[libFuzzer] Don't prefix absolute paths in fuchsia.

[charco]

Summary

The ExecuteCommand function in fuchsia used to prefix the
getOutputFile for each command run with the artifact_prefix flag if
it was available, because fuchsia components don't have a writable working
directory. However, if a file with a global path is provided, fuchsia
should honor that.

An example of this is using the global /tmp directory to store stuff.
In fuchsia it ended up being translated to data///tmp, whereas we want
to make sure it is using /tmp (which is available to components using the
isolated-temp feature).

To test this I made the change, compiled fuchsia with this toolchain and
ran a fuzzer with the -fork=1 flag (that mode makes use of the /tmp
directory). I also tested that normal fuzzing workflow was not affected
by this.

[phosek]

compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp

415

Nit: no curly braces for block with a single statement (LLVM style).

[charco]

Remove brackets on ifs conditions

[charco]

compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp

415

Thanks! I didn't know about this rule. Could you point me to where it is defined? I checked http://llvm.org/docs/CodingStandards.html#source-code-formatting and also tried to see if clang-format -style=LLVM catched it but nothing.

compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp

int ExecuteCommand(const Command &Cmd) {
   for (size_t i = 0; i < Argc; ++i)
     Argv[i] = Args[i].c_str();
   Argv[Argc] = nullptr;
 
   // Determine output.  On Fuchsia, the fuzzer is typically run as a component
   // that lacks a mutable working directory. Fortunately, when this is the case
   // a mutable output directory must be specified using "-artifact_prefix=...",
   // so write the log file(s) there.
+  // However, we don't want to apply this logic for absolute paths.
   int FdOut = STDOUT_FILENO;
   if (Cmd.hasOutputFile()) {
-    std::string Path;
-    if (Cmd.hasFlag("artifact_prefix"))
-      Path = Cmd.getFlagValue("artifact_prefix") + "/" + Cmd.getOutputFile();
-    else
-      Path = Cmd.getOutputFile();
+    std::string Path = Cmd.getOutputFile();
+    bool IsAbsolutePath = Path.length() > 1 && Path[0] == '/';
+    if (!IsAbsolutePath && Cmd.hasFlag("artifact_prefix")) {
+      Path = Cmd.getFlagValue("artifact_prefix") + "/" + Path;
+    }
     FdOut = open(Path.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
     if (FdOut == -1) {
       Printf("libFuzzer: failed to open %s: %s\n", Path.c_str(),
              strerror(errno));
       return ZX_ERR_IO;
     }
   }
   auto CloseFdOut = at_scope_exit([FdOut]() {