[PATCH v2] dim: Check that patches have git-format-patch default prefixes

[Javier Martinez Canillas]

By default the git-format-patch command generates patches with prefixes
for the source and destination (-p1), and is also what git-am uses as a
default. The command strips the first leading path component when patch
is applied (unless a different -p<num> argument is used).

But the patch generating behaviour can be changed with git-format-patch
--no-prefix argument (or setting 'diff.noprefix = true' in .gitconfig).

Patches with no source and destination prefixes can confuse the git-am
3-way merge logic, since stripping the first path component will lead
to wrong paths for newly added files.

To avoid this, check that patches to apply are using git-format-patch's
defaults prefixes to make sure that git-am defaults are safe to use too.

Since there are developers that generate patches without using git, make
this an error but allow to still continue when using the dim -f option.

Signed-off-by: Javier Martinez Canillas <javierm at redhat.com>
---

Changes in v2:
- Use warn_or_fail instead of always making this an error (Daniel).

 dim | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/dim b/dim
index fb0dce98fb5c..1193fe056bce 100755
--- a/dim
+++ b/dim
@@ -1113,6 +1113,18 @@ function check_merge_baseline
 	fi
 }
 
+# ensure the patch has prefixes (-p1), since otherwise it can confuse the git am
+# 3-way merge logic. check the default source (a/) and destination (b/) prefixes.
+function check_diff_prefix
+{
+	local rv
+	patch="$1"
+
+	rv=$(grep -q -E "^diff --git a\/.+ b\/.+$" $patch)
+
+	return $rv
+}
+
 # ensure we're on branch $1, and apply patches. the rest of the arguments are
 # passed to git am.
 dim_alias_ab=apply-branch
@@ -1139,6 +1151,11 @@ function dim_apply_branch
 	git mailsplit -b -o$dir $file > /dev/null
 
 	for patch in "$dir"/*; do
+
+		if ! check_diff_prefix "$patch"; then
+			warn_or_fail "Patch does not contain prefixes in its diff and can confuse git-am when applying"
+		fi
+
 		if ! apply_patch $patch "$@"; then
 			rv=1
 		fi
-- 
2.33.1