Can I configure git blame to always ignore certain commits? Want to fix git blame once and for all

Question

I am in a repository where git blame has effectively been broken.

There are two commits I want to ignore in git blame.

• Commit 1 destroyed a lot of files.
• Commit 2 immediately reverted commit 1.

Every time I git blame a line now I see the author of [commit 2] instead of the true logical author.

I end up having to do a git log [file in question] instead, or another one of the solutions listed in this question.

These two commits make me sad whenever I use the Annotate feature in Intellij (which is basically git blame).

Has anybody ever fixed this problem before without rewriting history?

Answer 1

These two commits make me sad whenever I use the Annotate feature in Intellij (which is basically git blame).
Has anybody ever fixed this problem before without rewriting history?

Before Q3 2019, no.
But with Git 2.23, you will be able to instruct git blame to ignore those two problematic commits. (IntelliJ "annotate" feature might take a while before catching up)

Michael Platings comments though:

git blame --ignore-rev works on the assumption that the specified commit made an uninteresting change (e.g. reformatting).
Unfortunately both removing and adding a file are quite drastic changes so --ignore-rev won't help here.

That being said, git blame can now ignore commits (even maybe not in this particular case).

In general, since Git 2.23:

"git blame" learned to "ignore" commits in the history, whose effects (as well as their presence) get ignored.

And you can register that in your git config! You don't even need to pass those commits in parameters on every git blame call.

See commit 78fafbb (30 Jun 2019), and commit 1d028dc (20 Jun 2019) by Michael Platings (``).
See commit 07a54dc (28 Jun 2019) by Jeff King (peff).
See commit f0cbe74, commit a07a977 (20 Jun 2019), and commit 1fc7338, commit 8934ac8, commit ae3f36d, commit 55f808f, commit f93895f, commit 24eb33e (15 May 2019) by Barret Rhoden (brho).
^{(Merged by Junio C Hamano -- gitster -- in commit 209f075, 19 Jul 2019)}

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.

---

git blame new documentation:

--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.

git config new documentation:

blame.ignoreRevsFile:

Ignore revisions listed in the file, one unabbreviated object name per line, in git blame.
Whitespace and comments beginning with # are ignored.
This option may be repeated multiple times.
Empty file names will reset the list of ignored revisions.
This option will be handled before the command line option --ignore-revs-file.

---

Since the line detection is not always perfect:

blame: add config options for the output of ignored or unblamable lines

When ignoring commits, the commit that is blamed might not be responsible for the change, due to the inaccuracy of our heuristic.
Users might want to know when a particular line has a potentially inaccurate blame.

Furthermore, guess_line_blames() may fail to find any parent commit for a given line touched by an ignored commit.
Those 'unblamable' lines remain blamed on an ignored commit.
Users might want to know if a line is unblamable so that they do not spend time investigating a commit they know is uninteresting.

This patch adds two config options to mark these two types of lines in the output of blame.

The first option can identify ignored lines by specifying blame.markIgnoredLines.
When this option is set, each blame line that was blamed on a commit other than the ignored commit is marked with a '?'.

For example:

278b6158d6fdb (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

appears as:

?278b6158d6fd (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

where the '?' is placed before the commit, and the hash has one fewer characters.

Sometimes we are unable to even guess at what ancestor commit touched a line.
These lines are 'unblamable.'
The second option, blame.markUnblamableLines, will mark the line with '*'.

For example, say we ignore e5e8d36d04cbe, yet we are unable to blame this line on another commit:

e5e8d36d04cbe (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

appears as:

*e5e8d36d04cb (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

When these config options are used together, every line touched by an ignored commit will be marked with either a '?' or a '*'.

That means the git config man page now has:

blame.markUnblamables: 

Mark lines that were changed by an ignored revision that we could not attribute to another commit with a '*' in the output of git blame.

blame.markIgnoredLines:

Mark lines that were changed by an ignored revision that we attributed to another commit with a '?' in the output of git blame.

---

Finally, to improve the git blame line detection:

blame: add a fingerprint heuristic to match ignored lines

This algorithm will replace the heuristic used to identify lines from ignored commits with one that finds likely candidate lines in the parent's version of the file.
The actual replacement occurs in an upcoming commit.

The old heuristic simply assigned lines in the target to the same line number (plus offset) in the parent. The new function uses a fingerprinting algorithm to detect similarity between lines.

The new heuristic is designed to accurately match changes made mechanically by formatting tools such as clang-format and clang-tidy.
These tools make changes such as breaking up lines to fit within a character limit or changing identifiers to fit with a naming convention.
The heuristic is not intended to match more extensive refactoring changes and may give misleading results in such cases.

In most cases formatting tools preserve line ordering, so the heuristic is optimized for such cases. (Some types of changes do reorder lines e.g. sorting keep the line content identical, the git blame -M option can already be used to address this).
The reason that it is advantageous to rely on ordering is due to source code repeating the same character sequences often e.g. declaring an identifier on one line and using that identifier on several subsequent lines.
This means that lines can look very similar to each other which presents a problem when doing fuzzy matching. Relying on ordering gives us extra clues to point towards the true match.

The heuristic operates on a single diff chunk change at a time.
It creates a “fingerprint” for each line on each side of the change.

Fingerprints are described in detail in the comment for struct fingerprint, but essentially are a multiset of the character pairs in a line.

• The heuristic first identifies the line in the target entry whose fingerprint is most clearly matched to a line fingerprint in the parent entry.
  Where fingerprints match identically, the position of the lines is used as a tie-break. - The heuristic locks in the best match, and subtracts the fingerprint of the line in the target entry from the fingerprint of the line in the parent entry to prevent other lines being matched on the same parts of that line.
• It then repeats the process recursively on the section of the chunk before the match, and then the section of the chunk after the match.

Here's an example of the difference the fingerprinting makes.
Consider a file with two commits:

    commit-a 1) void func_1(void *x, void *y);
    commit-b 2) void func_2(void *x, void *y);

After a commit 'X', we have:

    commit-X 1) void func_1(void *x,
    commit-X 2)             void *y);
    commit-X 3) void func_2(void *x,
    commit-X 4)             void *y);

When we blame-ignored with the old algorithm, we get:

    commit-a 1) void func_1(void *x,
    commit-b 2)             void *y);
    commit-X 3) void func_2(void *x,
    commit-X 4)             void *y);

Where commit-b is blamed for 2 instead of 3.

With the fingerprint algorithm, we get:

    commit-a 1) void func_1(void *x,
    commit-a 2)             void *y);
    commit-b 3) void func_2(void *x,
    commit-b 4)             void *y);

Note line 2 could be matched with either commit-a or commit-b as it is equally similar to both lines, but is matched with commit-a because its position as a fraction of the new line range is more similar to commit-a as a fraction of the old line range.
Line 4 is also equally similar to both lines, but as it appears after line 3 which will be matched first it cannot be matched with an earlier line.

For many more examples, see t/t8014-blame-ignore-fuzzy.sh which contains example parent and target files and the line numbers in the parent that must be matched.

Answer 2

if it were really immediately reverted, you can use git replace --edit $comment2 to fake parent of commit1 to be its parent.