git blame ignore whitespace option buggy?

Question

Based on my understanding, the command git blame is supposed to show, for each line in a file, the author and the commit in which the line was last modified. So for example, if I run git blame -- "<filename>" and get the following output for line 5:

106b77db (Walrus 2016-03-24 10:01:36 +0800   5) .root {

it means the line .root { originated from the author Walrus in the commit 106b77db. In other words, if I inspect the patch produced by 106b77db using git show -p 106b77db, I would expect the line +.root { to show up in the diff. Indeed, this is the case.

Snippet from 106b77db's diff for <filename>:

 /* JavaFX CSS - Leave this comment until you have at least create one rule       which uses -fx-Property */
+.root {
+   -fx-background-color: transparent;
+}
+

Now, when I run git blame -w -- "<filename>", (the -w option ignores whitespace changes, i.e. traces each line backwards in time to find the last author which introduced non-whitespace changes to that line), I now get the following output for line 5:

b6a6e8a2 (Walrus 2016-03-31 23:32:50 +0800   5) .root {

However, when I inspect the patch for b6a6e8a2 using git show -p b6a6e8a2, the diff shows .root { rather than +.root { as expected.

Snippet from b6a6e8a2's diff for <filename>:

+
+/* setting window to be transparent================================ */
 .root {
    -fx-background-color: transparent;
-}
-

Has Git given me erroneous output, because according to the diff, the line .root { was not modified at all in the commit b6a6e8a2?

I am using Git 2.13.3.windows.1.

EDIT: the repository is https://github.com/cs2103jan2016-f14-2j/main, and the file is JimplePlanner/src/application.css. After upgrading to Git 2.16.1.windows.4, the issue still persists.

Maraud

I can't reproduce this behavior in simple tests, so you'll probably need to give more specific information. Instead of describing the commands you issued and the results, you should show the exact command as it was typed and the exact output as it was printed. – Desinence

...or better yet, either link to the repository in question or set up a reproducible example to which we can have access. – Falla

@Falla Added the full commands, relevant output snippets, the repository link and filename. I did not show the full output as it is hundreds of lines long, and most of it is not relevant to the question. – Maraud

@MarkAdelsberger Updated. Could not tag you and larsks in the same comment. – Maraud

Can anyone at least verify that this example is reproducible on their machine? – Maraud

Bump, hope someone is willing and able to help. – Maraud

Answer 1

five years later I encountered the same issue and floundered in trying to understand what happened.

It turned out that when you ignore whitespace, git sometimes understands the change to have been a completely different one, because of a repeating pattern that, after whitespace elision, changes the location of the detected "smallest change".

Running git diff -w vs git diff on the "incorrect" revision (in your case, b6a6e8a) illustrates the problem:

git diff b6a6e8a~..b6a6e8a:

diff --git a/JimplePlanner/src/application.css b/JimplePlanner/src/application.css
index e4634ef..c0cc44b 100644
--- a/JimplePlanner/src/application.css
+++ b/JimplePlanner/src/application.css
@@ -1,248 +1,108 @@
 /* JavaFX CSS - Leave this comment until you have at least create one rule which uses -fx-Property */
+
+/* setting window to be transparent================================ */
 .root {
        -fx-background-color: transparent;
-}
-
-.today-pane {

git diff -w b6a6e8a~..b6a6e8a:

diff --git a/JimplePlanner/src/application.css b/JimplePlanner/src/application.css
index e4634ef..c0cc44b 100644
--- a/JimplePlanner/src/application.css
+++ b/JimplePlanner/src/application.css
@@ -1,248 +1,108 @@
 /* JavaFX CSS - Leave this comment until you have at least create one rule which uses -fx-Property */
-.root {
-       -fx-background-color: transparent;
-}
-
-.today-pane {
-       -fx-background-radius: 10px;
-    -fx-background-color: white;
-    -fx-border-color: grey;
-    -fx-border-radius: 10px;
-}
-
-.list-view {
-    -fx-padding: 3px ;
-    -fx-background-color: transparent;
-}
-
- .list-cell {
-    -fx-padding: 10px ;
-    -fx-background-color: transparent, -fx-background ;
-}

-
-.list-cell:empty {
-    -fx-padding: 10px;
+/* setting window to be transparent================================ */
+.root {
        -fx-background-color: transparent;
-    -fx-background-insets: 0 ;
-}
+}/*============================================++================== */

When whitespace is ignored, the diff algorithm ends up counting .root { as removed in one place, and added back in a different place.

This is just a consequence of ignoring whitespace - it will sometimes interpret changes that were made substantially differently to how you think about them; more often than when not ignoring whitespace.

Cidevant