Fix AllocateLikeTest gtests reading uninitialized null-mask

[davidwendt]

Description

Fixes a bug found in the gtests AllocateLikeTest.ColumnNumericTestSameSize and AllocateLikeTest.ColumnNumericTestSpecifiedSize where uninitialized column null-masks are used to verify null-counts.
This error was found by running the following in the libcudf build directory:

gtests/COPYING_TEST --rmm_mode=cuda

The AllocateLikeTest tests cudf::allocate_like() which produces an uninitialized fixed-width column which is usually filled in by internal functions. The uninitialized column data includes its null-mask. The gtest uses CUDF_TEST_EXPECT_COLUMN_PROPERTIES_EQUAL to verify the column returned by allocate_like() which also includes the null_count(). The null-count property returned by allocate_like() sets the null-count to UNKNOWN_NULL_COUNT which causes a future call to null_count() to calculate the count by reading the bits in the null-mask. Since the null-mask is uninitialized (garbage), the null-count would be invalid and likely not match any predictable value.
The gtests are updated to check for appropriate column properties excluding the null-count.

While debugging this issue, the allocate_like() logic was found to also attempt to build child columns incorrectly assuming the children will contain the same size and null-mask. Since the allocate_like() only supports fixed-width types which do not have children, this logic was removed.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

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Additional details and impacted files

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[vyasr]

Nice find! I see two other uses of allocate_like in column_device_view_test.cu, but in that case it looks like the columns are filled before any comparison so I think this isn't an issue, right?

[davidwendt]

Nice find! I see two other uses of allocate_like in column_device_view_test.cu, but in that case it looks like the columns are filled before any comparison so I think this isn't an issue, right?

Yes, it is not an issue there because the uninitialized data is not read and there is no null-mask.

[davidwendt]

/merge