Fix out-of-bounds error when a push doesn't contain commits

[typeless]

This happens when I push a new tag with git tag -af foo -m "bar" && git push --tags. Both the 'create' and 'push' events are triggered.
The log of the drone server would show an internal panic like the following:

2018/06/11 08:34:56 [Recovery] panic recovered:
POST /hook?access_token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ0ZXh0IjoibXVyYS9OVFREQVRBLXBsdWdpbiIsInR5cGUiOiJob29rIn0.IDq3cvR1LNHTvtrWWrmqW4GfyterT8B4tKS_MDUFBMQ HTTP/1.1
Host: 192.120.100.224
Accept-Encoding: gzip
Content-Length: 2902
Content-Type: application/json
User-Agent: GiteaServer
X-Gitea-Delivery: ad3bd9f2-571d-4dac-91d7-ec6b63baf5e7
X-Gitea-Event: push
X-Github-Delivery: ad3bd9f2-571d-4dac-91d7-ec6b63baf5e7
X-Github-Event: push
X-Gogs-Delivery: ad3bd9f2-571d-4dac-91d7-ec6b63baf5e7
X-Gogs-Event: push


runtime error: index out of range
/usr/local/go/src/runtime/panic.go:489 (0x430c7f)
/usr/local/go/src/runtime/panic.go:28 (0x42f8fe)
/go/src/github.com/drone/drone/remote/gitea/helper.go:91 (0xa1cf2f)
/go/src/github.com/drone/drone/remote/gitea/parse.go:72 (0xa1e61b)
/go/src/github.com/drone/drone/remote/gitea/parse.go:44 (0xa1e471)
/go/src/github.com/drone/drone/remote/gitea/gitea.go:338 (0xa1c4db)
/go/src/github.com/drone/drone/server/hook.go:72 (0x932cd4)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/router/middleware/token/token.go:31 (0x8a1620)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/router/middleware/session/user.go:82 (0x8a0ac2)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/router/middleware/store.go:29 (0x9a0fa7)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/contrib/ginrus/ginrus.go:26 (0x9a13ff)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/router/middleware/header/header.go:39 (0x898630)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/router/middleware/header/header.go:31 (0x8985d9)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/recovery.go:45 (0x89629a)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/context.go:97 (0x88800a)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/gin.go:284 (0x88de40)
/go/src/github.com/drone/drone/vendor/github.com/gin-gonic/gin/gin.go:265 (0x88d75b)
/usr/local/go/src/net/http/server.go:2568 (0x6cc862)
/usr/local/go/src/net/http/server.go:1825 (0x6c8a02)
/usr/local/go/src/runtime/asm_amd64.s:2197 (0x45f881)

[CLAassistant]

All committers have signed the CLA.

[typeless]

I tried to test this on my local machine. I put cmd/drone-server/drone-server and cmd/drone-agent/drone-agent into release/ and ran the Dockerfiles but it didn't work, not event started correctly.

Any pointers?

[tibkiss]

Hitting the same issue.
I'm not sure how to bring up the build, but extending the helper_tests.go with a testcase might be helpful to avoid regression in the future.

[tboerger]

https://github.com/drone/drone/blob/master/.drone.sh pretty clear shows how Drone gets built.

[tibkiss]

@tboerger : what would be the acceptance criteria to merge this issue? I'd love to see this in a new release as it blocks tag based builds using gitea.

[typeless]

@tboerger @tibkiss I am sorry I ignored @tboerger 's comment which was buried in the notifications. However, my problem was about creating the docker images rather than building the executables. Anyway, I had moved on and lost the dev setup so I am reluctant to proceed the testing. The fix is trivial so if anyone would like to proceed or submit a new PR by yourself I would appreciate.

[tboerger]

@bradrydzewski is this already covered by the new scm lib?

[bradrydzewski]

@tboerger nope, looks like it needs to be fixed in go-scm for pull requests. The relevant code can be found here:

https://github.com/drone/go-scm/blob/master/scm/driver/gitea/webhook.go#L197:L209