fix standalone generated quantities csv writer

src/stan/services/util/gq_writer.hpp

@@ -148,7 +148,10 @@ class gq_writer {
       logger_.info(e.what());
       throw;
     }
-    sample_writer_(values);
+    // drop the constrained parameters, as the std::vector overload does
+    plain_type_t<EigVec> gq_values
+        = values.tail(values.size() - num_constrained_params_);
+    sample_writer_(gq_values);
   }
 
   /**

src/test/unit/services/sample/standalone_gqs_parallel_test.cpp

@@ -92,7 +92,9 @@ Eigen::MatrixXd bernoulli_fit_draws() {
 }
 
 // drop the timing-dependent " Elapsed Time" line, which the test writers
-// emit without a comment prefix
+// emit without a comment prefix, and normalize the separator: the Eigen
+// writer path emits ", " where the std::vector path emits ",", which the
+// cross-path test below has to see through
 std::string data_lines(const std::string& csv) {
   std::stringstream in(csv), out;
   std::string line;
@@ -101,6 +103,7 @@ std::string data_lines(const std::string& csv) {
       continue;
     if (line.find("Elapsed Time") != std::string::npos)
       continue;
+    line.erase(std::remove(line.begin(), line.end(), ' '), line.end());
     out << line << "\n";
   }
   return out.str();
@@ -158,3 +161,68 @@ TEST_F(ServicesStandaloneGQ, genDraws_bernoulli_init_chain_id_offset) {
   EXPECT_EQ(from_3[1], from_1[3]);
   EXPECT_NE(from_1[0], from_1[1]);
 }
+
+// The multi-chain path must write only the generated quantities: one value
+// per header name.  Guards the Eigen `write_gq_values` overload, which used
+// to emit the constrained parameters as extra leading columns.
+TEST_F(ServicesStandaloneGQ, genDraws_bernoulli_multi_chain_column_count) {
+  Eigen::MatrixXd draws = bernoulli_fit_draws();
+  std::vector<std::stringstream> sample_ss(2);
+  std::vector<test_writer> sample_writer;
+  sample_writer.reserve(2);
+  std::vector<Eigen::MatrixXd> draws_vec;
+  for (int i = 0; i < 2; i++) {
+    sample_writer.emplace_back(
+        std::unique_ptr<std::stringstream, deleter_noop>(&sample_ss[i]), "");
+    draws_vec.push_back(draws);
+  }
+  EXPECT_EQ(stan::services::standalone_generate(
+                model, 2, draws_vec, 12345, interrupt, logger, sample_writer),
+            stan::services::error_codes::OK);
+  auto n_fields = [](const std::string& line) {
+    return std::count(line.begin(), line.end(), ',') + 1;
+  };
+  for (int i = 0; i < 2; i++) {
+    std::stringstream in(sample_ss[i].str());
+    std::string header;
+    ASSERT_TRUE(std::getline(in, header));
+    EXPECT_EQ(n_fields(header), 11);  // mu + y_rep.1 .. y_rep.10
+    std::string line;
+    int rows = 0;
+    while (std::getline(in, line)) {
+      if (line.empty() || line[0] == '#'
+          || line.find("Elapsed Time") != std::string::npos)
+        continue;
+      EXPECT_EQ(n_fields(line), n_fields(header)) << "row " << rows;
+      rows++;
+    }
+    EXPECT_EQ(rows, 1000);
+  }
+}
+
+// Cross-path check: the multi-chain and single-chain code paths must agree.
+TEST_F(ServicesStandaloneGQ, genDraws_bernoulli_multi_chain_matches_single) {
+  Eigen::MatrixXd draws = bernoulli_fit_draws();
+  std::vector<std::stringstream> sample_ss(num_chains);
+  std::vector<test_writer> sample_writer;
+  sample_writer.reserve(num_chains);
+  std::vector<Eigen::MatrixXd> draws_vec;
+  for (int i = 0; i < num_chains; i++) {
+    sample_writer.emplace_back(
+        std::unique_ptr<std::stringstream, deleter_noop>(&sample_ss[i]), "");
+    draws_vec.push_back(draws);
+  }
+  EXPECT_EQ(
+      stan::services::standalone_generate(model, num_chains, draws_vec, 12345,
+                                          interrupt, logger, sample_writer),
+      stan::services::error_codes::OK);
+  for (int i = 0; i < num_chains; i++) {
+    std::stringstream single_ss;
+    test_writer single_writer(
+        std::unique_ptr<std::stringstream, deleter_noop>(&single_ss), "");
+    EXPECT_EQ(stan::services::standalone_generate(
+                  model, draws, 12345, interrupt, logger, single_writer, 1 + i),
+              stan::services::error_codes::OK);
+    EXPECT_EQ(data_lines(single_ss.str()), data_lines(sample_ss[i].str()));
+  }
+}