Introduce small feature flag package

Feature flags on the Gitaly server weren't possible before and it was
suggested in https://gitlab.com/gitlab-org/gitaly/issues/1170 that
Gitaly could do this through gRPC metadata.

This gives control on wether of not to execute new code paths to Rails,
where the server needs to set a header in the form of
`gitaly-feature-<flag_name>` to "true".

3 files changed, 70 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 589720c3f..4a1a75bad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Gitaly changelog
 
+UNRELEASED
+
+- Introduce feature flag package based on gRPC metadata
+  https://gitlab.com/gitlab-org/gitaly/merge_requests/704
+
 v0.98.0
 
 - Server implementation for repository raw_changes
diff --git a/internal/featureflag/grpc_header.go b/internal/featureflag/grpc_header.go
new file mode 100644
index 000000000..8d63fb41f
--- /dev/null
+++ b/internal/featureflag/grpc_header.go
@@ -0,0 +1,34 @@
+package featureflag
+
+import (
+	"fmt"
+
+	"golang.org/x/net/context"
+	"google.golang.org/grpc/metadata"
+)
+
+// IsEnabled checks if the feature flag is enabled for the passed context.
+// Only return true if the metadata for the feature flag is set to "true"
+func IsEnabled(ctx context.Context, flag string) bool {
+	if flag == "" {
+		return false
+	}
+
+	md, ok := metadata.FromIncomingContext(ctx)
+	if !ok {
+		return false
+	}
+
+	headerKey := fmt.Sprintf("gitaly-feature-%s", flag)
+	val, ok := md[headerKey]
+	if !ok {
+		return false
+	}
+
+	return len(val) > 0 && val[0] == "true"
+}
+
+// IsDisabled is the inverse operation of IsEnabled
+func IsDisabled(ctx context.Context, flag string) bool {
+	return !IsEnabled(ctx, flag)
+}
diff --git a/internal/featureflag/grpc_header_test.go b/internal/featureflag/grpc_header_test.go
new file mode 100644
index 000000000..76e0925c5
--- /dev/null
+++ b/internal/featureflag/grpc_header_test.go
@@ -0,0 +1,31 @@
+package featureflag
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/net/context"
+	"google.golang.org/grpc/metadata"
+)
+
+func TestGRPCMetadataFeatureFlag(t *testing.T) {
+	testCases := []struct {
+		flag    string
+		headers map[string]string
+		enabled bool
+	}{
+		{"", nil, false},
+		{"flag", nil, false},
+		{"flag", map[string]string{"flag": "true"}, false},
+		{"flag", map[string]string{"gitaly-feature-flag": "TRUE"}, false},
+		{"flag", map[string]string{"gitaly-feature-flag": "true"}, true},
+	}
+
+	for _, tc := range testCases {
+		md := metadata.New(tc.headers)
+		ctx := metadata.NewIncomingContext(context.Background(), md)
+
+		assert.Equal(t, tc.enabled, IsEnabled(ctx, tc.flag))
+		assert.NotEqual(t, tc.enabled, IsDisabled(ctx, tc.flag))
+	}
+}