Adds cataloger for Azure Blob storage

pkg/storage/azureblob/cataloger.go

@@ -0,0 +1,58 @@
+package azureblob
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"github.com/Azure/azure-storage-blob-go/azblob"
+	"github.com/gomods/athens/pkg/config"
+	"github.com/gomods/athens/pkg/errors"
+	"github.com/gomods/athens/pkg/observ"
+	"github.com/gomods/athens/pkg/paths"
+)
+
+// Catalog implements the (./pkg/storage).Catalog interface
+// It returns a list of versions, if any, for a given module
+func (s *Storage) Catalog(ctx context.Context, token string, pageSize int) ([]paths.AllPathParams, string, error) {
+	const op errors.Op = "azblob.Catalog"
+	ctx, span := observ.StartSpan(ctx, op.String())
+	defer span.End()
+
+	res := make([]paths.AllPathParams, 0)
+
+	// one module@version consists of 3 pieces - info, mod, zip
+	objCount := 3 * pageSize
+
+	marker := azblob.Marker{&token}
+	blobs, err := s.client.containerURL.ListBlobsFlatSegment(ctx, marker, azblob.ListBlobsSegmentOptions{
+		MaxResults: int32(objCount),
+	})
+	if err != nil {
+		return nil, "",  errors.E(op, err)
+	}
+
+	nextToken := *blobs.NextMarker.Val
+
+	for _, blob := range blobs.Segment.BlobItems {
+	    if strings.HasSuffix(blob.Name, ".info") {
+		p, err := parsModVer(blob.Name)
+		if err != nil {
+		    continue
+		}
+		res = append(res, p)
+	    }
+	}
+
+	return res, nextToken, nil
+}
+
+func parsModVer(p string) (paths.AllPathParams, error) {
+	const op errors.Op = "azureblob.parseS3Key"
+	// github.com/gomods/testCatalogModule/@v/v1.2.0976.info
+	m, v := config.ModuleVersionFromPath(p)
+
+	if m == "" || v == "" {
+		return paths.AllPathParams{}, errors.E(op, fmt.Errorf("invalid object key format %s", p))
+	}
+	return paths.AllPathParams{Module: m, Version: v}, nil
+}