Added a simple test executable

Makefile

@@ -30,3 +30,9 @@ endif
 
 %.o: %.cpp
 	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $(PG_CPPFLAGS) -c -o $@ $<
+
+oauth_conn_test: src/oauth_conn_test.o
+	$(CXX) -o $@ $< -L$(libdir) -lpq
+
+src/oauth_conn_test.o: src/oauth_conn_test.cpp
+	$(CXX) $(CXXFLAGS) $(CPPFLAGS) -I$(includedir) -std=c++23 -c -o $@ $<

src/oauth_conn_test.cpp

@@ -0,0 +1,64 @@
+// This is  a simple demo/test executable, NOT part of the main pg_oidc_validator plugin.
+// Its only function is to execute a single SQL query using a pre obtained OAuth token.
+// With this, it is able to:
+// * provide a simple test interface for the validator, without relying on the device flow.
+//   (e.g. test/get-token.sh can obtain a token without user interaction)
+// * answering the common user questions of "How can we implement a client that do not rely on the device flow".
+#include <libpq-fe.h>
+
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
+static const char* g_token = nullptr;
+
+static int oauth_hook(PGauthData type, PGconn* conn, void* data) {
+  if (type == PQAUTHDATA_OAUTH_BEARER_TOKEN) {
+    auto* request = static_cast<PGoauthBearerRequest*>(data);
+    request->token = strdup(g_token);
+    request->cleanup = [](PGconn*, PGoauthBearerRequest* r) { free(r->token); };
+    return 1;
+  }
+  return 0;
+}
+
+int main(int argc, char* argv[]) {
+  if (argc != 4) {
+    fprintf(stderr, "Usage: %s <connstring> <sql_query> <access_token>\n", argv[0]);
+    return 1;
+  }
+
+  const char* connstring = argv[1];
+  const char* query = argv[2];
+  g_token = argv[3];
+
+  PQsetAuthDataHook(oauth_hook);
+
+  PGconn* conn = PQconnectdb(connstring);
+  if (PQstatus(conn) != CONNECTION_OK) {
+    fprintf(stderr, "Connection error: %s", PQerrorMessage(conn));
+    PQfinish(conn);
+    return 1;
+  }
+
+  PGresult* res = PQexec(conn, query);
+  if (PQresultStatus(res) != PGRES_TUPLES_OK) {
+    fprintf(stderr, "Query error: %s", PQresultErrorMessage(res));
+    PQclear(res);
+    PQfinish(conn);
+    return 1;
+  }
+
+  if (PQntuples(res) > 0) {
+    int ncols = PQnfields(res);
+    for (int i = 0; i < ncols; i++) {
+      if (i > 0) printf("\t");
+      printf("%s", PQgetvalue(res, 0, i));
+    }
+    printf("\n");
+  }
+
+  PQclear(res);
+  PQfinish(conn);
+  return 0;
+}