[geometry] Meshcat's server returns 404 (not 200) on failures (RobotL…

…ocomotion#20790)

This is also an opportunity to start to carve helper functions and
classes out into separate files for easier maintenance.

Two of the new functions are public and widely relevant:
ReadFile and ReadFileOrThrow.

bindings/pydrake/geometry/test/visualizers_test.py

@@ -261,6 +261,18 @@ def test_meshcat(self):
         # The pose is None because no meshcat session has broadcast its pose.
         self.assertIsNone(meshcat.GetTrackedCameraPose())
 
+    def test_meshcat_404(self):
+        meshcat = mut.Meshcat()
+
+        good_url = meshcat.web_url()
+        with urllib.request.urlopen(good_url) as response:
+            self.assertTrue(response.read(1))
+
+        bad_url = f"{good_url}/no_such_file"
+        with self.assertRaisesRegex(Exception, "HTTP.*404"):
+            with urllib.request.urlopen(bad_url) as response:
+                response.read(1)
+
     def test_meshcat_animation(self):
         animation = mut.MeshcatAnimation(frames_per_second=64)
         self.assertEqual(animation.frames_per_second(), 64)

common/BUILD.bazel

@@ -919,6 +919,7 @@ drake_cc_googletest(
         ":drake_path",
         ":find_resource",
         "//common/test_utilities:expect_no_throw",
+        "//common/test_utilities:expect_throws_message",
     ],
 )
 

common/find_resource.cc

@@ -1,7 +1,9 @@
 #include "drake/common/find_resource.h"
 
 #include <cstdlib>
-#include <filesystem>
+#include <fstream>
+#include <sstream>
+#include <stdexcept>
 #include <utility>
 #include <vector>
 
@@ -291,4 +293,24 @@ string FindResourceOrThrow(const string& resource_path) {
   return FindResource(resource_path).get_absolute_path_or_throw();
 }
 
+std::optional<string> ReadFile(const std::filesystem::path& path) {
+  std::optional<string> result;
+  std::ifstream input(path, std::ios::binary);
+  if (input.is_open()) {
+    std::stringstream content;
+    content << input.rdbuf();
+    result.emplace(std::move(content).str());
+  }
+  return result;
+}
+
+std::string ReadFileOrThrow(const std::filesystem::path& path) {
+  std::optional<string> result = ReadFile(path);
+  if (!result) {
+    throw std::runtime_error(
+        fmt::format("Error reading from '{}'", path.string()));
+  }
+  return std::move(*result);
+}
+
 }  // namespace drake

common/find_resource.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <filesystem>
 #include <optional>
 #include <string>
 #include <vector>
@@ -125,4 +126,12 @@ std::string FindResourceOrThrow(const std::string& resource_path);
 /// copy of Drake, in case other methods have failed.
 extern const char* const kDrakeResourceRootEnvironmentVariableName;
 
+/// Returns the content of the file at the given path, or nullopt if it cannot
+/// be read. Note that the path is a filesystem path, not a `resource_path`.
+std::optional<std::string> ReadFile(const std::filesystem::path& path);
+
+/// Returns the content of the file at the given path, or throws if it cannot
+/// be read. Note that the path is a filesystem path, not a `resource_path`.
+std::string ReadFileOrThrow(const std::filesystem::path& path);
+
 }  // namespace drake

common/test/find_resource_test.cc

@@ -2,10 +2,8 @@
 
 #include <cstdlib>
 #include <filesystem>
-#include <fstream>
 #include <functional>
 #include <memory>
-#include <sstream>
 #include <stdexcept>
 #include <string>
 
@@ -16,6 +14,7 @@
 #include "drake/common/drake_path.h"
 #include "drake/common/drake_throw.h"
 #include "drake/common/test_utilities/expect_no_throw.h"
+#include "drake/common/test_utilities/expect_throws_message.h"
 
 using std::string;
 
@@ -84,13 +83,8 @@ GTEST_TEST(FindResourceTest, FoundDeclaredData) {
   // The path is the correct answer.
   ASSERT_FALSE(absolute_path.empty());
   EXPECT_EQ(absolute_path[0], '/');
-  std::ifstream input(absolute_path, std::ios::binary);
-  ASSERT_TRUE(input);
-  std::stringstream buffer;
-  buffer << input.rdbuf();
-  EXPECT_EQ(
-      buffer.str(),
-      "Test data for drake/common/test/find_resource_test.cc.\n");
+  EXPECT_EQ(ReadFileOrThrow(absolute_path),
+            "Test data for drake/common/test/find_resource_test.cc.\n");
 
   // Sugar works the same way.
   EXPECT_EQ(FindResourceOrThrow(relpath), absolute_path);
@@ -120,5 +114,11 @@ GTEST_TEST(GetDrakePathTest, PathIncludesDrake) {
   EXPECT_TRUE(std::filesystem::exists(expected));
 }
 
+GTEST_TEST(ReadFileTest, NoSuchPath) {
+  EXPECT_FALSE(ReadFile("/foo/bar/missing"));
+  DRAKE_EXPECT_THROWS_MESSAGE(ReadFileOrThrow("/foo/bar/missing"),
+                              "Error reading.*/foo/bar/missing.*");
+}
+
 }  // namespace
 }  // namespace drake

common/yaml/test/yaml_doxygen_test.cc

@@ -30,16 +30,6 @@ std::string WriteTemp(const std::string& data) {
   return filename;
 }
 
-// Read data from a scratch file.
-// (This is a test helper, not part of the Doxygen header.)
-std::string ReadTemp(const std::string& filename) {
-  std::ifstream input(filename, std::ios::binary);
-  DRAKE_DEMAND(!input.fail());
-  std::stringstream buffer;
-  buffer << input.rdbuf();
-  return buffer.str();
-}
-
 // This is an example from the Doxygen header.
 struct MyData {
   template <typename Archive>
@@ -80,7 +70,7 @@ GTEST_TEST(YamlDoxygenTest, ExamplesSaving) {
   SaveYamlFile(output_filename, data);
 
   // This data is an example from the Doxygen header.
-  const std::string output = ReadTemp(output_filename);
+  const std::string output = ReadFileOrThrow(output_filename);
   const std::string expected_output = R"""(
 foo: 4.0
 bar: [5.0, 6.0]

common/yaml/test/yaml_io_test.cc

@@ -21,14 +21,6 @@ namespace yaml {
 namespace test {
 namespace {
 
-std::string ReadTestFileAsString(const std::string& filename) {
-  std::ifstream input(filename, std::ios::binary);
-  DRAKE_DEMAND(!input.fail());
-  std::stringstream buffer;
-  buffer << input.rdbuf();
-  return buffer.str();
-}
-
 GTEST_TEST(YamlIoTest, LoadString) {
   const std::string data = R"""(
 value:
@@ -182,7 +174,7 @@ GTEST_TEST(YamlIoTest, SaveFile) {
   const std::string filename = temp_directory() + "/SaveFile1.yaml";
   const StringStruct data{.value = "save_file_1"};
   SaveYamlFile(filename, data);
-  const auto result = ReadTestFileAsString(filename);
+  const auto result = ReadFileOrThrow(filename);
   EXPECT_EQ(result, "value: save_file_1\n");
 }
 
@@ -194,7 +186,7 @@ GTEST_TEST(YamlIoTest, SaveFileAllArgs) {
   data.value.insert({"save_file_4", 1.0});
   ASSERT_EQ(data.value.size(), 2);
   SaveYamlFile(filename, data, child_name, {defaults});
-  const auto result = ReadTestFileAsString(filename);
+  const auto result = ReadFileOrThrow(filename);
   EXPECT_EQ(result, "some_child:\n  value:\n    save_file_4: 1.0\n");
 }
 

geometry/BUILD.bazel

@@ -452,13 +452,18 @@ drake_cc_googletest(
 
 drake_cc_library(
     name = "meshcat",
-    srcs = ["meshcat.cc"],
+    srcs = [
+        "meshcat.cc",
+        "meshcat_internal.cc",
+    ],
     hdrs = [
         "meshcat.h",
+        "meshcat_internal.h",
         "meshcat_types_internal.h",
     ],
     data = [":meshcat_resources"],
     install_hdrs_exclude = [
+        "meshcat_internal.h",
         "meshcat_types_internal.h",
     ],
     interface_deps = [
@@ -535,6 +540,13 @@ drake_cc_googletest(
     ],
 )
 
+drake_cc_googletest(
+    name = "meshcat_internal_test",
+    deps = [
+        ":meshcat",
+    ],
+)
+
 drake_cc_googletest(
     name = "meshcat_denied_test",
     allow_network = ["none"],