Re: [PATCH] libstdc++: testsuite: avoid predictable mkstemp

[Alexandre Oliva <oliva@adacore.com>]

On Jun 22, 2022, Jonathan Wakely <jwakely@redhat.com> wrote:

> On Wed, 22 Jun 2022 at 07:05, Alexandre Oliva via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:

>> It was prompted by a target system with a non-random implementation of
>> mkstemp, that returns a predictable sequence of filenames and selects
>> the first one that isn't already taken.

> OK

And here's the patch that enabled me to stop worrying about the above.
Regstrapped on x86_64-linux-gnu, also tested with a cross to
aarch64-rtems6.  Ok to install?


__gnu_test::nonexistent_path: Always include counter in filename returned

From: Joel Brobecker <brobecker@adacore.com>

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.

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 |   31 ++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/testsuite/util/testsuite_fs.h b/libstdc++-v3/testsuite/util/testsuite_fs.h
index 037d9ffc0f429..206ea67779003 100644
--- a/libstdc++-v3/testsuite/util/testsuite_fs.h
+++ b/libstdc++-v3/testsuite/util/testsuite_fs.h
@@ -38,9 +38,9 @@ namespace test_fs = std::experimental::filesystem;
 
 #if defined(_GNU_SOURCE) || _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
 #include <stdlib.h> // mkstemp
-#else
-#include <random>   // std::random_device
+#include <cstring> // strcpy
 #endif
+#include <random>   // std::random_device
 
 #if defined(__MINGW32__) || defined(__MINGW64__) \
   || !defined (_GLIBCXX_HAVE_SYMLINK)
@@ -125,8 +125,32 @@ namespace __gnu_test
       file.erase(0, pos+1);
 
     test_fs::path p;
+    // A counter, starting from a random value, to be included as part
+    // of the filename being returned, and incremented each time
+    // this method is used.  It allows us to ensure that two calls
+    // to this method 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{}();
+
 #if defined(_GNU_SOURCE) || _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
-    char tmp[] = "filesystem-test.XXXXXX";
+    // 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 include a counter in the template passed to mkstemp.
+    std::string mkstemp_template ("filesystem-test.");
+    mkstemp_template.append(std::to_string (counter++));
+    mkstemp_template.append(".XXXXXX");
+
+    char tmp[mkstemp_template.length() + 1];
+    std::strcpy (tmp, mkstemp_template.c_str());
+
     int fd = ::mkstemp(tmp);
     if (fd == -1)
       throw test_fs::filesystem_error("mkstemp failed",
@@ -141,7 +165,6 @@ namespace __gnu_test
     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


-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>