Implement $? for ninja

src/command.cc

diff --git a/src/command.cc b/src/command.cc
index 41393ab..ca8a86e 100644
--- a/src/command.cc
+++ b/src/command.cc
@@ -127,14 +127,34 @@
   pat.Stem(n->output.str()).AppendToString(s);
 }
 
-void AutoQuestionVar::Eval(Evaluator*, string* s) const {
+void AutoQuestionVar::Eval(Evaluator* ev, string* s) const {
   unordered_set<StringPiece> seen;
-  WordWriter ww(s);
-  double target_age = GetTimestamp(ce_->current_dep_node()->output.str());
-  for (Symbol ai : ce_->current_dep_node()->actual_inputs) {
-    if (seen.insert(ai.str()).second
-      && GetTimestamp(ai.str()) > target_age) {
+
+  if (ev->avoid_io()) {
+    // Postpone checking timestamps to the start of rule execution using
+    // the helper application `ckati-newer'.
+    *s += "$KATI_NEW_INPUTS";
+    if (ev->env_vars().find("KATI_NEW_INPUTS") == ev->env_vars().end()) {
+      string def;
+
+      WordWriter ww(&def);
+      ww.Write("$(ckati-newer");
+      ww.Write(ce_->current_dep_node()->output.str());
+      for (Symbol ai : ce_->current_dep_node()->actual_inputs) {
+        if (seen.insert(ai.str()).second) {
+          ww.Write(ai.str());
+        }
+      }
+      def += ")";
+      ev->set_env_var("KATI_NEW_INPUTS", def);
+    }
+  } else {
+    WordWriter ww(s);
+    double target_age = GetTimestamp(ce_->current_dep_node()->output.str());
+    for (Symbol ai : ce_->current_dep_node()->actual_inputs) {
+      if (seen.insert(ai.str()).second && GetTimestamp(ai.str()) > target_age) {
         ww.Write(ai.str());
+      }
     }
   }
 }
@@ -193,11 +213,7 @@
   INSERT_AUTO_VAR(AutoHatVar, "^");
   INSERT_AUTO_VAR(AutoPlusVar, "+");
   INSERT_AUTO_VAR(AutoStarVar, "*");
-  if (!g_flags.generate_ninja) {
-    INSERT_AUTO_VAR(AutoQuestionVar, "?");
-  } else {
-    INSERT_AUTO_VAR(AutoNotImplementedVar, "?");
-  }
+  INSERT_AUTO_VAR(AutoQuestionVar, "?");
   // TODO: Implement them.
   INSERT_AUTO_VAR(AutoNotImplementedVar, "%");
   INSERT_AUTO_VAR(AutoNotImplementedVar, "|");
@@ -242,8 +258,15 @@
     continue;
   }
 
-  if (!ev_->delayed_output_commands().empty()) {
+  if (!ev_->delayed_output_commands().empty() || !ev_->env_vars().empty()) {
     std::vector<Command> output_commands;
+    for (auto& var : ev_->env_vars()) {
+      Command& c = output_commands.emplace_back(n.output);
+      c.cmd = var.first + "=" + var.second + " && export " + var.first;
+      c.echo = false;
+      c.ignore_error = false;
+      c.updates_environment = true;
+    }
     for (const string& cmd : ev_->delayed_output_commands()) {
       Command& c = output_commands.emplace_back(n.output);
       c.cmd = cmd;
@@ -254,6 +277,7 @@
     result.swap(output_commands);
     copy(output_commands.begin(), output_commands.end(), back_inserter(result));
     ev_->clear_delayed_output_commands();
+    ev_->clear_env_vars();
   }
 
   ev_->set_current_scope(NULL);

src/command.h

diff --git a/src/command.h b/src/command.h
index 017f6bb..215502f 100644
--- a/src/command.h
+++ b/src/command.h
@@ -25,11 +25,12 @@
 class Evaluator;
 
 struct Command {
-  explicit Command(Symbol o) : output(o), echo(true), ignore_error(false) {}
+  explicit Command(Symbol o) : output(o), echo(true), ignore_error(false), updates_environment(false) {}
   Symbol output;
   string cmd;
   bool echo;
   bool ignore_error;
+  bool updates_environment;
 };
 
 class CommandEvaluator {

src/eval.h

diff --git a/src/eval.h b/src/eval.h
index fc7fcff..1959ef3 100644
--- a/src/eval.h
+++ b/src/eval.h
@@ -173,6 +173,14 @@
   }
   void clear_delayed_output_commands() { delayed_output_commands_.clear(); }
 
+  const unordered_map<string, string>& env_vars() const {
+    return env_vars_;
+  }
+  void set_env_var(const string& name, const string& value) {
+    env_vars_.insert_or_assign(name, value);
+  }
+  void clear_env_vars() { env_vars_.clear(); }
+
   static const SymbolSet& used_undefined_vars() { return used_undefined_vars_; }
 
   int eval_depth() const { return eval_depth_; }
@@ -270,6 +278,7 @@
   // Commands which should run at ninja-time (i.e., info, warning, and
   // error).
   vector<string> delayed_output_commands_;
+  unordered_map<string, string> env_vars_;
 
   Symbol posix_sym_;
   bool is_posix_;

src/newer.cc

diff --git a/src/newer.cc b/src/newer.cc
new file mode 100644
index 0000000..ef3f535
--- /dev/null
+++ b/src/newer.cc
@@ -0,0 +1,53 @@
+// Copyright 2022 Google Inc. All rights reserved
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build ignore
+
+// This is the only source file for ckati-newer, a helper utility
+// invoked by generated Ninja files to evaluate `$?`.
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <sys/stat.h>
+
+double GetTimestamp(const char* filename) {
+  struct stat st;
+  if (stat(filename, &st) < 0) {
+    return -2.0;
+  }
+#if defined(__linux__)
+  return st.st_mtime + st.st_mtim.tv_nsec * 0.001 * 0.001 * 0.001;
+#else
+  return st.st_mtime;
+#endif
+}
+
+int main(int argc, const char* argv[]) {
+  if (argc < 2) {
+    return 1;
+  }
+  double target_age = GetTimestamp(argv[1]);
+  bool first = true;
+  for (int i = 2; i < argc; i++) {
+    if (GetTimestamp(argv[i]) > target_age) {
+      if (first) {
+        first = false;
+      } else {
+        putchar(' ');
+      }
+      puts(argv[i]);
+    }
+  }
+  return 0;
+}

src/ninja.cc

diff --git a/src/ninja.cc b/src/ninja.cc
index ee4a87f..aa6a08f 100644
--- a/src/ninja.cc
+++ b/src/ninja.cc
@@ -414,7 +414,7 @@
       while (isspace(*in))
         in++;
 
-      bool needs_subshell = (command_count > 1 || c.ignore_error);
+      bool needs_subshell = (command_count > 1 || c.ignore_error) && !c.updates_environment;
 
       if (needs_subshell)
         *cmd_buf += '(';