difference of info from diff and blame

BUG Semyon Kirnosenko 2 participants 4 messages Jun 15, 2016

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Semyon Kirnosenko

author

2016-06-15 22:50 UTC #0 lore raw

Hi.

I'm working on automatic processing of info from git. And I have some problems. Let me show it on an axample.

I have jquery repo (https://github.com/jquery/jquery.git)
Let's get blame for some file in some revision:
git blame -l -s 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js According to blame, line 127 was added in revision 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6. Let's get diff for that revision:
git diff-tree -p 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js We can see this:
@@ -105,19 +120,16 @@ jQuery.event = {

// Handle triggering a single element else {
- var handler = element["on" + type ], val,
- fn = jQuery.isFunction( element[ type
+ var val, ret, fn = jQuery.isFunction( element
+
+ // Pass along a fake event
+ data.unshift( this.fix({ type: type, target:

- if ( handler ) {

As you can see line 127 is not marked with '+' char, which means it was not added in this revision. But blame sad otherwise.

I had the same problem on a wide range of repos.
I think it's a bug.

Thomas Rast

2016-06-15 22:50 UTC #1 lore raw

Semyon Kirnosenko wrote:
[Word wrap fixed.]

Quoted 6 lines

I have jquery repo (https://github.com/jquery/jquery.git)
Let's get blame for some file in some revision:
git blame -l -s 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js According to blame, line 127 was added in revision 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6.

The surrounding context (with authorship and some whitespace snipped, obviously it's always the same) is

2ad223ae (124)
2ad223ae (125) // Pass along a fake event
2ad223ae (126) data.unshift( this.fix({ type: type, target: element }) );
2ad223ae (127)
2ad223ae (128) // Trigger the event
2ad223ae (129) if ( (val = this.handle.apply( element, data )) !== false )
2ad223ae (130) this.triggered = true;

Quoted 4 lines

Let's get diff for that revision:
git diff-tree -p 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js We can see this:

Review jQuery.event = { · line 127

Quoted 10 lines

(quoted diff) +4 -2

@@ -105,19 +120,16 @@ jQuery.event = {
 
  		// Handle triggering a single element
  		else {
-			var handler = element["on" + type ], val,
-				fn = jQuery.isFunction( element[ type
+			var val, ret, fn = jQuery.isFunction( element
+			
+			// Pass along a fake event
+			data.unshift( this.fix({ type: type, target:

[this blank context line is line 127 in the postimage]

Quoted 4 lines

(quoted diff) +0 -1
-			if ( handler ) {
 
As you can see line 127 is not marked with '+' char, which means it was not added in this revision. But blame sad otherwise.

git-blame internally runs a diff with no context lines to "pass blame". On all lines in this diff, the current commit can pass on blame to the parent, thus avoiding having to take it for itself.

And indeed, running

git show -U0 2ad223ae -- src/event/event.js

in your repository gives a hunk

@@ -108,11 +123,8 @@ jQuery.event = {
- var handler = element["on" + type ], val,
- fn = jQuery.isFunction( element[ type ] );
-
- if ( handler ) {
- // Pass along a fake event
- data.unshift( this.fix({ type: type, target: element }) );
-
- // Trigger the event
- if ( (val = handler.apply( element, data )) !== false )
- this.triggered = true;
- }
+ var val, ret, fn = jQuery.isFunction( element[ type ] );
+
+ // Pass along a fake event
+ data.unshift( this.fix({ type: type, target: element }) );
+
+ // Trigger the event
+ if ( (val = this.handle.apply( element, data )) !== false )
+ this.triggered = true;

So I would tend to agree with the blame implementation. If anything it's a bug in git-diff when not using any context.

Do you have a reproduction recipe that exhibits the flaw on a non-whitespace line? I'm not up to speed on the diff implementation (maybe someone else can help?), but I wouldn't be too surprised if it had heuristics that put lines consisting only of whitespace at a lower importance than "actual" lines.

Signature · 2 lines

Thomas Rast
trast@{inf,student}.ethz.ch

Semyon Kirnosenko

author

2016-06-15 22:50 UTC #2 lore raw

11.01.2011 16:40, Thomas Rast пишет:

Quoted 72 lines

Semyon Kirnosenko wrote:
[Word wrap fixed.]
Quoted 6 lines
I have jquery repo (https://github.com/jquery/jquery.git)
Let's get blame for some file in some revision:
git blame -l -s 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js According to blame, line 127 was added in revision 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6.
The surrounding context (with authorship and some whitespace snipped, obviously it's always the same) is
2ad223ae (124)
2ad223ae (125) // Pass along a fake event
2ad223ae (126) data.unshift( this.fix({ type: type, target: element }) );
2ad223ae (127)
2ad223ae (128) // Trigger the event
2ad223ae (129) if ( (val = this.handle.apply( element, data )) !== false )
2ad223ae (130) this.triggered = true;
Quoted 4 lines
Let's get diff for that revision:
git diff-tree -p 2ad223aedd1f93c783d98d60adc9fda3bdfbb4b6 -- src/event/event.js We can see this:
Review jQuery.event = { · line 127
Quoted 10 lines
(quoted diff) +4 -2
@@ -105,19 +120,16 @@ jQuery.event = {
 
   		// Handle triggering a single element
   		else {
-			var handler = element["on" + type ], val,
-				fn = jQuery.isFunction( element[ type
+			var val, ret, fn = jQuery.isFunction( element
+			
+			// Pass along a fake event
+			data.unshift( this.fix({ type: type, target:
 
[this blank context line is line 127 in the postimage]
Quoted 4 lines
(quoted diff) +0 -1
-			if ( handler ) {
 
As you can see line 127 is not marked with '+' char, which means it was not added in this revision. But blame sad otherwise.
git-blame internally runs a diff with no context lines to "pass blame". On all lines in this diff, the current commit can pass on blame to the parent, thus avoiding having to take it for itself.
And indeed, running
   git show -U0 2ad223ae -- src/event/event.js
in your repository gives a hunk
(quoted diff) +8 ~~-11~~
@@ -108,11 +123,8 @@ jQuery.event = {
-			var handler = element["on" + type ], val,
-				fn = jQuery.isFunction( element[ type ] );
-
-			if ( handler ) {
-				// Pass along a fake event
-				data.unshift( this.fix({ type: type, target: element }) );
-	
-				// Trigger the event
-				if ( (val = handler.apply( element, data )) !== false )
-					this.triggered = true;
-			}
+			var val, ret, fn = jQuery.isFunction( element[ type ] );
+			
+			// Pass along a fake event
+			data.unshift( this.fix({ type: type, target: element }) );
+
+			// Trigger the event
+			if ( (val = this.handle.apply( element, data )) !== false )
+				this.triggered = true;

Quoted 9 lines

So I would tend to agree with the blame implementation. If anything it's a bug in git-diff when not using any context.
Do you have a reproduction recipe that exhibits the flaw on a non-whitespace line? I'm not up to speed on the diff implementation (maybe someone else can help?), but I wouldn't be too surprised if it had heuristics that put lines consisting only of whitespace at a lower importance than "actual" lines.

All cases I have seen were about whitespace lines.

Signature · 3 lines

Regards,
Semyon Kirnosenko
[email protected]

Thomas Rast

2016-06-15 22:50 UTC #3 lore raw

diff -U0 occasionally misses a chance to make empty lines context [was: Re: [BUG] difference of info from diff and blame]

Ok, so to summarize for late joiners and the new Ccs:

git diff -U0 occasionally gives suboptimal results in hunks like

Quoted 13 lines · Semyon Kirnosenko

11.01.2011 16:40, Thomas Rast пишет:

Quoted 12 lines

(quoted diff) +0 ~~-11~~

@@ -108,11 +123,8 @@ jQuery.event = {
-			var handler = element["on" + type ], val,
-				fn = jQuery.isFunction( element[ type ] );
-
-			if ( handler ) {
-				// Pass along a fake event
-				data.unshift( this.fix({ type: type, target: element }) );
-	
-				// Trigger the event
-				if ( (val = handler.apply( element, data )) !== false )
-					this.triggered = true;
-			}

Quoted 8 lines

Review jQuery.event = { · line 130

Quoted 8 lines

(quoted diff) +8 -0

+			var val, ret, fn = jQuery.isFunction( element[ type ] );
+			
+			// Pass along a fake event
+			data.unshift( this.fix({ type: type, target: element }) );
+
+			// Trigger the event
+			if ( (val = this.handle.apply( element, data )) !== false )
+				this.triggered = true;

where two of the whitespace lines are the same and thus could have been made context. Oddly enough, this specific line *is* made context for the default -U3. This occasionally causes git-blame (which uses the equivalent of 'git diff -U0' internally) to misattribute blank lines. This does *not* affect correctness of the diff, it is just suboptimal.

My own guess at the problem is

Quoted 3 lines

Quoted 3 lines
I wouldn't be too surprised if it had heuristics that put lines consisting only of whitespace at a lower importance than "actual" lines.

i.e. that it's whitespace-related, and indeed Semyon says

Quoted 1 line

All cases I have seen were about whitespace lines.

I don't really care, since I haven't seen a single use-case where the attribution of empty lines matters. Still, let's at least bring it to the attention of the people who have worked on libxdiff.

Signature · 2 lines

Thomas Rast
trast@{inf,student}.ethz.ch