Re: [PUSHED] gdb/testsuite: Fix race condition in gdb.base/skip.exp

[Simon Marchi <simark at simark dot ca>]

On 2020-01-09 6:04 p.m., Andrew Burgess wrote:
> In this commit:
> 
>   commit 5024637fac653914d471808288dc3221bc7ec089
>   Date:   Sun Dec 15 11:05:47 2019 +0100
> 
>       Fix skip.exp test failure observed with gcc-9.2.0
> 
> A race condition was introduced into the gdb.base/skip.exp test when
> this line:
> 
>     gdb_test "step" "foo \\(\\) at.*" "step 3"
> 
> Was changed to this:
> 
>     gdb_test "step" "foo \\(\\) at.*" "step 3" "main \\(\\) at .*" "step"
> 
> Before the above change we expected GDB to behave like this:
> 
>   (gdb) step
>   foo () at /path/to/gdb/testsuite/gdb.base/skip.c:42
>   42        return 0;
>   (gdb)
> 
> However, when the test is compiled with GCC 9.2.0 we get a different
> behaviour, and so we need a second 'step', like this:
> 
>   (gdb) step
>   main () at /path/to/gdb.base/skip.c:32
>   32        x = baz ((bar (), foo ()));
>   (gdb) step
>   foo () at /path/to/gdb/testsuite/gdb.base/skip.c:42
>   42        return 0;
>   (gdb)
> 
> Now the change to the test matches against 'main () at .*', however if
> GDB or expect is being slow then we might only get to see output like
> this:
> 
>   (gdb) step
>   main () at /path/to/g
> 
> This will happily match the question pattern, so we send 'step' to GDB
> again.  Now GDB continues to produce output which expect accepts, we
> now see this:
> 
>   b.base/skip.c:32
>   32        x = baz ((bar (), foo ()));
>   (gdb)
> 
> This has carried on from where the previous block of output left off.
> This doesn't match the final pattern 'foo \\(\\) at.*', but it does
> match the prompt pattern that gdb_test_multiple adds, and so we report
> the test as failing.
> 
> The solution is to simply ensure that the question consumes everything
> up to, and including the prompt.  This ensures that the prompt can't
> then match the failure case.  The new test line becomes:
> 
>     gdb_test "step" "foo \\(\\) at.*" "step 3" \
>        "main \\(\\) at .*\r\n$gdb_prompt " "step"
> 
> gdb/testsuite/ChangeLog:
> 
> 	* gdb.base/skip.exp: Fix race condition in test.
> 
> Change-Id: I9f0b0b52ef1b4f980bfaa8fe405ff06d520f3482

Ah, thanks for fixing this, I missed it when looking at the patch.  I don't know
if you used it, but this kind of issue is also reproducible using "make check-read1".

Simon