Does increasing the number of unified diff context lines have any downsides?

Question

By default, diff -u and git diff produce unified diffs with context lines. Apart from the size of the diff file itself, is there any disadvantage to increasing the number of context lines? I assume that it may help in cases where the file(s) to be patched have been modified since the patch was made. Specifically, if you increase the number of context lines, are there cases where patch will fail, where it wouldn't have if you hadn't done that?

Answer 1

Yes. Consider the following case:

There's a file f1:

a
b
c
d
e
f
g

You modify the f line, and get either

--- f1  2013-04-15 13:23:57.524966109 +0200
+++ f2  2013-04-15 13:25:17.832965720 +0200
@@ -5,3 +5,3 @@
 e
-f
+f2
 g

or

--- f1  2013-04-15 13:23:57.524966109 +0200
+++ f2  2013-04-15 13:25:17.832965720 +0200
@@ -1,7 +1,7 @@
 a
 b
 c
 d
 e
-f
+f2
 g

depending on whether you use the -U1 or the -U5 option with diff. Assume now that someone else edited the upper section of the file as follows:

a
b1
c
d
e
f
g

Here's the output of the two patch commands:

$ patch f3 < u1.patch 
patching file f3
$ patch f3 < u5.patch 
patching file f3
Hunk #1 succeeded at 1 with fuzz 2.

The patch was successfully applied in both scenarios, however, in the second run we had to use a fuzz value of 2. What does that mean?

First patch looks for a place where all lines of the context match. If no such place is found, and it's a context diff, and the maximum fuzz factor is set to 1 or more, then another scan takes place ignoring the first and last line of context. If that fails, and the maximum fuzz factor is set to 2 or more, the first two and last two lines of context are ignored, and another scan is made.

As you can see from this description from man patch, the patch created with the -U5 version will take longer to apply in such a scenario, or even worse, if the fuzz value used by patch isn't big enough, applying the patch will fail.