[LibFuzzer] Fix GCC `-Wunused-result` warning

[delcypher]

Summary

[LibFuzzer] Fix GCC -Wunused-result warning in
Fuzzer::RawPrint(const char*) and make the implementation more robust by handling partial writes.

The warning was

./FuzzerIOPosix.cpp:118:29: warning: ignoring return value of ‘ssize_t write(int, const void*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]
   write(2, Str, strlen(Str));

[kcc]

I understand why this is desired in theory, but do we hit any related problem in practice?
If so, is a test possible?

[delcypher]

In D35245#805183, @kcc wrote:

I understand why this is desired in theory, but do we hit any related problem in practice?
If so, is a test possible?

I've not hit problems in practice. My only real aim was to fix the warning. We could just do

(void) write(2, Str, strlen(str));

instead to suppress the warning. This is "technically wrong" though due to the possibility of not all the bytes of Str being written. So I thought I'd code up my solution to that.

I would understand if you don't want to use because it does over complicated the code without any (so far) observable benefit.

[joerg]

A better construct would be to compute the full length and change both it and Str in the loop. No point in the complex arithmetic. I generally do prefer this as it improves interaction with pipes etc.

lib/Fuzzer/FuzzerIOPosix.cpp

 #include "FuzzerDefs.h"
 #if LIBFUZZER_POSIX
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 #include <cstdarg>
 #include <cstdio>
 #include <dirent.h>
+#include <errno.h>
 #include <fstream>
 #include <iterator>
 #include <libgen.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 namespace fuzzer {
if (FileName.find("/usr/lib/") != std::string::npos)
     return false;
   if (FileName.find("/usr/include/") != std::string::npos)
     return false;
   if (FileName == "<null>")
     return false;
   return true;
 }
 
-
 void RawPrint(const char *Str) {
-  write(2, Str, strlen(Str));
+  const size_t StrLength = strlen(Str);
+  size_t RemainingBytes = StrLength;
+  while (RemainingBytes > 0) {
+    ssize_t BytesWritten =
+        write(2, Str + (StrLength - RemainingBytes), RemainingBytes);
+    if (BytesWritten == -1) {
+      if (errno == EINTR) {
+        // Syscall was interrupted. Try again.
+        continue;
+      }
+      // Otherwise just give up.
+      break;
+    }
+    if (BytesWritten == 0) {
+      // Give up.
+      break;
+    }
+    RemainingBytes -= BytesWritten;
+  }
 }
 
 }  // namespace fuzzer
 
 #endif // LIBFUZZER_POSIX