[BUG] difference of info from diff and blame

[BUG] difference of info from diff and blame

[Semyon Kirnosenko]

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.

Re: [BUG] difference of info from diff and blame

[Thomas Rast]

Semyon Kirnosenko wrote:
[Word wrap fixed.]
> 
> 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;


> 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:
> 
[this blank context line is line 127 in the postimage]
> -			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.

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

Re: [BUG] difference of info from diff and blame

[Semyon Kirnosenko]

11.01.2011 16:40, Thomas Rast пишет:
> Semyon Kirnosenko wrote:
> [Word wrap fixed.]
>>
>> 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;
>
>
>> 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:
>>
> [this blank context line is line 127 in the postimage]
>> -			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.
>

All cases I have seen were about whitespace lines.

-- 
Regards,
Semyon Kirnosenko
kirnosenko@mail.ru

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

[Thomas Rast]

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

git diff -U0 occasionally gives suboptimal results in hunks like

Semyon Kirnosenko wrote:
> 11.01.2011 16:40, Thomas Rast пишет:
> > @@ -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;

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

> > 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

> 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.

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