plumbing: diff, fix crash when a small ending equal-chunk

[irias]

Signed-off-by: Mechiel Lukkien [email protected]

note: i'm not familiar with the code base. please review fix, and whether needs fixing in other places.

to reproduce, first create a git repo with a commit that has a small trailing equals-chunk:

mkdir git-test-repo
cd git-test-repo/
git init
yes | head -n10 >test.txt
git add test.txt
git commit -m test
(yes | head -n8; echo n; echo y) >test.txt
git commit -m test test.txt

now run the test program at the bottom, and observe the crash:

$ go run main.go -- git-test-repo/
2018/02/14 18:22:25 patch between commit
commit beccfaa9484487c5adad075ac96308ed9ab897d2
Author: Mechiel Lukkien <[email protected]>
Date:   Wed Feb 14 18:10:24 2018 +0100

    test


and
commit c408553d8c54d91bf197fad054541722d16d471e
Author: Mechiel Lukkien <[email protected]>
Date:   Wed Feb 14 18:10:24 2018 +0100

    test


panic: runtime error: slice bounds out of range

goroutine 1 [running]:
gopkg.in/src-d/go-git.v4/plumbing/format/diff.(*hunksGenerator).processEqualsLines(0xc4200edd00, 0xc4201b4540, 0x1, 0x2, 0x3)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/format/diff/unified_encoder.go:265 +0x59d
gopkg.in/src-d/go-git.v4/plumbing/format/diff.(*hunksGenerator).Generate(0xc4200edd00, 0xc42005bdc0, 0x4, 0x4)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/format/diff/unified_encoder.go:180 +0x3f2
gopkg.in/src-d/go-git.v4/plumbing/format/diff.(*UnifiedEncoder).encodeFilePatch(0xc4200d6400, 0xc420124770, 0x1, 0x1, 0x1, 0xc4200d6400)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/format/diff/unified_encoder.go:85 +0x208
gopkg.in/src-d/go-git.v4/plumbing/format/diff.(*UnifiedEncoder).Encode(0xc4200d6400, 0x1612a00, 0xc42013e7b0, 0x13e40e0, 0x101)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/format/diff/unified_encoder.go:68 +0x65
gopkg.in/src-d/go-git.v4/plumbing/object.(*Patch).Encode(0xc42013e7b0, 0x160e980, 0xc4201b23f0, 0xc4200ede70, 0x10cb200)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/object/patch.go:107 +0x7c
gopkg.in/src-d/go-git.v4/plumbing/object.(*Patch).String(0xc42013e7b0, 0x1f, 0xc4200edf50)
	/Users/mjl/go/src/gopkg.in/src-d/go-git.v4/plumbing/object/patch.go:116 +0x5a
main.main()
	/Users/mjl/go/src/test/go-git-diff-bug/main.go:51 +0x3d8
exit status 2

then apply the patch in this pull request, and see the expected patch:

$ go run main.go -- git-test-repo/
2018/02/14 18:12:35 patch between commit
commit beccfaa9484487c5adad075ac96308ed9ab897d2
Author: Mechiel Lukkien <[email protected]>
Date:   Wed Feb 14 18:10:24 2018 +0100

    test


and
commit c408553d8c54d91bf197fad054541722d16d471e
Author: Mechiel Lukkien <[email protected]>
Date:   Wed Feb 14 18:10:24 2018 +0100

    test


2018/02/14 18:12:35 diff --git a/test.txt b/test.txt
index db7cbfc682f6343bb0e71e0cf774ba7a1ca1a37b..67c82c94116850ddfcf70fd43ae4ab2eb2242503 100644
--- a/test.txt
+++ b/test.txt
@@ -6,5 +6,5 @@ y
 y
 y
 y
-y
+n
 y

and the attached program that triggers the bug:

package main

import (
	"flag"
	"log"

	"gopkg.in/src-d/go-git.v4"
	"gopkg.in/src-d/go-git.v4/plumbing/object"
)


func check(err error, msg string) {
	if err != nil {
		log.Fatalf("%s: %s\n", msg, err)
	}
}

func main() {
	flag.Parse()
	args := flag.Args()
	if len(args) != 1 {
		log.Fatalf("pass git repo directory as parameter")
	}
	dir := args[0]
	repo, err := git.PlainOpen(dir)
	check(err, "plainopen")

	iter, err := repo.CommitObjects()
	check(err, "commitobjects")

	c0, err := iter.Next()
	check(err, "commit1")
	c1, err := iter.Next()
	check(err, "commit2")

	commits := []*object.Commit{c0, c1}
	if len(commits[0].ParentHashes) != 0 {
		commits = []*object.Commit{c1, c0}
	}

	t0, err := repo.TreeObject(commits[0].TreeHash)
	check(err, "treeobject first commit")
	t1, err := repo.TreeObject(commits[1].TreeHash)
	check(err, "treeobject second commit")

	p, err := t0.Patch(t1)
	check(err, "patch")

	log.Printf("patch between commit\n%s\nand\n%s\n", commits[0], commits[1])

	diff := p.String() // crashes

	log.Print(diff)
}

[ajnavarro]

Could you add a test for this case please?

[irias]

test has been added.

[mjl-]

fyi, PR and comments are showing up as created by "ghost" because i turned my company github user account i was using into an organization.

fix addressed