Factor out backup.refs loading code from restore

In the next patch we will need to load backup.refs in the beginning of
pull too. Factored function changed to return regular error instead of
raising exception (which will be the general plan from now on).

git-backup.go

@@ -858,37 +858,8 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
     HEAD := xgitSha1("rev-parse", "--verify", HEAD_)
 
     // read backup refs index
-    repotab := map[string]*BackupRepo{} // repo.path -> repo
-    backup_refs := xgit("cat-file", "blob", fmt.Sprintf("%s:backup.refs", HEAD))
-    for _, refentry := range xstrings.SplitLines(backup_refs, "\n") {
-        // sha1 prefix+refname (sha1_)
-        badentry := func() { exc.Raisef("E: invalid backup.refs entry: %q", refentry) }
-        refentryv := strings.Fields(refentry)
-        if !(2 <= len(refentryv) && len(refentryv) <= 3) {
-            badentry()
-        }
-        sha1, err := Sha1Parse(refentryv[0])
-        sha1_, err_ := sha1, err
-        if len(refentryv) == 3 {
-            sha1_, err_ = Sha1Parse(refentryv[2])
-        }
-        if err != nil || err_ != nil {
-            badentry()
-        }
-        reporef := refentryv[1]
-        repopath, ref := reporef_split(reporef)
-
-        repo := repotab[repopath]
-        if repo == nil {
-            repo = &BackupRepo{repopath, RefMap{}}
-            repotab[repopath] = repo
-        }
-
-        if _, alreadyin := repo.refs[ref]; alreadyin {
-            exc.Raisef("E: duplicate ref %s in backup.refs", reporef)
-        }
-        repo.refs[ref] = BackupRefSha1{sha1, sha1_}
-    }
+    repotab, err := loadBackupRefs(fmt.Sprintf("%s:backup.refs", HEAD))
+    exc.Raiseif(err)
 
     // flattened & sorted repotab
     // NOTE sorted - to process repos always in the same order & for searching
@@ -1093,6 +1064,51 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
     }
 }
 
+// loadBackupRefs loads 'backup.ref' content from a git object.
+//
+// an example of object is e.g. "HEAD:backup.ref".
+func loadBackupRefs(object string) (repotab map[string]*BackupRepo, err error) {
+    defer xerr.Contextf(&err, "load backup.refs %q", object)
+
+    gerr, backup_refs, _ := ggit("cat-file", "blob", object)
+    if gerr != nil {
+        return nil, gerr
+    }
+
+    repotab = make(map[string]*BackupRepo)
+    for _, refentry := range xstrings.SplitLines(backup_refs, "\n") {
+        // sha1 prefix+refname (sha1_)
+        badentry := func() error { return fmt.Errorf("invalid entry: %q", refentry) }
+        refentryv := strings.Fields(refentry)
+        if !(2 <= len(refentryv) && len(refentryv) <= 3) {
+            return nil, badentry()
+        }
+        sha1, err := Sha1Parse(refentryv[0])
+        sha1_, err_ := sha1, err
+        if len(refentryv) == 3 {
+            sha1_, err_ = Sha1Parse(refentryv[2])
+        }
+        if err != nil || err_ != nil {
+            return nil, badentry()
+        }
+        reporef := refentryv[1]
+        repopath, ref := reporef_split(reporef)
+
+        repo := repotab[repopath]
+        if repo == nil {
+            repo = &BackupRepo{repopath, RefMap{}}
+            repotab[repopath] = repo
+        }
+
+        if _, alreadyin := repo.refs[ref]; alreadyin {
+            return nil, fmt.Errorf("duplicate ref %q", ref)
+        }
+        repo.refs[ref] = BackupRefSha1{sha1, sha1_}
+    }
+
+    return repotab, nil
+}
+
 var commands = map[string]func(*git.Repository, []string){
     "pull":    cmd_pull,
     "restore": cmd_restore,