go-github: add support for commit reference SHA-1 method

Fixes #297
Fixes #298

Change-Id: Ib17c43b368a0d0ee1a33690c17ad7c0f4dab55e5

github/github.go

@@ -59,6 +59,9 @@ const (
 
 	// https://developer.github.com/changes/2016-02-11-issue-locking-api/
 	mediaTypeIssueLockingPreview = "application/vnd.github.the-key-preview+json"
+
+	// https://developer.github.com/changes/2016-02-24-commit-reference-sha-api/
+	mediaTypeCommitReferenceSHAPreview = "application/vnd.github.chitauri-preview+sha"
 )
 
 // A Client manages communication with the GitHub API.

github/repos_commits.go

@@ -6,6 +6,7 @@
 package github
 
 import (
+	"bytes"
 	"fmt"
 	"time"
 )
@@ -146,6 +147,33 @@ func (s *RepositoriesService) GetCommit(owner, repo, sha string) (*RepositoryCom
 	return commit, resp, err
 }
 
+// GetCommitSHA1 gets the SHA-1 of a commit reference.  If a last-known SHA1 is
+// supplied and no new commits have occurred, a 304 Unmodified response is returned.
+//
+// GitHub API docs: https://developer.github.com/v3/repos/commits/#get-the-sha-1-of-a-commit-reference
+func (s *RepositoriesService) GetCommitSHA1(owner, repo, ref, lastSHA string) (string, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/commits/%v", owner, repo, ref)
+
+	req, err := s.client.NewRequest("GET", u, nil)
+	if err != nil {
+		return "", nil, err
+	}
+	if lastSHA != "" {
+		req.Header.Set("If-None-Match", `"`+lastSHA+`"`)
+	}
+
+	// TODO: remove custom Accept header when this API fully launches.
+	req.Header.Set("Accept", mediaTypeCommitReferenceSHAPreview)
+
+	var buf bytes.Buffer
+	resp, err := s.client.Do(req, &buf)
+	if err != nil {
+		return "", resp, err
+	}
+
+	return buf.String(), resp, err
+}
+
 // CompareCommits compares a range of commits with each other.
 // todo: support media formats - https://github.com/google/go-github/issues/6
 //

github/repos_commits_test.go

@@ -119,6 +119,47 @@ func TestRepositoriesService_GetCommit(t *testing.T) {
 	}
 }
 
+func TestRepositoriesService_GetCommitSHA1(t *testing.T) {
+	setup()
+	defer teardown()
+	const sha1 = "01234abcde"
+
+	mux.HandleFunc("/repos/o/r/commits/master", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "GET")
+		testHeader(t, r, "Accept", mediaTypeCommitReferenceSHAPreview)
+
+		fmt.Fprintf(w, sha1)
+	})
+
+	got, _, err := client.Repositories.GetCommitSHA1("o", "r", "master", "")
+	if err != nil {
+		t.Errorf("Repositories.GetCommitSHA1 returned error: %v", err)
+	}
+
+	want := sha1
+	if got != want {
+		t.Errorf("Repositories.GetCommitSHA1 = %v, want %v", got, want)
+	}
+
+	mux.HandleFunc("/repos/o/r/commits/tag", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "GET")
+		testHeader(t, r, "Accept", mediaTypeCommitReferenceSHAPreview)
+		testHeader(t, r, "If-None-Match", `"`+sha1+`"`)
+
+		w.WriteHeader(http.StatusNotModified)
+	})
+
+	got, _, err = client.Repositories.GetCommitSHA1("o", "r", "tag", sha1)
+	if err == nil {
+		t.Errorf("Expected HTTP 304 response")
+	}
+
+	want = ""
+	if got != want {
+		t.Errorf("Repositories.GetCommitSHA1 = %v, want %v", got, want)
+	}
+}
+
 func TestRepositoriesService_CompareCommits(t *testing.T) {
 	setup()
 	defer teardown()