common: Add stub implementation of database diff function

This adds a stub implementation of database diffing. Currently this
function is not doing anything useful but it can be extended later.

Author: MKleusberg

common/diff.go

@@ -0,0 +1,67 @@
+package common
+
+import (
+	"fmt"
+	"log"
+)
+
+type Diffs struct {
+	Dummy string `json:"dummy"`	// TODO: This is only used for debugging purposes and needs to be removed later
+}
+
+// Diff
+func Diff(ownerA string, folderA string, nameA string, commitA string, ownerB string, folderB string, nameB string, commitB string, loggedInUser string) (Diffs, error) {
+	// Check if the user has access to the requested databases
+	bucketA, idA, _, err := MinioLocation(ownerA, folderA, nameA, commitA, loggedInUser)
+	if err != nil {
+		return Diffs{}, err
+	}
+	bucketB, idB, _, err := MinioLocation(ownerB, folderB, nameB, commitB, loggedInUser)
+	if err != nil {
+		return Diffs{}, err
+	}
+
+	// Sanity check
+	if idA == "" {
+		// The requested database wasn't found, or the user doesn't have permission to access it
+		err = fmt.Errorf("Requested database not found")
+		log.Printf("Requested database not found: '%s%s%s'", ownerA, folderA, nameA)
+		return Diffs{}, err
+	}
+	if idB == "" {
+		// The requested database wasn't found, or the user doesn't have permission to access it
+		err = fmt.Errorf("Requested database not found")
+		log.Printf("Requested database not found: '%s%s%s'", ownerB, folderB, nameB)
+		return Diffs{}, err
+	}
+
+	// Retrieve database files from Minio, using locally cached version if it's already there
+	dbA, err := RetrieveDatabaseFile(bucketA, idA)
+	if err != nil {
+		return Diffs{}, err
+	}
+	dbB, err := RetrieveDatabaseFile(bucketB, idB)
+	if err != nil {
+		return Diffs{}, err
+	}
+
+	// Call dbDiff which does the actual diffing of the database files
+	return dbDiff(dbA, dbB)
+}
+
+// dbDiff
+func dbDiff(dbA string, dbB string) (Diffs, error) {
+	var diff Diffs
+
+	// Check if this is the same database and exit early
+	if dbA == dbB {
+		diff.Dummy = "same dbs"
+		return diff, nil
+	}
+
+	// TODO
+	diff.Dummy = "different dbs"
+
+	// Return
+	return diff, nil
+}