no error when getting a variable that doesn't exist in current step

[germasch]

To reproduce:

TEST_F(ADIOS2_CXX11_API_IO, StreamingWrite)
{
    adios2::Engine writer = m_Io.Open("xxx.bp", adios2::Mode::Write);

    double x[3];
    auto var = m_Io.DefineVariable<double>("x", {3}, {0}, {3});
    for (int step = 0; step < 3; step++) {
      x[0] = 10 + step;
      x[1] = 20 + step;
      x[2] = 30 + step;
      writer.BeginStep();
      if (step > 0) {
	writer.Put(var, x);
      }
      writer.EndStep();
    }
    writer.Close();
}

TEST_F(ADIOS2_CXX11_API_IO, StreamingRead)
{
    adios2::Engine reader = m_Io.Open("xxx.bp", adios2::Mode::Read);

    double x[3];
    auto var = m_Io.InquireVariable<double>("x");
    for (int step = 0; step < 3; step++) {
      reader.BeginStep();
      reader.Get(var, x);
      reader.EndStep();
      printf("step %d: %g %g %g\n", step, x[0], x[1], x[2]);
    }
    reader.Close();
}

The second test, when run, gives

[ RUN      ] ADIOS2_CXX11_API_IO.StreamingRead
step 0: 6.95312e-310 2.19424e-314 5.2286e-310
step 1: 11 21 31
step 2: 12 22 32
[       OK ] ADIOS2_CXX11_API_IO.StreamingRead (2 ms)

The file doesn't contain the variable x in step 0, so I don't expect it to read anything (ie, non-initialized nonsense in x is fine), but I do expect to get some kind of an error when the Get call does not succeed.

[williamfgc]

@germasch "safe" code can be written by only doing a Get if the variable returned by InquireVariable is true. adios2 doesn't handle internal references/pointers to the user in an "unsafe" mode. We pass objects that can be used safely/unsafely by using/not-using the bool operator. In any case, after the refactoring separation proposed in #1384 the exception should be thrown by BeginStep since InquireVariable is requested in a Random access fashion (before BeginStep). The latter won't work with streaming engines. InquireVariable should be called between BeginStep/EndStep, for portability and intended "step" scope.

Safe streaming mode:

adios2::Engine reader = m_Io.Open("xxx.bp", adios2::Mode::Read);
double x[3];
for (int step = 0; step < 3; step++) {
    reader.BeginStep(); //streaming mode
    auto var = m_Io.InquireVariable<double>("x"); //check x for current step only  
    if(var) reader.Get(var, x);  //safe using bool operator of var
    reader.EndStep();
    printf("step %d: %g %g %g\n", step, x[0], x[1], x[2]);
}
    reader.Close();

Now, I agree, if the variable doesn't exist Get should be throwing an exception. But in this case, we are passing a valid Variable object due to mixing random access and streaming modes. Hence, I agree with @pnorbert in that the separation is required as it also allows to distinguish portable (file and streaming) and non-portable (file only) code across engines.

[williamfgc]

Related to #1384 . Will refactor using lazy evaluation to separate random access (file engines only) from step-by-step BeginStep/EndStep (all engines) usage.

[germasch]

Okay, so I hadn't realized it's that closely related to #1384, I thought I was doing something valid. Let me comment there.

[williamfgc]

This is good, actually showing code allow us to see how enforcing a certain behavior will look like. Thanks.