d 16) some_line_d

Commit X nets two additionally lines: 13 and 14.  The current
guess_line_blames() algorithm will not attribute these to the parent,
whose diff chunk is only two lines - not four.

When we ignore with the current algorithm, we get:

commit-a 11) void new_func_1(void *x,
commit-b 12)                 void *y);
commit-X 13) void new_func_2(void *x,
commit-X 14)                 void *y);
commit-c 15) some_line_c
commit-d 16) some_line_d

Note that line 12 was blamed on B, though B was the commit for
new_func_2(), not new_func_1().  Even when guess_line_blames() finds a
line in the parent, it may still be incorrect.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2019-05-16 11:36:23 +09:00

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-b::
Show blank SHA-1 for boundary commits. This can also
be controlled via the `blame.blankboundary` config option.
--root::
Do not treat root commits as boundaries. This can also be
controlled via the `blame.showRoot` config option.
--show-stats::
Include additional statistics at the end of blame output.
-L <start>,<end>::
-L :<funcname>::
Annotate only the given line range. May be specified multiple times.
Overlapping ranges are allowed.
+
<start> and <end> are optional. ``-L <start>'' or ``-L <start>,'' spans from
<start> to end of file. ``-L ,<end>'' spans from start of file to <end>.
+
include::line-range-format.txt[]
-l::
Show long rev (Default: off).
-t::
Show raw timestamp (Default: off).
-S <revs-file>::
Use revisions from revs-file instead of calling linkgit:git-rev-list[1].
--reverse <rev>..<rev>::
Walk history forward instead of backward. Instead of showing
the revision in which a line appeared, this shows the last
revision in which a line has existed. This requires a range of
revision like START..END where the path to blame exists in
START. `git blame --reverse START` is taken as `git blame
--reverse START..HEAD` for convenience.
-p::
--porcelain::
Show in a format designed for machine consumption.
--line-porcelain::
Show the porcelain format, but output commit information for
each line, not just the first time a commit is referenced.
Implies --porcelain.
--incremental::
Show the result incrementally in a format designed for
machine consumption.
--encoding=<encoding>::
Specifies the encoding used to output author names
and commit summaries. Setting it to `none` makes blame
output unconverted data. For more information see the
discussion about encoding in the linkgit:git-log[1]
manual page.
--contents <file>::
When <rev> is not specified, the command annotates the
changes starting backwards from the working tree copy.
This flag makes the command pretend as if the working
tree copy has the contents of the named file (specify
`-` to make the command read from the standard input).
--date <format>::
Specifies the format used to output dates. If --date is not
provided, the value of the blame.date config variable is
used. If the blame.date config variable is also not set, the
iso format is used. For supported values, see the discussion
of the --date option at linkgit:git-log[1].
--[no-]progress::
Progress status is reported on the standard error stream
by default when it is attached to a terminal. This flag
enables progress reporting even if not attached to a
terminal. Can't use `--progress` together with `--porcelain`
or `--incremental`.
-M[<num>]::
Detect moved or copied lines within a file. When a commit
moves or copies a block of lines (e.g. the original file
has A and then B, and the commit changes it to B and then
A), the traditional 'blame' algorithm notices only half of
the movement and typically blames the lines that were moved
up (i.e. B) to the parent and assigns blame to the lines that
were moved down (i.e. A) to the child commit. With this
option, both groups of lines are blamed on the parent by
running extra passes of inspection.
+
<num> is optional but it is the lower bound on the number of
alphanumeric characters that Git must detect as moving/copying
within a file for it to associate those lines with the parent
commit. The default value is 20.
-C[<num>]::
In addition to `-M`, detect lines moved or copied from other
files that were modified in the same commit. This is
useful when you reorganize your program and move code
around across files. When this option is given twice,
the command additionally looks for copies from other
files in the commit that creates the file. When this
option is given three times, the command additionally
looks for copies from other files in any commit.
+
<num> is optional but it is the lower bound on the number of
alphanumeric characters that Git must detect as moving/copying
between files for it to associate those lines with the parent
commit. And the default value is 40. If there are more than one
`-C` options given, the <num> argument of the last `-C` will
take effect.
--ignore-rev <rev>::
Ignore changes made by the revision when assigning blame, as if the
change never happened. Lines that were changed or added by an ignored
commit will be blamed on the previous commit that changed that line or
nearby lines. This option may be specified multiple times to ignore
more than one revision.
--ignore-revs-file <file>::
Ignore revisions listed in `file`, which must be in the same format as an
`fsck.skipList`. This option may be repeated, and these files will be
processed after any files specified with the `blame.ignoreRevsFile` config
option. An empty file name, `""`, will clear the list of revs from
previously processed files.
-h::
Show help message.
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