examples: Add example for SHA256 repositories

Add a new example on initializing a new repository with object format SHA256.
A new Makefile target was created to run the example to ensure that new changes
won't break SHA256 support going forwards.

Relates to the SHA256 implementation, defined in #706.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

3 files changed, 75 insertions, 0 deletions

diff --git a/.github/workflows/git.yml b/.github/workflows/git.yml
index c945e72..3a40c0c 100644
--- a/.github/workflows/git.yml
+++ b/.github/workflows/git.yml
@@ -42,6 +42,9 @@ jobs:
     - name: Test
       run: make test-coverage
 
+    - name: Test SHA256
+      run: make test-sha256
+
     - name: Build go-git with CGO disabled
       run: go build ./...
       env:
diff --git a/Makefile b/Makefile
index 2acb8bc..f0c1abb 100644
--- a/Makefile
+++ b/Makefile
@@ -29,6 +29,12 @@ test:
 	@echo "running against `git version`"; \
 	$(GOTEST) -race ./...
 
+TEMP_REPO := $(shell mktemp)
+test-sha256:
+	$(GOCMD) run -tags sha256 _examples/sha256/main.go $(TEMP_REPO)
+	cd $(TEMP_REPO) && git fsck
+	rm -rf $(TEMP_REPO)
+
 test-coverage:
 	@echo "running against `git version`"; \
 	echo "" > $(COVERAGE_REPORT); \
diff --git a/_examples/sha256/main.go b/_examples/sha256/main.go
new file mode 100644
index 0000000..2a257e8
--- /dev/null
+++ b/_examples/sha256/main.go
@@ -0,0 +1,66 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/go-git/go-git/v5"
+	. "github.com/go-git/go-git/v5/_examples"
+	"github.com/go-git/go-git/v5/plumbing/format/config"
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+// This example requires building with the sha256 tag for it to work:
+// go run -tags sha256 main.go /tmp/repository
+
+// Basic example of how to initialise a repository using sha256 as the hashing algorithmn.
+func main() {
+	CheckArgs("<directory>")
+	directory := os.Args[1]
+
+	os.RemoveAll(directory)
+
+	// Init a new repository using the ObjectFormat SHA256.
+	r, err := git.PlainInitWithOptions(directory, &git.PlainInitOptions{ObjectFormat: config.SHA256})
+	CheckIfError(err)
+
+	w, err := r.Worktree()
+	CheckIfError(err)
+
+	// ... we need a file to commit so let's create a new file inside of the
+	// worktree of the project using the go standard library.
+	Info("echo \"hello world!\" > example-git-file")
+	filename := filepath.Join(directory, "example-git-file")
+	err = ioutil.WriteFile(filename, []byte("hello world!"), 0644)
+	CheckIfError(err)
+
+	// Adds the new file to the staging area.
+	Info("git add example-git-file")
+	_, err = w.Add("example-git-file")
+	CheckIfError(err)
+
+	// Commits the current staging area to the repository, with the new file
+	// just created. We should provide the object.Signature of Author of the
+	// commit Since version 5.0.1, we can omit the Author signature, being read
+	// from the git config files.
+	Info("git commit -m \"example go-git commit\"")
+	commit, err := w.Commit("example go-git commit", &git.CommitOptions{
+		Author: &object.Signature{
+			Name:  "John Doe",
+			Email: "john@doe.org",
+			When:  time.Now(),
+		},
+	})
+
+	CheckIfError(err)
+
+	// Prints the current HEAD to verify that all worked well.
+	Info("git show -s")
+	obj, err := r.CommitObject(commit)
+	CheckIfError(err)
+
+	fmt.Println(obj)
+}