[OTLP/HTTP] Honor Retry-After header when retrying exports

CHANGELOG.md

@@ -15,6 +15,9 @@ Increment the:
 
 ## [Unreleased]
 
+* [OTLP/HTTP] Honor `Retry-After` response header when retrying exports,
+  supporting both delay-seconds and HTTP-date formats per RFC 7231 §7.1.3.
+  [#4172](https://github.com/open-telemetry/opentelemetry-cpp/issues/4172)
 * [CODE HEALTH] Move api/test and sdk/test classes into anonymous namespace
   [#4217](https://github.com/open-telemetry/opentelemetry-cpp/pull/4217)
 

api/include/opentelemetry/common/timestamp.h

@@ -3,9 +3,17 @@
 
 #pragma once
 
+#include <cctype>
 #include <chrono>
 #include <cstdint>
+#include <ctime>
+#include <iomanip>
+#include <limits>
+#include <sstream>
+#include <string>
 
+#include "opentelemetry/common/string_util.h"
+#include "opentelemetry/nostd/string_view.h"
 #include "opentelemetry/version.h"
 
 OPENTELEMETRY_BEGIN_NAMESPACE
@@ -202,5 +210,103 @@ class DurationUtil
   }
 };
 
+/**
+ * @brief Parsing helpers for RFC 7231 value formats (HTTP-date, section
+ * 7.1.1.1, and delay-seconds, section 7.1.3) that appear in headers such
+ * as Retry-After, Date, Last-Modified, and Expires.
+ */
+class HttpUtil
+{
+public:
+  static bool ParseDelaySeconds(nostd::string_view value, std::chrono::seconds &delay) noexcept
+  {
+    value = StringUtil::Trim(value);
+    if (value.empty())
+    {
+      return false;
+    }
+
+    std::chrono::seconds::rep result = 0;
+    for (const char c : value)
+    {
+      if (!std::isdigit(static_cast<unsigned char>(c)))
+      {
+        return false;
+      }
+      auto digit = c - '0';
+      if (result > (std::numeric_limits<std::chrono::seconds::rep>::max() - digit) / 10)
+      {
+        return false;
+      }
+      result = result * 10 + digit;
+    }
+
+    delay = std::chrono::seconds(result);
+    return true;
+  }
+
+  static bool ParseHttpDate(nostd::string_view value, std::chrono::system_clock::time_point &date)
+  {
+    value = StringUtil::Trim(value);
+    std::string str(value.data(), value.size());
+    std::tm tm = {};
+    std::istringstream ss(str);
+
+    ss >> std::get_time(&tm, "%a, %d %b %Y %H:%M:%S");
+    if (!ss.fail())
+    {
+      date = std::chrono::system_clock::from_time_t(PortableTimegm(&tm));
+      return true;
+    }
+
+    ss.clear();
+    ss.str(str);
+    tm = {};
+    ss >> std::get_time(&tm, "%A, %d-%b-%y %H:%M:%S");
+    if (!ss.fail())
+    {
+      std::time_t now_t = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
+      int current_year  = 1970 + static_cast<int>(now_t / 31556952);
+      int full_year     = 1900 + tm.tm_year;
+      if (full_year - current_year > 50)
+      {
+        full_year -= 100;
+      }
+      tm.tm_year = full_year - 1900;
+      date       = std::chrono::system_clock::from_time_t(PortableTimegm(&tm));
+      return true;
+    }
+
+    ss.clear();
+    ss.str(str);
+    tm = {};
+    ss >> std::get_time(&tm, "%a %b %d %H:%M:%S %Y");
+    if (!ss.fail())
+    {
+      date = std::chrono::system_clock::from_time_t(PortableTimegm(&tm));
+      return true;
+    }
+
+    return false;
+  }
+
+private:
+  static std::time_t PortableTimegm(std::tm *tm) noexcept
+  {
+    int year  = tm->tm_year + 1900;
+    int month = tm->tm_mon + 1;
+    if (month <= 2)
+    {
+      year -= 1;
+      month += 12;
+    }
+    int day  = tm->tm_mday;
+    int days = 365 * year + year / 4 - year / 100 + year / 400 + (153 * (month - 3) + 2) / 5 + day -
+               719469;
+    return static_cast<std::time_t>(days) * 86400 + tm->tm_hour * 3600 + tm->tm_min * 60 +
+           tm->tm_sec;
+  }
+};
+
 }  // namespace common
 OPENTELEMETRY_END_NAMESPACE

ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h

@@ -348,6 +348,7 @@ class HttpOperation
   const RetryPolicy retry_policy_;
   decltype(RetryPolicy::max_attempts) retry_attempts_;
   std::chrono::system_clock::time_point last_attempt_time_;
+  std::chrono::system_clock::time_point retry_after_time_point_{};
 
   // Processed response headers and body
   // See CURLINFO_RESPONSE_CODE, type is long

ext/src/http/client/curl/http_operation_curl.cc

@@ -8,8 +8,10 @@
 #  include <array>
 #endif  // ENABLE_OTLP_RETRY_PREVIEW
 
+#include <stdint.h>
 #include <algorithm>
 #include <atomic>
+#include <cctype>
 #include <chrono>
 #include <cmath>
 #include <cstring>
@@ -23,6 +25,7 @@
 #include <utility>
 #include <vector>
 
+#include "opentelemetry/common/timestamp.h"
 #include "opentelemetry/ext/http/client/curl/http_client_curl.h"
 #include "opentelemetry/ext/http/client/curl/http_operation_curl.h"
 #include "opentelemetry/ext/http/client/http_client.h"
@@ -35,6 +38,55 @@
 #  define CURL_VERSION_BITS(x, y, z) ((x) << 16 | (y) << 8 | (z))
 #endif
 
+namespace
+{
+
+bool FindRetryAfterValue(const std::vector<uint8_t> &raw_headers, std::string &value)
+{
+  if (raw_headers.empty())
+  {
+    return false;
+  }
+
+  const char *data = reinterpret_cast<const char *>(raw_headers.data());
+  const char *end  = data + raw_headers.size();
+  const char *line = data;
+
+  while (line < end)
+  {
+    const char *line_end = line;
+    while (line_end < end && *line_end != '\n')
+    {
+      ++line_end;
+    }
+
+    static const char kRetryAfterHeader[] = "retry-after:";
+    static const size_t kRetryAfterLen    = sizeof(kRetryAfterHeader) - 1;
+
+    size_t line_len = static_cast<size_t>(line_end - line);
+    if (line_len > kRetryAfterLen)
+    {
+      bool match = true;
+      for (size_t i = 0; i < kRetryAfterLen && match; ++i)
+      {
+        match = (std::tolower(static_cast<unsigned char>(line[i])) == kRetryAfterHeader[i]);
+      }
+
+      if (match)
+      {
+        value = std::string(line + kRetryAfterLen, line_end);
+        return true;
+      }
+    }
+
+    line = (line_end < end) ? line_end + 1 : end;
+  }
+
+  return false;
+}
+
+}  // namespace
+
 OPENTELEMETRY_BEGIN_NAMESPACE
 namespace ext
 {
@@ -560,6 +612,11 @@ bool HttpOperation::IsRetryable()
 
 std::chrono::system_clock::time_point HttpOperation::NextRetryTime()
 {
+  if (retry_after_time_point_ != std::chrono::system_clock::time_point{})
+  {
+    return retry_after_time_point_;
+  }
+
   static std::random_device rd;
   static std::mt19937 gen(rd());
   static std::uniform_real_distribution<float> dis(0.8f, 1.2f);
@@ -1463,8 +1520,9 @@ void HttpOperation::Abort()
 void HttpOperation::PerformCurlMessage(CURLcode code)
 {
   ++retry_attempts_;
-  last_attempt_time_ = std::chrono::system_clock::now();
-  last_curl_result_  = code;
+  last_attempt_time_      = std::chrono::system_clock::now();
+  last_curl_result_       = code;
+  retry_after_time_point_ = std::chrono::system_clock::time_point{};
 
   if (code != CURLE_OK)
   {
@@ -1513,6 +1571,27 @@ void HttpOperation::PerformCurlMessage(CURLcode code)
 
   if (IsRetryable())
   {
+
+    std::string retry_after;
+    if (FindRetryAfterValue(response_headers_, retry_after))
+    {
+      std::chrono::seconds delay;
+
+      if (opentelemetry::common::HttpUtil::ParseDelaySeconds(retry_after, delay))
+      {
+        retry_after_time_point_ = std::chrono::system_clock::now() + delay;
+      }
+      else
+      {
+        std::chrono::system_clock::time_point date;
+        if (opentelemetry::common::HttpUtil::ParseHttpDate(retry_after, date))
+        {
+          auto now                = std::chrono::system_clock::now();
+          retry_after_time_point_ = (date > now) ? date : now;
+        }
+      }
+    }
+
     // Clear any response data received in previous attempt
     ReleaseResponse();
     // Rewind request data so that read callback can re-transfer the payload