Spurious libstdc++ testsuite failures because of truncated buffered output

[Paolo Carlini]

Hi
> I am getting several failures in decimal/mixed-mode_neg.cc:
>
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 195)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 196)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 197)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 198)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 199)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 200)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 201)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 202)
> FAIL: decimal/mixed-mode_neg.cc  (test for errors, line 203)
>
> This test expects syntax errors in all these lines, and the compiler
> is indeed flagging them but the output from the compiler is so big
> that dejagnu truncates it.  The testsuite never sees the error
> messages starting at line 195 (I can see the output truncated in
> libstdc++.log).
>
> Is there a way to increase dejagnu's buffering?  This does not happen,
> if I simply run the build in a shallower build tree, but that is not a
> viable alternative.
Honestly, I know very little about the dejagnu buffering, but I'm having 
a quick look to the issue and I'm rather surprised that you are seeing 
it for these particular errors and nowhere else in the testsuite: here, 
on the libstdc++.log of x86_64-linux, I see just 9 candidates listed, 
and I'm pretty sure that in terms of error messages we have much, much, 
longer examples, when templates are involved in particular. What I'm 
missing?

Paolo.