[libFuzzer] Don't complain about lack of interesting inputs when -runs=0.

[Dor1s]

Summary

The most common usecase for -runs=0 is for generating code coverage
over some corpus. Coverage reports based on sancov are about to be deprecated,
which means some external coverage solution will be used, e.g. Clang source
based code coverage, which does not use any sancov instrumentations and thus
libFuzzer would consider any input to be not interesting in that case.

[Dor1s]

FYI, this breaks some tests, but I can fix that if we agree that we need this change.

[Dor1s]

Stop being stupid and write conditions right (i.e. remove "!")

[Dor1s]

Remove unnecessary test modification, the default value works as well.

[Dor1s]

An update: no tests seem to be failing due to this, but I have some build issue with dataflow.test:

Exit Code: 1

Command Output (stderr):
--
clang-6.0: warning: -Z-reserved-lib-stdc++: 'linker' input unused [-Wunused-command-line-argument]
<...>/build/projects/compiler-rt/test/fuzzer/Output/dataflow.test.tmp-ThreeFunctionsTest.o: In function `dfs$_Z5Func2PKhm':
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:19: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp8'
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:19: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
<...>/build/projects/compiler-rt/test/fuzzer/Output/dataflow.test.tmp-ThreeFunctionsTest.o: In function `LLVMFuzzerTestOneInput':
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:23: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp8'
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:24: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:25: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:26: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:27: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
<...>/build/projects/compiler-rt/test/fuzzer/Output/dataflow.test.tmp-ThreeFunctionsTest.o: In function `dfs$_ZL5Func1PKhm':
<...>/llvm/projects/compiler-rt/test/fuzzer/ThreeFunctionsTest.cpp:14: undefined reference to `dfs$__sanitizer_cov_trace_const_cmp1'
clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation)

[kcc]

LGTM

For data flow test, make sure to rebuild dfsan (ninja check-dfsan)

[Dor1s]

Thanks for the hint! All tests passed now, I'm committing.

lib/fuzzer/FuzzerLoop.cpp

if (SizedFiles.empty()) {
     }
   }
 
   PrintStats("INITED");
   if (!Options.FocusFunction.empty())
     Printf("INFO: %zd/%zd inputs touch the focus function\n",
            Corpus.NumInputsThatTouchFocusFunction(), Corpus.size());
 
-  if (Corpus.empty()) {
+  if (Corpus.empty() && Options.MaxNumberOfRuns) {
     Printf("ERROR: no interesting inputs were found. "
            "Is the code instrumented for coverage? Exiting.\n");
     exit(1);
   }
 }
 
 void Fuzzer::Loop(const Vector<std::string> &CorpusDirs) {
   ReadAndExecuteSeedCorpora(CorpusDirs);