Keep Mongo operations online across Vault credential rotation

backend/shared/database/source/source.go

@@ -0,0 +1,30 @@
+// Package source owns Mongo connection generations and operation lifetimes.
+package source
+
+import (
+	"context"
+
+	merrors "git.sendico.io/tech/sxchnge/shared/errors"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+)
+
+// Source pins one database/client until release. The returned context must be
+// passed to nested operations so transactions never cross client generations.
+type Source interface {
+	Acquire(context.Context) (*mongo.Database, context.Context, func(), error)
+}
+
+type fixed struct{ database *mongo.Database }
+
+// Fixed adapts an externally owned database, e.g. a test fixture. It never closes it.
+func Fixed(database *mongo.Database) Source { return &fixed{database: database} }
+
+func (s *fixed) Acquire(ctx context.Context) (*mongo.Database, context.Context, func(), error) {
+	if s.database == nil {
+		return nil, ctx, nil, merrors.FailedPrecondition("database is not configured")
+	}
+	if session := mongo.SessionFromContext(ctx); session != nil && session.Client() != s.database.Client() {
+		return nil, ctx, nil, merrors.FailedPrecondition("transaction belongs to another database client")
+	}
+	return s.database, ctx, func() {}, nil
+}