1 files changed, 24 insertions, 24 deletions
diff --git a/app/services/prometheus/proxy_variable_substitution_service.rb b/app/services/prometheus/proxy_variable_substitution_service.rb
index 240586c8419..aa3a09ba05c 100644
--- a/app/services/prometheus/proxy_variable_substitution_service.rb
+++ b/app/services/prometheus/proxy_variable_substitution_service.rb
@@ -4,11 +4,20 @@ module Prometheus
   class ProxyVariableSubstitutionService < BaseService
     include Stepable
 
+    VARIABLE_INTERPOLATION_REGEX = /
+      {{                  # Variable needs to be wrapped in these chars.
+        \s*               # Allow whitespace before and after the variable name.
+          (?<variable>    # Named capture.
+            \w+           # Match one or more word characters.
+          )
+        \s*
+      }}
+    /x.freeze
+
     steps :validate_variables,
       :add_params_to_result,
       :substitute_params,
-      :substitute_ruby_variables,
-      :substitute_liquid_variables
+      :substitute_variables
 
     def initialize(environment, params = {})
       @environment, @params = environment, params.deep_dup
@@ -46,37 +55,28 @@ module Prometheus
       success(result)
     end
 
-    def substitute_liquid_variables(result)
+    def substitute_variables(result)
       return success(result) unless query(result)
 
-      result[:params][:query] =
-        TemplateEngines::LiquidService.new(query(result)).render(full_context)
+      result[:params][:query] = gsub(query(result), full_context)
 
       success(result)
-    rescue TemplateEngines::LiquidService::RenderError => e
-      error(e.message)
     end
 
-    def substitute_ruby_variables(result)
-      return success(result) unless query(result)
-
-      # The % operator doesn't replace variables if the hash contains string
-      # keys.
-      result[:params][:query] = query(result) % predefined_context.symbolize_keys
-
-      success(result)
-    rescue TypeError, ArgumentError => exception
-      log_error(exception.message)
-      Gitlab::ErrorTracking.track_exception(exception, {
-        template_string: query(result),
-        variables: predefined_context
-      })
-
-      error(_('Malformed string'))
+    def gsub(string, context)
+      # Search for variables of the form `{{variable}}` in the string and replace
+      # them with their value.
+      string.gsub(VARIABLE_INTERPOLATION_REGEX) do |match|
+        # Replace with the value of the variable, or if there is no such variable,
+        # replace the invalid variable with itself. So,
+        # `up{instance="{{invalid_variable}}"}` will remain
+        # `up{instance="{{invalid_variable}}"}` after substitution.
+        context.fetch($~[:variable], match)
+      end
     end
 
     def predefined_context
-      @predefined_context ||= Gitlab::Prometheus::QueryVariables.call(@environment)
+      Gitlab::Prometheus::QueryVariables.call(@environment).stringify_keys
     end
 
     def full_context