fix: avoid enumerating every public repository in issue search (#38992)

Both issue search endpoints resolve their repository filter with
`SearchRepositoryIDs` and pass the result to the indexer as `RepoIDs`.
They mean
to leave public repositories to the indexer, but
`SearchRepoOptions.AllPublic` is
only read when `OwnerID > 0`, so without an `owner` filter the flag does
nothing
and every public repository is enumerated, without a `LIMIT`, into
`repo_id IN (...)`.

Those IDs are redundant, as `allPublic` is passed to the indexer, which
already
matches every public repository. On a large instance this binds tens of
thousands
of parameters and can fail in the driver, making the endpoint return 500
for every
filter. Admins are worst hit, as `SearchRepositoryCondition` skips their
accessible-repository condition and enumerates the whole table.

Restrict the enumeration to private repositories. The result set is
unchanged, as
the dropped IDs are a subset of what `allPublic` matches.

Both endpoints held copies of this block, so it moves to
`routers/common`.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/repo/repo_list.go

@@ -768,6 +768,15 @@ func PublicRepoUnderPublicOwnerCond() builder.Cond {
 	)
 }
 
+// NotPublicRepoUnderPublicOwnerCond complements PublicRepoUnderPublicOwnerCond. Spelled positively so
+// the owner subquery hashes the limited/private minority, not every public user.
+func NotPublicRepoUnderPublicOwnerCond() builder.Cond {
+	return builder.Or(
+		builder.Eq{"`repository`.is_private": true},
+		builder.In("`repository`.owner_id", builder.Select("id").From("`user`").Where(builder.Neq{"visibility": structs.VisibleTypePublic})),
+	)
+}
+
 // UserActionsAccessibleOwnerRepoCond selects the repos owned by ownerID whose Actions `user` may read.
 // It is used to list an org/user's Actions runs and jobs (see the callers in routers/api/v1/shared).
 //   - owner_id = ownerID: only that owner's repos.