Support fetching/checkouts of non-branch/-tag git refs (#322)

* Support fetching/checkouts of non-branch/-tag git refs

Provides a `source.git.ref` field for specifying a specific non-branch
non-tag git ref at which to perform a checkout. Providing one will
result in a shallow (depth 1) clone of the repo and shallow subsequent
fetches.

The motivation for this change is to support sourcing of trigger
templates from Gerrit patchsets. Gerrit uses non-branch/-tag refs
extensively and specifically in this case stores change patchsets in
`refs/changes/[slice]/[change]/[patchset]`. While arbitrary refs are
kind of an obscure git feature, there very well may be other git based
systems utilizing them.

* Fixed protobuf field index for Ref

pkg/apis/sensor/v1alpha1/types.go

@@ -466,10 +466,15 @@ type GitArtifact struct {
 	// +optional
 	Tag string `json:"tag,omitempty" protobuf:"bytes,8,opt,name=tag"`
 
+	// Ref to use to pull trigger resource. Will result in a shallow clone and
+	// fetch.
+	// +optional
+	Ref string `json:"ref,omitempty" protobuf:"bytes,9,opt,name=ref"`
+
 	// Remote to manage set of tracked repositories. Defaults to "origin".
 	// Refer https://git-scm.com/docs/git-remote
 	// +optional
-	Remote *GitRemoteConfig `json:"remote" protobuf:"bytes,9,opt,name=remote"`
+	Remote *GitRemoteConfig `json:"remote" protobuf:"bytes,10,opt,name=remote"`
 }
 
 // GitRemoteConfig contains the configuration of a Git remote

store/git.go

@@ -138,6 +138,13 @@ func (g *GitArtifactReader) readFromRepository(r *git.Repository) ([]byte, error
 		fetchOptions.Auth = auth
 	}
 
+	// In the case of a specific given ref, it isn't necessary to fetch anything
+	// but the single ref
+	if g.artifact.Ref != "" {
+		fetchOptions.Depth = 1
+		fetchOptions.RefSpecs = []config.RefSpec{config.RefSpec(g.artifact.Ref + ":" + g.artifact.Ref)}
+	}
+
 	if err := r.Fetch(fetchOptions); err != nil && err != git.NoErrAlreadyUpToDate {
 		return nil, fmt.Errorf("failed to fetch. err: %v", err)
 	}
@@ -146,17 +153,20 @@ func (g *GitArtifactReader) readFromRepository(r *git.Repository) ([]byte, error
 		return nil, fmt.Errorf("failed to checkout. err: %+v", err)
 	}
 
-	pullOpts := &git.PullOptions{
-		RecurseSubmodules: git.DefaultSubmoduleRecursionDepth,
-		ReferenceName:     g.getBranchOrTag().Branch,
-		Force:             true,
-	}
-	if auth != nil {
-		pullOpts.Auth = auth
-	}
+	// In the case of a specific given ref, it shouldn't be necessary to pull
+	if g.artifact.Ref != "" {
+		pullOpts := &git.PullOptions{
+			RecurseSubmodules: git.DefaultSubmoduleRecursionDepth,
+			ReferenceName:     g.getBranchOrTag().Branch,
+			Force:             true,
+		}
+		if auth != nil {
+			pullOpts.Auth = auth
+		}
 
-	if err := w.Pull(pullOpts); err != nil && err != git.NoErrAlreadyUpToDate {
-		return nil, fmt.Errorf("failed to pull latest updates. err: %+v", err)
+		if err := w.Pull(pullOpts); err != nil && err != git.NoErrAlreadyUpToDate {
+			return nil, fmt.Errorf("failed to pull latest updates. err: %+v", err)
+		}
 	}
 
 	return ioutil.ReadFile(fmt.Sprintf("%s/%s", g.artifact.CloneDirectory, g.artifact.FilePath))
@@ -173,6 +183,9 @@ func (g *GitArtifactReader) getBranchOrTag() *git.CheckoutOptions {
 	if g.artifact.Tag != "" {
 		opts.Branch = plumbing.NewTagReferenceName(g.artifact.Tag)
 	}
+	if g.artifact.Ref != "" {
+		opts.Branch = plumbing.ReferenceName(g.artifact.Ref)
+	}
 
 	return opts
 }
@@ -197,6 +210,12 @@ func (g *GitArtifactReader) Read() ([]byte, error) {
 			cloneOpt.Auth = auth
 		}
 
+		// In the case of a specific given ref, it isn't necessary to have branch
+		// histories
+		if g.artifact.Ref != "" {
+			cloneOpt.Depth = 1
+		}
+
 		r, err = git.PlainClone(g.artifact.CloneDirectory, false, cloneOpt)
 		if err != nil {
 			return nil, fmt.Errorf("failed to clone repository. err: %+v", err)

store/git_test.go

@@ -94,4 +94,10 @@ func TestGetBranchOrTag(t *testing.T) {
 		tag := gar.getBranchOrTag()
 		convey.So(tag.Branch, convey.ShouldNotEqual, "refs/heads/master")
 	})
+
+	convey.Convey("Given a git artifact with a specific ref, get the ref", t, func() {
+		gar.artifact.Ref = "refs/something/weird/or/specific"
+		br := gar.getBranchOrTag()
+		convey.So(br.Branch, convey.ShouldEqual, "refs/something/weird/or/specific")
+	})
 }