Merge pull request #399 from redpanda-data/td-stable-avrogen-include-guard

[c++] avrogencpp: emit deterministic include guard

Author: travisdowns

lang/c++/impl/avrogencpp.cc

@@ -17,9 +17,6 @@
  */
 
 #include <cctype>
-#ifndef _WIN32
-#include <ctime>
-#endif
 #include <fstream>
 #include <iostream>
 #include <map>
@@ -30,7 +27,6 @@
 #include <boost/lexical_cast.hpp>
 #include <boost/program_options.hpp>
 
-#include <boost/random/mersenne_twister.hpp>
 #include <utility>
 
 #include "Compiler.hh"
@@ -92,7 +88,6 @@ class CodeGen {
     const std::string includePrefix_;
     const bool noUnion_;
     const std::string guardString_;
-    boost::mt19937 random_;
 
     vector<PendingSetterGetter> pendingGettersAndSetters;
     vector<PendingConstructor> pendingConstructors;
@@ -123,8 +118,7 @@ class CodeGen {
             std::string includePrefix, bool noUnion) : unionTracker_(schemaFile), os_(os), inNamespace_(false), ns_(std::move(ns)),
                                                        schemaFile_(std::move(schemaFile)), headerFile_(std::move(headerFile)),
                                                        includePrefix_(std::move(includePrefix)), noUnion_(noUnion),
-                                                       guardString_(std::move(guardString)),
-                                                       random_(static_cast<uint32_t>(::time(nullptr))) {
+                                                       guardString_(std::move(guardString)) {
     }
 
     void generate(const ValidSchema &schema);
@@ -770,7 +764,14 @@ void CodeGen::emitGeneratedWarning() {
 string CodeGen::guard() {
     string h = headerFile_;
     makeCanonical(h, true);
-    return h + "_" + lexical_cast<string>(random_()) + "_H";
+    // headerFile_ is already a unique-per-output path, so the canonicalised
+    // form is already a valid, unique include guard. Avoid mixing in a
+    // time-seeded RNG here so the generated output is byte-deterministic
+    // across invocations -- otherwise build systems that key their cache on
+    // input-content digests (e.g. Bazel remote cache, Nix store paths) end
+    // up rebuilding every downstream consumer on every invocation, even on
+    // byte-identical schemas.
+    return h + "_H";
 }
 
 void CodeGen::generate(const ValidSchema &schema) {