Fix restore repo bug, clarify the problem of ForeignIndex (#22776)

Fix #22581

TLDR: #18446 made a mess with ForeignIndex and triggered a design
flaw/bug of #16356, then a quick patch #21271 helped #18446, then the
the bug was re-triggered by #21721 .

Related:
* #16356
* BasicIssueContext
https://github.com/go-gitea/gitea/pull/16356/files#diff-7938eb670d42a5ead6b08121e16aa4537a4d716c1cf37923c70470020fb9d036R16-R27
* #18446 
* If some issues were dumped without ForeignIndex, then they would be
imported as ForeignIndex=0
https://github.com/go-gitea/gitea/pull/18446/files#diff-1624a3e715d8fc70edf2db1630642b7d6517f8c359cc69d58c3958b34ba4ce5eR38-R39
* #21271
* It patched the above bug (somewhat), made the issues without
ForeignIndex could have the same value as LocalIndex
* #21721 
    * It re-triggered the zero-ForeignIndex bug.


ps: I am not sure whether the changes in `GetForeignIndex` are ideal (at
least, now it has almost the same behavior as BasicIssueContext in
#16356), it's just a quick fix. Feel free to edit on this PR directly or
replace it.

Co-authored-by: zeripath <art27@cantab.net>

modules/migration/comment.go

@@ -8,8 +8,7 @@ import "time"
 
 // Commentable can be commented upon
 type Commentable interface {
-	GetLocalIndex() int64
-	GetForeignIndex() int64
+	Reviewable
 	GetContext() DownloaderContext
 }
 

modules/migration/issue.go

@@ -34,6 +34,15 @@ func (issue *Issue) GetExternalName() string { return issue.PosterName }
 // GetExternalID ExternalUserMigrated interface
 func (issue *Issue) GetExternalID() int64 { return issue.PosterID }
 
-func (issue *Issue) GetLocalIndex() int64          { return issue.Number }
-func (issue *Issue) GetForeignIndex() int64        { return issue.ForeignIndex }
+func (issue *Issue) GetLocalIndex() int64 { return issue.Number }
+
+func (issue *Issue) GetForeignIndex() int64 {
+	// see the comment of Reviewable.GetForeignIndex
+	// if there is no ForeignIndex, then use LocalIndex
+	if issue.ForeignIndex == 0 {
+		return issue.Number
+	}
+	return issue.ForeignIndex
+}
+
 func (issue *Issue) GetContext() DownloaderContext { return issue.Context }

modules/migration/review.go

@@ -8,6 +8,16 @@ import "time"
 // Reviewable can be reviewed
 type Reviewable interface {
 	GetLocalIndex() int64
+
+	// GetForeignIndex presents the foreign index, which could be misused:
+	// For example, if there are 2 Gitea sites: site-A exports a dataset, then site-B imports it:
+	// * if site-A exports files by using its LocalIndex
+	// * from site-A's view, LocalIndex is site-A's IssueIndex while ForeignIndex is site-B's IssueIndex
+	// * but from site-B's view, LocalIndex is site-B's IssueIndex while ForeignIndex is site-A's IssueIndex
+	//
+	// So the exporting/importing must be paired, but the meaning of them looks confusing then:
+	// * either site-A and site-B both use LocalIndex during dumping/restoring
+	// * or site-A and site-B both use ForeignIndex
 	GetForeignIndex() int64
 }
 
@@ -37,7 +47,7 @@ type Review struct {
 // GetExternalName ExternalUserMigrated interface
 func (r *Review) GetExternalName() string { return r.ReviewerName }
 
-// ExternalID ExternalUserMigrated interface
+// GetExternalID ExternalUserMigrated interface
 func (r *Review) GetExternalID() int64 { return r.ReviewerID }
 
 // ReviewComment represents a review comment