[v6,3/6] blame: add the ability to ignore commits and their changes

Commit Message

Barret Rhoden April 10, 2019, 4:24 p.m. UTC

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 optional 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
is marked as 'unblamable', and we output a hash of all zeros.

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);
00000000 13) void new_func_2(void *x,
00000000 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 <[e-mail]>
---
 Documentation/blame-options.txt |  14 +++
 Documentation/config/blame.txt  |   7 ++
 Documentation/git-blame.txt     |   1 +
 blame.c                         | 181 ++++++++++++++++++++++++++++++--
 blame.h                         |   3 +
 builtin/blame.c                 |  42 ++++++++
 t/t8013-blame-ignore-revs.sh    | 168 +++++++++++++++++++++++++++++
 7 files changed, 406 insertions(+), 10 deletions(-)
 create mode 100755 t/t8013-blame-ignore-revs.sh

Comments

Ævar Arnfjörð Bjarmason April 10, 2019, 7 p.m. UTC | #1

On Wed, Apr 10 2019, Barret Rhoden wrote:

(Just skimming)

> revisions for commits that perform mass reformatting, and their users
> have the optional to ignore all of the commits in that file.

s/have the optional/have the option/

> +--ignore-revs-file <file>::
> +	Ignore revisions listed in `file`, one unabbreviated object name per line.
> +	Whitespace and comments beginning with `#` are ignored.

Maybe just say "Ignore revisions listed in `file`, which is expected to
be in the same format as an `fsck.skipList`.".

> +	the `blame.ignoreRevsFile` config option.  An empty file name, `""`, will
> +	clear the list of revs from previously processed files.

Maybe I haven't read this carefully enough but the use-case for this
doesn't seem to be explained, you need this for the option, but the
config file too? If I want to override fsck.skipList I do
`fsck.skipList=/dev/zero`. Isn't that enough for this use-case without
introducing config state-machine magic?

> +	split[0].unblamable = e->unblamable;
> +	split[1].unblamable = e->unblamable;
> +	split[2].unblamable = e->unblamable;

I wonder what the comfort level for people in general is before turning
this sort of thing into a for-loop, 4? :)

> +	nr_lines = e->num_lines;	// e changes in the loop

A C++-like trailing comment.

> +	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
> +	git rev-parse X >expect &&
> +	test_cmp expect actual &&
> +
> +	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
> +	git rev-parse X >expect &&
> +	test_cmp expect actual

The grep here is a bug. See my 4abf20f004 ("tests: fix unportable "\?"
and "\+" regex syntax", 2019-02-21).

Michael Platings April 14, 2019, 10:42 a.m. UTC | #2

> > +     the `blame.ignoreRevsFile` config option.  An empty file name, `""`, will
> > +     clear the list of revs from previously processed files.
>
> Maybe I haven't read this carefully enough but the use-case for this
> doesn't seem to be explained, you need this for the option, but the
> config file too? If I want to override fsck.skipList I do
> `fsck.skipList=/dev/zero`. Isn't that enough for this use-case without
> introducing config state-machine magic?

The difference between blame.ignoreRevsFile and fsck.skipList is that
ignoreRevsFile can be specified repeatedly. This is useful if you have
one file listing reformatting commits, another listing renaming
commits etc. Or maybe a checked-in list of commits to ignore, and a
personal list of commits to ignore. However sometimes you're going to
want to *not* ignore those commits, so you need a way to discard the
previously specified options. To accommodate all operating systems an
empty string seems the best way to do this.

Barret Rhoden April 15, 2019, 1:32 p.m. UTC | #3

On 4/14/19 6:42 AM, Michael Platings wrote:
>>> +     the `blame.ignoreRevsFile` config option.  An empty file name, `""`, will
>>> +     clear the list of revs from previously processed files.
>>
>> Maybe I haven't read this carefully enough but the use-case for this
>> doesn't seem to be explained, you need this for the option, but the
>> config file too? If I want to override fsck.skipList I do
>> `fsck.skipList=/dev/zero`. Isn't that enough for this use-case without
>> introducing config state-machine magic?
> 
> The difference between blame.ignoreRevsFile and fsck.skipList is that
> ignoreRevsFile can be specified repeatedly. This is useful if you have
> one file listing reformatting commits, another listing renaming
> commits etc. Or maybe a checked-in list of commits to ignore, and a
> personal list of commits to ignore. However sometimes you're going to
> want to *not* ignore those commits, so you need a way to discard the
> previously specified options. To accommodate all operating systems an
> empty string seems the best way to do this.
> 

In a previous round of reviews[1], this style was recommended.  It's 
based on what credential.helper does.

The main thing I've been using the --ignore-revs-file="" for is to turn 
off my default ignore list for debugging.  =)

Thanks,

Barret


[1] 
https://public-inbox.org/git/[e-mail]/

Barret Rhoden April 15, 2019, 1:34 p.m. UTC | #4

On 4/10/19 3:00 PM, Ævar Arnfjörð Bjarmason wrote:
[snip]

>> +	split[0].unblamable = e->unblamable;
>> +	split[1].unblamable = e->unblamable;
>> +	split[2].unblamable = e->unblamable;
> 
> I wonder what the comfort level for people in general is before turning
> this sort of thing into a for-loop, 4? :)

4 sounds good to me.  =)

>> +	nr_lines = e->num_lines;	// e changes in the loop
> 
> A C++-like trailing comment.
> 
>> +	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
>> +	git rev-parse X >expect &&
>> +	test_cmp expect actual &&
>> +
>> +	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
>> +	git rev-parse X >expect &&
>> +	test_cmp expect actual
> 
> The grep here is a bug. See my 4abf20f004 ("tests: fix unportable "\?"
> and "\+" regex syntax", 2019-02-21).

Thanks - will fix up this stuff in the next round.

Patch

diff --git a/Documentation/blame-options.txt b/Documentation/blame-options.txt
index dc41957afab2..8f155196c6fe 100644
--- a/Documentation/blame-options.txt
+++ b/Documentation/blame-options.txt
@@ -110,5 +110,19 @@  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`, one unabbreviated object name per line.
+	Whitespace and comments beginning with `#` are ignored.  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.
diff --git a/Documentation/config/blame.txt b/Documentation/config/blame.txt
index 67b5c1d1e02a..4da2788f306d 100644
--- a/Documentation/config/blame.txt
+++ b/Documentation/config/blame.txt
@@ -19,3 +19,10 @@  blame.showEmail::
 blame.showRoot::
 	Do not treat root commits as boundaries in linkgit:git-blame[1].
 	This option defaults to false.
+
+blame.ignoreRevsFile::
+	Ignore revisions listed in the file, one unabbreviated object name per
+	line, in linkgit:git-blame[1].  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`.
diff --git a/Documentation/git-blame.txt b/Documentation/git-blame.txt
index 16323eb80e31..7e8154199635 100644
--- a/Documentation/git-blame.txt
+++ b/Documentation/git-blame.txt
@@ -10,6 +10,7 @@  SYNOPSIS
 [verse]
 'git blame' [-c] [-b] [-l] [--root] [-t] [-f] [-n] [-s] [-e] [-p] [-w] [--incremental]
 	    [-L <range>] [-S <revs-file>] [-M] [-C] [-C] [-C] [--since=<date>]
+	    [--ignore-rev <rev>] [--ignore-revs-file <file>]
 	    [--progress] [--abbrev=<n>] [<rev> | --contents <file> | --reverse <rev>..<rev>]
 	    [--] <file>
 
diff --git a/blame.c b/blame.c
index 9555a9420836..0bbb86ad5985 100644
--- a/blame.c
+++ b/blame.c
@@ -480,7 +480,8 @@  void blame_coalesce(struct blame_scoreboard *sb)
 
 	for (ent = sb->ent; ent && (next = ent->next); ent = next) {
 		if (ent->suspect == next->suspect &&
-		    ent->s_lno + ent->num_lines == next->s_lno) {
+		    ent->s_lno + ent->num_lines == next->s_lno &&
+		    ent->unblamable == next->unblamable) {
 			ent->num_lines += next->num_lines;
 			ent->next = next->next;
 			blame_origin_decref(next->suspect);
@@ -732,6 +733,10 @@  static void split_overlap(struct blame_entry *split,
 	int chunk_end_lno;
 	memset(split, 0, sizeof(struct blame_entry [3]));
 
+	split[0].unblamable = e->unblamable;
+	split[1].unblamable = e->unblamable;
+	split[2].unblamable = e->unblamable;
+
 	if (e->s_lno < tlno) {
 		/* there is a pre-chunk part not blamed on parent */
 		split[0].suspect = blame_origin_incref(e->suspect);
@@ -852,6 +857,7 @@  static struct blame_entry *split_blame_at(struct blame_entry *e, int len,
 	struct blame_entry *n = xcalloc(1, sizeof(struct blame_entry));
 
 	n->suspect = new_suspect;
+	n->unblamable = e->unblamable;
 	n->lno = e->lno + len;
 	n->s_lno = e->s_lno + len;
 	n->num_lines = e->num_lines - len;
@@ -860,6 +866,109 @@  static struct blame_entry *split_blame_at(struct blame_entry *e, int len,
 	return n;
 }
 
+struct blame_line_tracker {
+	int is_parent;
+	int s_lno;
+};
+
+static int are_lines_adjacent(struct blame_line_tracker *first,
+			      struct blame_line_tracker *second)
+{
+	return first->is_parent == second->is_parent &&
+	       first->s_lno + 1 == second->s_lno;
+}
+
+/*
+ * This cheap heuristic assigns lines in the chunk to their relative location in
+ * the parent's chunk.  Any additional lines are left with the target.
+ */
+static void guess_line_blames(struct blame_entry *e,
+			      struct blame_origin *parent,
+			      struct blame_origin *target,
+			      int offset, int parent_slno, int parent_len,
+			      struct blame_line_tracker *line_blames)
+{
+	int i, parent_idx;
+
+	for (i = 0; i < e->num_lines; i++) {
+		parent_idx = e->s_lno + i + offset;
+		if (parent_slno <= parent_idx &&
+		    parent_idx < parent_slno + parent_len) {
+			line_blames[i].is_parent = 1;
+			line_blames[i].s_lno = parent_idx;
+		} else {
+			line_blames[i].is_parent = 0;
+			line_blames[i].s_lno = e->s_lno + i;
+		}
+	}
+}
+
+/*
+ * This decides which parts of a blame entry go to the parent (added to the
+ * ignoredp list) and which stay with the target (added to the diffp list).  The
+ * actual decision is made in a separate heuristic function.  This consumes e,
+ * essentially putting it on a list.
+ *
+ * Note that the blame entries on the ignoredp list are not necessarily sorted
+ * with respect to the parent's line numbers yet.
+ */
+static void ignore_blame_entry(struct blame_entry *e,
+			       struct blame_origin *parent,
+			       struct blame_origin *target,
+			       int offset, int parent_slno, int parent_len,
+			       struct blame_entry **diffp,
+			       struct blame_entry **ignoredp)
+{
+	struct blame_line_tracker *line_blames;
+	int entry_len, nr_lines, i;
+
+	line_blames = xcalloc(sizeof(struct blame_line_tracker),
+			      e->num_lines);
+	guess_line_blames(e, parent, target, offset, parent_slno, parent_len,
+			  line_blames);
+	/*
+	 * We carve new entries off the front of e.  Each entry comes from a
+	 * contiguous chunk of lines: adjacent lines from the same origin
+	 * (either the parent or the target).
+	 */
+	entry_len = 1;
+	nr_lines = e->num_lines;	// e changes in the loop
+	for (i = 0; i < nr_lines; i++) {
+		struct blame_entry *next = NULL;
+
+		/*
+		 * We are often adjacent to the next line - only split the blame
+		 * entry when we have to.
+		 */
+		if (i + 1 < nr_lines) {
+			if (are_lines_adjacent(&line_blames[i],
+					       &line_blames[i + 1])) {
+				entry_len++;
+				continue;
+			}
+			next = split_blame_at(e, entry_len,
+					      blame_origin_incref(e->suspect));
+		}
+		if (line_blames[i].is_parent) {
+			blame_origin_decref(e->suspect);
+			e->suspect = blame_origin_incref(parent);
+			e->s_lno = line_blames[i - entry_len + 1].s_lno;
+			e->next = *ignoredp;
+			*ignoredp = e;
+		} else {
+			e->unblamable = 1;
+			/* e->s_lno is already in the target's address space. */
+			e->next = *diffp;
+			*diffp = e;
+		}
+		assert(e->num_lines == entry_len);
+		e = next;
+		entry_len = 1;
+	}
+	assert(!e);
+	free(line_blames);
+}
+
 /*
  * Process one hunk from the patch between the current suspect for
  * blame_entry e and its parent.  This first blames any unfinished
@@ -869,13 +978,19 @@  static struct blame_entry *split_blame_at(struct blame_entry *e, int len,
  * -C options may lead to overlapping/duplicate source line number
  * ranges, all we can rely on from sorting/merging is the order of the
  * first suspect line number.
+ *
+ * tlno: line number in the target where this chunk begins
+ * same: line number in the target where this chunk ends
+ * offset: add to tlno to get the chunk starting point in the parent
+ * parent_len: number of lines in the parent chunk
  */
 static void blame_chunk(struct blame_entry ***dstq, struct blame_entry ***srcq,
-			int tlno, int offset, int same,
-			struct blame_origin *parent)
+			int tlno, int offset, int same, int parent_len,
+			struct blame_origin *parent,
+			struct blame_origin *target, int ignore_diffs)
 {
 	struct blame_entry *e = **srcq;
-	struct blame_entry *samep = NULL, *diffp = NULL;
+	struct blame_entry *samep = NULL, *diffp = NULL, *ignoredp = NULL;
 
 	while (e && e->s_lno < tlno) {
 		struct blame_entry *next = e->next;
@@ -943,10 +1058,29 @@  static void blame_chunk(struct blame_entry ***dstq, struct blame_entry ***srcq,
 			n->next = samep;
 			samep = n;
 		}
-		e->next = diffp;
-		diffp = e;
+		if (ignore_diffs) {
+			ignore_blame_entry(e, parent, target, offset,
+					   tlno + offset, parent_len, &diffp,
+					   &ignoredp);
+		} else {
+			e->next = diffp;
+			diffp = e;
+		}
 		e = next;
 	}
+	if (ignoredp) {
+		/*
+		 * Note ignoredp is not sorted yet, and thus neither is dstq.
+		 * That list must be sorted before we queue_blames().  We defer
+		 * sorting until after all diff hunks are processed, so that
+		 * guess_line_blames() can pick *any* line in the parent.  The
+		 * slight drawback is that we end up sorting all blame entries
+		 * passed to the parent, including those that are unrelated to
+		 * changes made by the ignored commit.
+		 */
+		**dstq = reverse_blame(ignoredp, **dstq);
+		*dstq = &ignoredp->next;
+	}
 	**srcq = reverse_blame(diffp, reverse_blame(samep, e));
 	/* Move across elements that are in the unblamable portion */
 	if (diffp)
@@ -955,7 +1089,9 @@  static void blame_chunk(struct blame_entry ***dstq, struct blame_entry ***srcq,
 
 struct blame_chunk_cb_data {
 	struct blame_origin *parent;
+	struct blame_origin *target;
 	long offset;
+	int ignore_diffs;
 	struct blame_entry **dstq;
 	struct blame_entry **srcq;
 };
@@ -968,7 +1104,8 @@  static int blame_chunk_cb(long start_a, long count_a,
 	if (start_a - start_b != d->offset)
 		die("internal error in blame::blame_chunk_cb");
 	blame_chunk(&d->dstq, &d->srcq, start_b, start_a - start_b,
-		    start_b + count_b, d->parent);
+		    start_b + count_b, count_a, d->parent, d->target,
+		    d->ignore_diffs);
 	d->offset = start_a + count_a - (start_b + count_b);
 	return 0;
 }
@@ -980,7 +1117,7 @@  static int blame_chunk_cb(long start_a, long count_a,
  */
 static void pass_blame_to_parent(struct blame_scoreboard *sb,
 				 struct blame_origin *target,
-				 struct blame_origin *parent)
+				 struct blame_origin *parent, int ignore_diffs)
 {
 	mmfile_t file_p, file_o;
 	struct blame_chunk_cb_data d;
@@ -990,7 +1127,9 @@  static void pass_blame_to_parent(struct blame_scoreboard *sb,
 		return; /* nothing remains for this target */
 
 	d.parent = parent;
+	d.target = target;
 	d.offset = 0;
+	d.ignore_diffs = ignore_diffs;
 	d.dstq = &newdest; d.srcq = &target->suspects;
 
 	fill_origin_blob(&sb->revs->diffopt, parent, &file_p, &sb->num_read_blob);
@@ -1002,8 +1141,13 @@  static void pass_blame_to_parent(struct blame_scoreboard *sb,
 		    oid_to_hex(&parent->commit->object.oid),
 		    oid_to_hex(&target->commit->object.oid));
 	/* The rest are the same as the parent */
-	blame_chunk(&d.dstq, &d.srcq, INT_MAX, d.offset, INT_MAX, parent);
+	blame_chunk(&d.dstq, &d.srcq, INT_MAX, d.offset, INT_MAX, 0,
+		    parent, target, 0);
 	*d.dstq = NULL;
+	if (ignore_diffs)
+		newdest = llist_mergesort(newdest, get_next_blame,
+					  set_next_blame,
+					  compare_blame_suspect);
 	queue_blames(sb, parent, newdest);
 
 	return;
@@ -1507,11 +1651,28 @@  static void pass_blame(struct blame_scoreboard *sb, struct blame_origin *origin,
 			blame_origin_incref(porigin);
 			origin->previous = porigin;
 		}
-		pass_blame_to_parent(sb, origin, porigin);
+		pass_blame_to_parent(sb, origin, porigin, 0);
 		if (!origin->suspects)
 			goto finish;
 	}
 
+	/*
+	 * Pass remaining suspects for ignored commits to their parents.
+	 */
+	if (oidset_contains(&sb->ignore_list, &commit->object.oid)) {
+		for (i = 0, sg = first_scapegoat(revs, commit, sb->reverse);
+		     i < num_sg && sg;
+		     sg = sg->next, i++) {
+			struct blame_origin *porigin = sg_origin[i];
+
+			if (!porigin)
+				continue;
+			pass_blame_to_parent(sb, origin, porigin, 1);
+			if (!origin->suspects)
+				goto finish;
+		}
+	}
+
 	/*
 	 * Optionally find moves in parents' files.
 	 */
diff --git a/blame.h b/blame.h
index be3a895043e0..91664913d7c4 100644
--- a/blame.h
+++ b/blame.h
@@ -92,6 +92,7 @@  struct blame_entry {
 	 * scanning the lines over and over.
 	 */
 	unsigned score;
+	int unblamable;
 };
 
 /*
@@ -117,6 +118,8 @@  struct blame_scoreboard {
 	/* linked list of blames */
 	struct blame_entry *ent;
 
+	struct oidset ignore_list;
+
 	/* look-up a line in the final buffer */
 	int num_lines;
 	int *lineno;
diff --git a/builtin/blame.c b/builtin/blame.c
index 177c1022a0c4..b48842d8459b 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -52,6 +52,7 @@  static int no_whole_file_rename;
 static int show_progress;
 static char repeated_meta_color[COLOR_MAXLEN];
 static int coloring_mode;
+static struct string_list ignore_revs_file_list = STRING_LIST_INIT_NODUP;
 
 static struct date_mode blame_date_mode = { DATE_ISO8601 };
 static size_t blame_date_width;
@@ -346,6 +347,8 @@  static void emit_porcelain(struct blame_scoreboard *sb, struct blame_entry *ent,
 	char hex[GIT_MAX_HEXSZ + 1];
 
 	oid_to_hex_r(hex, &suspect->commit->object.oid);
+	if (ent->unblamable)
+		memset(hex, '0', strlen(hex));
 	printf("%s %d %d %d\n",
 	       hex,
 	       ent->s_lno + 1,
@@ -479,6 +482,8 @@  static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
 			}
 		}
 
+		if (ent->unblamable)
+			memset(hex, '0', length);
 		printf("%.*s", length, hex);
 		if (opt & OUTPUT_ANNOTATE_COMPAT) {
 			const char *name;
@@ -695,6 +700,16 @@  static int git_blame_config(const char *var, const char *value, void *cb)
 		parse_date_format(value, &blame_date_mode);
 		return 0;
 	}
+	if (!strcmp(var, "blame.ignorerevsfile")) {
+		const char *str;
+		int ret;
+
+		ret = git_config_pathname(&str, var, value);
+		if (ret)
+			return ret;
+		string_list_insert(&ignore_revs_file_list, str);
+		return 0;
+	}
 	if (!strcmp(var, "color.blame.repeatedlines")) {
 		if (color_parse_mem(value, strlen(value), repeated_meta_color))
 			warning(_("invalid color '%s' in color.blame.repeatedLines"),
@@ -774,6 +789,27 @@  static int is_a_rev(const char *name)
 	return OBJ_NONE < oid_object_info(the_repository, &oid, NULL);
 }
 
+static void build_ignorelist(struct blame_scoreboard *sb,
+			     struct string_list *ignore_revs_file_list,
+			     struct string_list *ignore_rev_list)
+{
+	struct string_list_item *i;
+	struct object_id oid;
+
+	oidset_init(&sb->ignore_list, 0);
+	for_each_string_list_item(i, ignore_revs_file_list) {
+		if (!strcmp(i->string, ""))
+			oidset_clear(&sb->ignore_list);
+		else
+			oidset_parse_file(&sb->ignore_list, i->string);
+	}
+	for_each_string_list_item(i, ignore_rev_list) {
+		if (get_oid_committish(i->string, &oid))
+			die(_("Cannot find revision %s to ignore"), i->string);
+		oidset_insert(&sb->ignore_list, &oid);
+	}
+}
+
 int cmd_blame(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info revs;
@@ -785,6 +821,7 @@  int cmd_blame(int argc, const char **argv, const char *prefix)
 	struct progress_info pi = { NULL, 0 };
 
 	struct string_list range_list = STRING_LIST_INIT_NODUP;
+	struct string_list ignore_rev_list = STRING_LIST_INIT_NODUP;
 	int output_option = 0, opt = 0;
 	int show_stats = 0;
 	const char *revs_file = NULL;
@@ -806,6 +843,8 @@  int cmd_blame(int argc, const char **argv, const char *prefix)
 		OPT_BIT('s', NULL, &output_option, N_("Suppress author name and timestamp (Default: off)"), OUTPUT_NO_AUTHOR),
 		OPT_BIT('e', "show-email", &output_option, N_("Show author email instead of name (Default: off)"), OUTPUT_SHOW_EMAIL),
 		OPT_BIT('w', NULL, &xdl_opts, N_("Ignore whitespace differences"), XDF_IGNORE_WHITESPACE),
+		OPT_STRING_LIST(0, "ignore-rev", &ignore_rev_list, N_("rev"), N_("Ignore <rev> when blaming")),
+		OPT_STRING_LIST(0, "ignore-revs-file", &ignore_revs_file_list, N_("file"), N_("Ignore revisions from <file>")),
 		OPT_BIT(0, "color-lines", &output_option, N_("color redundant metadata from previous line differently"), OUTPUT_COLOR_LINE),
 		OPT_BIT(0, "color-by-age", &output_option, N_("color lines by age"), OUTPUT_SHOW_AGE_WITH_COLOR),
 
@@ -999,6 +1038,9 @@  int cmd_blame(int argc, const char **argv, const char *prefix)
 	sb.contents_from = contents_from;
 	sb.reverse = reverse;
 	sb.repo = the_repository;
+	build_ignorelist(&sb, &ignore_revs_file_list, &ignore_rev_list);
+	string_list_clear(&ignore_revs_file_list, 0);
+	string_list_clear(&ignore_rev_list, 0);
 	setup_scoreboard(&sb, path, &o);
 	lno = sb.num_lines;
 
diff --git a/t/t8013-blame-ignore-revs.sh b/t/t8013-blame-ignore-revs.sh
new file mode 100755
index 000000000000..df4993f98682
--- /dev/null
+++ b/t/t8013-blame-ignore-revs.sh
@@ -0,0 +1,168 @@ 
+#!/bin/sh
+
+test_description='ignore revisions when blaming'
+. ./test-lib.sh
+
+# Creates:
+# 	A--B--X
+# A added line 1 and B added line 2.  X makes changes to those lines.  Sanity
+# check that X is blamed for both lines.
+test_expect_success setup '
+	test_commit A file line1 &&
+
+	echo line2 >>file &&
+	git add file &&
+	test_tick &&
+	git commit -m B &&
+	git tag B &&
+
+	test_write_lines line-one line-two >file &&
+	git add file &&
+	test_tick &&
+	git commit -m X &&
+	git tag X &&
+
+	git blame --line-porcelain file >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse X >expect &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse X >expect &&
+	test_cmp expect actual
+	'
+
+# Ignore X, make sure A is blamed for line 1 and B for line 2.
+test_expect_success ignore_rev_changing_lines '
+	git blame --line-porcelain --ignore-rev X file >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse A >expect &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse B >expect &&
+	test_cmp expect actual
+	'
+
+# For ignored revs that have added 'unblamable' lines, blame those lines on an
+# all-zeros rev.
+# 	A--B--X--Y
+# Where Y changes lines 1 and 2, and adds lines 3 and 4.  The added lines ought
+# to have nothing in common with "line-one" or "line-two", to keep any
+# heuristics from matching them with any lines in the parent.
+test_expect_success ignore_rev_adding_unblamable_lines '
+	test_write_lines line-one-change line-two-changed y3 y4 >file &&
+	git add file &&
+	test_tick &&
+	git commit -m Y &&
+	git tag Y &&
+
+	git rev-parse Y >y_rev &&
+	sed -e "s/[0-9a-f]/0/g" y_rev >expect &&
+	git blame --line-porcelain file --ignore-rev Y >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 3" blame_raw | sed -e "s/ .*//" >actual &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 4" blame_raw | sed -e "s/ .*//" >actual &&
+	test_cmp expect actual
+	'
+
+# Ignore X and Y, both in separate files.  Lines 1 == A, 2 == B.
+test_expect_success ignore_revs_from_files '
+	git rev-parse X >ignore_x &&
+	git rev-parse Y >ignore_y &&
+	git blame --line-porcelain file --ignore-revs-file ignore_x --ignore-revs-file ignore_y >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse A >expect &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse B >expect &&
+	test_cmp expect actual
+	'
+
+# Ignore X from the config option, Y from a file.
+test_expect_success ignore_revs_from_configs_and_files '
+	git config --add blame.ignoreRevsFile ignore_x &&
+	git blame --line-porcelain file --ignore-revs-file ignore_y >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse A >expect &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse B >expect &&
+	test_cmp expect actual
+	'
+
+# Override blame.ignoreRevsFile (ignore_x) with an empty string.  X should be
+# blamed now for lines 1 and 2, since we are no longer ignoring X.
+test_expect_success override_ignore_revs_file '
+	git blame --line-porcelain file --ignore-revs-file "" --ignore-revs-file ignore_y >blame_raw &&
+	git rev-parse X >expect &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 2" blame_raw | sed -e "s/ .*//" >actual &&
+	test_cmp expect actual
+	'
+test_expect_success bad_files_and_revs '
+	test_must_fail git blame file --ignore-rev NOREV 2>err &&
+	test_i18ngrep "Cannot find revision NOREV to ignore" err &&
+
+	test_must_fail git blame file --ignore-revs-file NOFILE 2>err &&
+	test_i18ngrep "Could not open object name list: NOFILE" err &&
+
+	echo NOREV >ignore_norev &&
+	test_must_fail git blame file --ignore-revs-file ignore_norev 2>err &&
+	test_i18ngrep "Invalid object name: NOREV" err
+	'
+
+# Resetting the repo and creating:
+#
+# A--B--M
+#  \   /
+#   C-+
+#
+# 'A' creates a file.  B changes line 1, and C changes line 9.  M merges.
+test_expect_success ignore_merge '
+	rm -rf .git/ &&
+	git init &&
+
+	test_write_lines L1 L2 L3 L4 L5 L6 L7 L8 L9 >file &&
+	git add file &&
+	test_tick &&
+	git commit -m A &&
+	git tag A &&
+
+	test_write_lines BB L2 L3 L4 L5 L6 L7 L8 L9 >file &&
+	git add file &&
+	test_tick &&
+	git commit -m B &&
+	git tag B &&
+
+	git reset --hard A &&
+	test_write_lines L1 L2 L3 L4 L5 L6 L7 L8 CC >file &&
+	git add file &&
+	test_tick &&
+	git commit -m C &&
+	git tag C &&
+
+	test_merge M B &&
+	git blame --line-porcelain file --ignore-rev M >blame_raw &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 1" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse B >expect &&
+	test_cmp expect actual &&
+
+	grep "^[0-9a-f]\+ [0-9]\+ 9" blame_raw | sed -e "s/ .*//" >actual &&
+	git rev-parse C >expect &&
+	test_cmp expect actual
+	'
+
+test_done