From 0685889e45977086f2ecd968c85b52b0bb4bc81c Mon Sep 17 00:00:00 2001
From: Will Gittoes <will.gittoes@datadoghq.com>
Date: Fri, 15 Feb 2019 14:52:05 +1100
Subject: [PATCH] Ennicen the error message

---
 src/encoder.cpp            | 15 +++++++++++++--
 test/agent_writer_test.cpp | 37 ++++++++++++++++++++++++++++++++-----
 2 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/src/encoder.cpp b/src/encoder.cpp
index 42626b61..c80fce15 100644
--- a/src/encoder.cpp
+++ b/src/encoder.cpp
@@ -16,6 +16,8 @@ const std::string header_dd_meta_lang = "Datadog-Meta-Lang";
 const std::string header_dd_meta_lang_version = "Datadog-Meta-Lang-Version";
 const std::string header_dd_meta_tracer_version = "Datadog-Meta-Tracer-Version";
 const std::string header_dd_trace_count = "X-Datadog-Trace-Count";
+
+const size_t RESPONSE_ERROR_REGION_SIZE = 50;
 }  // namespace
 
 AgentHttpEncoder::AgentHttpEncoder(std::shared_ptr<SampleProvider> sampler)
@@ -59,8 +61,17 @@ void AgentHttpEncoder::handleResponse(const std::string& response) {
         return;  // No priority sampling info.
       }
       sampler_->configure(config[priority_sampling_key]);
-    } catch (const json::parse_error&) {
-      std::cerr << "Unable to parse response from agent. Response was: " << response << std::endl;
+    } catch (const json::parse_error& error) {
+      size_t start = (error.byte > (RESPONSE_ERROR_REGION_SIZE / 2))
+                         ? error.byte - (RESPONSE_ERROR_REGION_SIZE / 2)
+                         : 0;
+      size_t size = std::min(response.length() - start, RESPONSE_ERROR_REGION_SIZE);
+      std::string prefix = (start > 0) ? "..." : "";
+      std::string suffix = ((start + size) < response.length()) ? "..." : "";
+      std::string response_region = response.substr(start, size);
+      std::cerr << "Unable to parse response from agent." << std::endl
+                << "Error was: " << error.what() << std::endl
+                << "Error near: " << prefix << response_region << suffix << std::endl;
       return;
     }
   }
diff --git a/test/agent_writer_test.cpp b/test/agent_writer_test.cpp
index 59d54225..6e7993cc 100644
--- a/test/agent_writer_test.cpp
+++ b/test/agent_writer_test.cpp
@@ -85,17 +85,44 @@ TEST_CASE("writer") {
   }
 
   SECTION("handle dodgy responses") {
-    handle->response =
-        "// What?! This isn't JSON! Everyone knows real JSON doesn't have comments...";
+    struct BadResponseTest {
+      std::string response;
+      std::string error;
+    };
+
+    auto bad_response_test_case = GENERATE(values<BadResponseTest>(
+        {{"// Error at start, short body",
+          ("Unable to parse response from agent.\n"
+           "Error was: [json.exception.parse_error.101] parse error at 1: syntax error - "
+           "invalid literal; last read: '/'\n"
+           "Error near: // Error at start, short body\n")},
+         {"{\"lol\" // Error near start, error message should have truncated "
+          "body. 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9",
+          "Unable to parse response from agent.\n"
+          "Error was: [json.exception.parse_error.101] parse error at 8: syntax error - invalid "
+          "literal; last read: '\"lol\" /'; expected ':'\n"
+          "Error near: {\"lol\" // Error near start, error message should h...\n"},
+         {"{\"Error near the end, should be truncated. 0 1 2 3 4 5 6 7 8 9 \", oh noes",
+          "Unable to parse response from agent.\n"
+          "Error was: [json.exception.parse_error.101] parse error at 65: syntax error - "
+          "unexpected ','; expected ':'\n"
+          "Error near: ...d. 0 1 2 3 4 5 6 7 8 9 \", oh noes\n"},
+         {"{\"Error in the middle, truncated from both ends\" lol 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 "
+          "6 7 8 9",
+          "Unable to parse response from agent.\n"
+          "Error was: [json.exception.parse_error.101] parse error at 50: syntax error - invalid "
+          "literal; last read: '\"Error in the middle, truncated from both ends\" l'; expected "
+          "':'\n"
+          "Error near: ...uncated from both ends\" lol 0 1 2 3 4 5 6 7 8 9 0 ...\n"}}));
+
+    handle->response = bad_response_test_case.response;
     writer.write(make_trace(
         {TestSpanData{"web", "service", "resource", "service.name", 1, 1, 0, 69, 420, 0}}));
 
     std::stringstream error_message;
     std::streambuf* stderr = std::cerr.rdbuf(error_message.rdbuf());
     writer.flush(std::chrono::seconds(10));
-    REQUIRE(error_message.str() ==
-            ("Unable to parse response from agent. Response was: "
-             "// What?! This isn't JSON! Everyone knows real JSON doesn't have comments...\n"));
+    REQUIRE(error_message.str() == bad_response_test_case.error);
     std::cerr.rdbuf(stderr);  // Restore stderr.
     REQUIRE(sampler->config == "");
   }