Use separate SQLite connection pools for reading and writing.

By only allowing a single write connection, we can leverage database/sql's
auto-blocking behavior to avoid SQLITE_BUSY errors on multiple write requests.

database.go

@@ -86,10 +86,16 @@ func initializeDatabase(path string) (*sql.DB, error) {
         accessible = true
     }
 
-	db, err := sql.Open("sqlite", path)
-	if err != nil {
-		return nil, fmt.Errorf("failed to create SQLite file at %q; %w", path, err)
-	}
+    // This object's connections are intended for serial writing, so we set
+    // IMMEDIATE transactions to make debugging of locking issues easier.
+    db, err := sql.Open("sqlite", path + "?_txlock=immediate")
+    if err != nil {
+        return nil, fmt.Errorf("failed to open read/write SQLite handle; %w", err)
+    }
+
+    // Maxing out at one connection so that there can only be one write at any
+    // time; everyone else will have to block on the connection availability.
+    db.SetMaxOpenConns(1)
 
     // Make sure to eventually close all idle connections in the pool so that
     // SQLite actually commits its WAL journal. 
@@ -165,6 +171,19 @@ CREATE INDEX index_links ON links(tid, fid);
     return db, nil
 }
 
+func initializeReadOnlyDatabase(path string) (*sql.DB, error) {
+    ro_db, err := sql.Open("sqlite", path + "?_pragma=query_only(1)")
+    if err != nil {
+        return nil, fmt.Errorf("failed to open read-only SQLite handle; %w", err)
+    }
+
+    // Make sure to eventually close all idle connections in the pool so that
+    // SQLite actually commits its WAL journal. 
+    ro_db.SetConnMaxIdleTime(1 * time.Minute)
+
+    return ro_db, nil
+}
+
 /**********************************************************************/
 
 // Pre-building the insertion statements for efficiency when iterating over and

main.go

@@ -23,10 +23,16 @@ func main() {
 
     db, err := initializeDatabase(dbpath)
     if err != nil {
-        log.Fatalf("failed to create the initial SQLite file at %q; %v", dbpath, err)
+        log.Fatalf("failed to initialize SQLite file at %q; %v", dbpath, err)
     }
     defer db.Close()
 
+    ro_db, err := initializeReadOnlyDatabase(dbpath)
+    if err != nil {
+        log.Fatalf("failed to create read-only connections to %q; %w", dbpath, err)
+    }
+    defer ro_db.Close()
+
     tokenizer, err := newUnicodeTokenizer(false)
     if err != nil {
         log.Fatalf("failed to create the default tokenizer; %v", err)
@@ -49,14 +55,14 @@ func main() {
     // Setting up the endpoints.
     http.HandleFunc("POST " + prefix + "/register/start", newRegisterStartHandler(verifier))
     http.HandleFunc("POST " + prefix + "/register/finish", newRegisterFinishHandler(db, verifier, tokenizer, timeout))
-    http.HandleFunc("POST " + prefix + "/deregister/start", newDeregisterStartHandler(db, verifier))
+    http.HandleFunc("POST " + prefix + "/deregister/start", newDeregisterStartHandler(ro_db, verifier))
     http.HandleFunc("POST " + prefix + "/deregister/finish", newDeregisterFinishHandler(db, verifier, timeout))
 
-    http.HandleFunc(prefix + "/registered", newListRegisteredDirectoriesHandler(db))
-    http.HandleFunc(prefix + "/query", newQueryHandler(db, tokenizer, wild_tokenizer, "/query"))
-    http.HandleFunc(prefix + "/retrieve/metadata", newRetrieveMetadataHandler(db))
-    http.HandleFunc(prefix + "/retrieve/file", newRetrieveFileHandler(db))
-    http.HandleFunc(prefix + "/list", newListFilesHandler(db))
+    http.HandleFunc(prefix + "/registered", newListRegisteredDirectoriesHandler(ro_db))
+    http.HandleFunc(prefix + "/query", newQueryHandler(ro_db, tokenizer, wild_tokenizer, "/query"))
+    http.HandleFunc(prefix + "/retrieve/metadata", newRetrieveMetadataHandler(ro_db))
+    http.HandleFunc(prefix + "/retrieve/file", newRetrieveFileHandler(ro_db))
+    http.HandleFunc(prefix + "/list", newListFilesHandler(ro_db))
 
     http.HandleFunc(prefix + "/", newDefaultHandler())