test: Add RAII utility classes for testing

- TempFile holds specific contents
 - EnvValue sets an environment variable

test/ares-test.cc

@@ -6,6 +6,8 @@
 #include "ares_dns.h"
 
 #include <netdb.h>
+#include <stdio.h>
+#include <stdlib.h>
 
 #include <functional>
 #include <sstream>
@@ -389,5 +391,30 @@ void NameInfoCallback(void *data, int status, int timeouts,
   if (verbose) std::cerr << "NameInfoCallback(" << *result << ")" << std::endl;
 }
 
+TempFile::TempFile(const std::string& contents)
+  : filename_(tempnam(nullptr, "ares")) {
+  if (!filename_) {
+    std::cerr << "Error: failed to generate temporary filename" << std::endl;
+    return;
+  }
+  FILE *f = fopen(filename_, "w");
+  if (!f) {
+    std::cerr << "Error: failed to create temporary file " << filename_ << std::endl;
+    return;
+  }
+  int rc = fwrite(contents.data(), 1, contents.size(), f);
+  if (rc < (int)contents.size()) {
+    std::cerr << "Error: failed to store data in temporary file " << filename_ << std::endl;
+  }
+  fclose(f);
+}
+
+TempFile::~TempFile() {
+  if (filename_) {
+    unlink(filename_);
+    free(filename_);
+  }
+}
+
 }  // namespace test
 }  // namespace ares

test/ares-test.h

@@ -223,6 +223,41 @@ void SearchCallback(void *data, int status, int timeouts,
 void NameInfoCallback(void *data, int status, int timeouts,
                       char *node, char *service);
 
+// RAII class for a temporary file with the given contents.
+class TempFile {
+ public:
+  TempFile(const std::string& contents);
+  ~TempFile();
+  const char *filename() const {
+    return filename_;
+  }
+ private:
+  char *filename_;
+};
+
+// RAII class for a temporary environment variable value.
+class EnvValue {
+ public:
+  EnvValue(const char *name, const char *value) : name_(name), restore_(false) {
+    char *original = getenv(name);
+    if (original) {
+      restore_ = true;
+      original_ = original;
+    }
+    setenv(name_.c_str(), value, 1);
+  }
+  ~EnvValue() {
+    if (restore_) {
+      setenv(name_.c_str(), original_.c_str(), 1);
+    } else {
+      unsetenv(name_.c_str());
+    }
+  }
+ private:
+  std::string name_;
+  bool restore_;
+  std::string original_;
+};
 
 }  // namespace test
 }  // namespace ares