libstdc++: testsuite: avoid predicable mkstemp

We have noticed that, on RTEMS, a small number of testscases are
failing because two calls to this method return the same filename.
This happens for instance in 27_io/filesystem/operations/copy_file.cc
where it does:

  auto from = __gnu_test::nonexistent_path();
  auto to = __gnu_test::nonexistent_path();

We tracked this issue down to the fact that the implementation of
mkstemp on that system appears to use a very predictable algorithm
for chosing the name of the temporary file, where the same filename
appears to be tried in the same order, regardless of past calls.
So, as long as the file gets deleted after a call to mkstemp (something
we do here in our nonexistent_path method), the next call to mkstemps
ends up returning the same filename, causing the collision we se above.

This commit enhances the __gnu_test::nonexistent_path method to
introduce in the filename being returned a counter which gets
incremented at every call of this method.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_fs.h (__gnu_test::nonexistent_path):
	Always include a counter in the filename returned.

libstdc++-v3/testsuite/util/testsuite_fs.h

@@ -32,14 +32,14 @@ namespace test_fs = std::experimental::filesystem;
 #endif
 #include <algorithm>
 #include <fstream>
+#include <random>   // std::random_device
 #include <string>
+#include <system_error>
 #include <cstdio>
 #include <unistd.h> // unlink, close, getpid, geteuid
 
 #if defined(_GNU_SOURCE) || _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
 #include <stdlib.h> // mkstemp
-#else
-#include <random>   // std::random_device
 #endif
 
 #ifndef _GLIBCXX_HAVE_SYMLINK
@@ -123,32 +123,51 @@ namespace __gnu_test
     if (pos != file.npos)
       file.erase(0, pos+1);
 
+    file.reserve(file.size() + 40);
+    file.insert(0, "filesystem-test.");
+
+    // A counter, starting from a random value, to be included as part
+    // of the filename being returned, and incremented each time
+    // this function is used.  It allows us to ensure that two calls
+    // to this function can never return the same filename, something
+    // testcases do when they need multiple non-existent filenames
+    // for their purposes.
+    static unsigned counter = std::random_device{}();
+    file += '.';
+    file += std::to_string(counter++);
+    file += '.';
+
     test_fs::path p;
 #if defined(_GNU_SOURCE) || _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
-    char tmp[] = "filesystem-test.XXXXXX";
-    int fd = ::mkstemp(tmp);
+
+    // Use mkstemp to determine the name of a file which does not exist yet.
+    //
+    // Note that we have seen on some systems (such as RTEMS, for instance)
+    // that mkstemp behaves very predictably, causing it to always try
+    // the same sequence of file names.  In other words, if we call mkstemp
+    // with a pattern, delete the file it created (which is what we do, here),
+    // and call mkstemp with the same pattern again, it returns the same
+    // filename once more.  While most implementations introduce a degree
+    // of randomness, it is not mandated by the standard, and this is why
+    // we also include a counter in the template passed to mkstemp.
+    file += "XXXXXX";
+    int fd = ::mkstemp(&file[0]);
     if (fd == -1)
       throw test_fs::filesystem_error("mkstemp failed",
 	  std::error_code(errno, std::generic_category()));
-    ::unlink(tmp);
+    ::unlink(file.c_str());
     ::close(fd);
-    if (!file.empty())
-      file.insert(0, 1, '-');
-    file.insert(0, tmp);
-    p = file;
+    p = std::move(file);
 #else
     if (file.length() > 64)
       file.resize(64);
-    char buf[128];
-    static unsigned counter = std::random_device{}();
-#if _GLIBCXX_USE_C99_STDIO
-    std::snprintf(buf, 128,
-#else
-    std::sprintf(buf,
-#endif
-      "filesystem-test.%u.%lu-%s", counter++, (unsigned long) ::getpid(),
-      file.c_str());
-    p = buf;
+    // The combination of random counter and PID should be unique for a given
+    // run of the testsuite.
+    file += std::to_string(::getpid());
+    p = std::move(file);
+    if (test_fs::exists(p))
+      throw test_fs::filesystem_error("Failed to generate unique pathname", p,
+	  std::make_error_code(std::errc::file_exists));
 #endif
     return p;
   }