refactor: tracing injection mechanism (#79)

This commit introduces a refined header injection mechanism that modify
the request structure with headers to inject, instead of mutating nginx's
configuration with `proxy_set_header`, `fastcgi_param`, `grpc_set_header`
or `uwsgi_param`.

Resolves #19
Resolves #43

Changes:
  - Add a DictWriter implementation for nginx.
  - Add a test covering issue #19.
  - Deprecate `opentracing_*_propagate_context`.
  - Remove tests no longer necessary.
  - Update existing tests to align with the new implementation.

Author: dmehala

CMakeLists.txt

@@ -94,7 +94,6 @@ target_sources(ngx_http_datadog_module
     src/ngx_http_datadog_module.cpp
     src/ngx_logger.cpp
     src/ngx_script.cpp
-    src/propagation_header_querier.cpp
     src/request_tracing.cpp
     src/string_util.cpp
     src/tracing_library.cpp

doc/API.md

@@ -539,19 +539,6 @@ the environment, then the variable expands to a hyphen character (`-`) instead.
 
 This family of variables is used in the tests for the Datadog nginx module.
 
-### `datadog_propagation_header_*`
-`$datadog_propagation_header_<header>` expands to the value of the specified
-HTTP `<header>` as it would appear in a request to a service proxied using the
-`proxy_pass` directive.
-
-`<header>` is transformed into the name of an HTTP request header by replacing
-underscores with hyphens, e.g. `$datadog_propagation_header_x_datadog_origin`
-expands to what would be the value of the `X-Datadog-Origin` header were trace
-context to be propagated to a service via `proxy_pass`.
-
-This family of variables is used in the implementation of the Datadog nginx
-module.
-
 ### `datadog_auth_request_hook`
 This is an implementation detail of the module and should not be used.
 

nginx-module.cmake

@@ -10,9 +10,13 @@ separate_arguments(NGINX_CONF_ARGS NATIVE_COMMAND "${NGINX_CONF_ARGS}")
 
 list(APPEND NGINX_CONF_ARGS
   "--add-dynamic-module=${CMAKE_SOURCE_DIR}/module/"
-  "--with-threads"
   "--with-compat"
 )
+
+if (NGINX_DATADOG_ASM_ENABLED)
+  list(APPEND NGINX_CONF_ARGS "--with-threads")
+endif ()
+
 if(CMAKE_BUILD_TYPE STREQUAL "Debug")
   list(APPEND NGINX_CONF_ARGS "--with-debug")
 endif()

src/datadog_conf.h

@@ -86,7 +86,6 @@ struct datadog_main_conf_t {
   // (e.g. before the first `server` block, before the first `access_log`
   // directive).
   bool are_log_formats_defined;
-  std::vector<std::string_view> span_context_keys;
   // This module automates the forwarding of the environment variables in
   // `TracingLibrary::environment_variable_names()`. Rather than injecting
   // `env` directives into the configuration, or mucking around with the core
@@ -109,13 +108,6 @@ struct datadog_main_conf_t {
   std::optional<configured_value_t> environment;
   // `agent_url` is set by the `datadog_agent_url` directive.
   std::optional<configured_value_t> agent_url;
-  //  `is_sampling_delegation_response_header_added` is whether we have added an
-  //  `add_header` directive before the first `server` block within the
-  //  configuration. The directive is used to include the
-  //  X-Datadog-Trace-Sampling-Decision response header when the client
-  //  requested trace sampling delegation. This `bool` ensures that the
-  //  directive is not added more than once.
-  bool is_sampling_delegation_response_header_added;
 
 #ifdef WITH_WAF
   // DD_APPSEC_ENABLED
@@ -196,10 +188,6 @@ struct datadog_loc_conf_t {
   NgxScript loc_resource_name_script;
   ngx_flag_t trust_incoming_span = NGX_CONF_UNSET;
   ngx_array_t *tags;
-  // `response_info_script` is a script containing variables that refer
-  // to HTTP response headers.  This is used by sampling delegation to
-  // retrieve the sampling decision made by an upstream service.
-  NgxScript response_info_script;
   // `proxy_directive` is the name of the configuration directive used to proxy
   // requests at this location, i.e. `proxy_pass`, `grpc_pass`, or
   // `fastcgi_pass`.  If this location does not have such a directive directly
@@ -226,15 +214,6 @@ struct datadog_loc_conf_t {
   // `sampling_delegation_directive` is the source location of the
   // `datadog_delegate_sampling` directive that applies this location, if any.
   conf_directive_source_location_t sampling_delegation_directive;
-  //  `is_sampling_delegation_response_header_added` is whether we have added an
-  //  `add_header` directive before the first user-specified `add_header`
-  //  directive within this server/location block. The directive is used to
-  //  include the X-Datadog-Trace-Sampling-Decision response header when the
-  //  client requested trace sampling delegation. The inserted `add_header` is
-  //  necessary only if there are user-defined `add_header` directives at the
-  //  same level; if not, then the `add_header` is inherited from the `http`
-  //  block.
-  bool is_sampling_delegation_response_header_added;
   // `block_type` is the name of the kind of configuration block we're in, e.g.
   // "http", "server", "location", or "if". `block_type` is used by some
   // configuration directives to alter their behavior based on the current

src/datadog_context.cpp

@@ -86,17 +86,6 @@ void DatadogContext::on_log_request(ngx_http_request_t *request) {
 #endif
 }
 
-ngx_str_t DatadogContext::lookup_propagation_header_variable_value(
-    ngx_http_request_t *request, std::string_view key) {
-  auto trace = find_trace(request);
-  if (trace == nullptr) {
-    throw std::runtime_error{
-        "lookup_propagation_header_variable_value failed: could not find "
-        "request trace"};
-  }
-  return trace->lookup_propagation_header_variable_value(key);
-}
-
 ngx_str_t DatadogContext::lookup_span_variable_value(
     ngx_http_request_t *request, std::string_view key) {
   auto trace = find_trace(request);
@@ -107,17 +96,6 @@ ngx_str_t DatadogContext::lookup_span_variable_value(
   return trace->lookup_span_variable_value(key);
 }
 
-ngx_str_t DatadogContext::lookup_sampling_delegation_response_variable_value(
-    ngx_http_request_t *request) {
-  auto trace = find_trace(request);
-  if (trace == nullptr) {
-    throw std::runtime_error{
-        "lookup_sampling_delegation_response_variable_value failed: could not "
-        "find request trace"};
-  }
-  return trace->lookup_sampling_delegation_response_variable_value();
-}
-
 RequestTracing *DatadogContext::find_trace(ngx_http_request_t *request) {
   const auto found = std::find_if(
       traces_.begin(), traces_.end(),

src/datadog_context.h

@@ -6,7 +6,6 @@
 #include <vector>
 
 #include "datadog_conf.h"
-#include "propagation_header_querier.h"
 #include "request_tracing.h"
 #ifdef WITH_WAF
 #include "security/context.h"
@@ -41,15 +40,9 @@ class DatadogContext {
 
   void on_log_request(ngx_http_request_t* request);
 
-  ngx_str_t lookup_propagation_header_variable_value(
-      ngx_http_request_t* request, std::string_view key);
-
   ngx_str_t lookup_span_variable_value(ngx_http_request_t* request,
                                        std::string_view key);
 
-  ngx_str_t lookup_sampling_delegation_response_variable_value(
-      ngx_http_request_t* request);
-
   RequestTracing& single_trace();
 
  private: