[lib/fuzzer] Fix logging for Fuchsia

[aarongreen]

Summary

This change fixes two aspects of logging on Fuchsia:

1. When logging to stdout, the output file descriptor should not be closed in ExecuteCommand, as it prevent fdio_spawn_etc from succeeding in subsequent calls.
2. When logging to a file in anything other than standalone Zircon, the log file needs to be created in mutable storage. The best indicator where this is relative to the Fuchsia component will be given by '-artifact_prefix=...', so save the log file relative to that path.

[mcgrathr]

lgtm

compiler-rt/trunk/lib/fuzzer/FuzzerCommand.h

public:
   // Removes the given argument from the command argument list.  Ignores any
   // occurrences after "-ignore_remaining_args=1", if present.
   void removeArgument(const std::string &Arg) {
     auto i = endMutableArgs();
     Args.erase(std::remove(Args.begin(), i, Arg), i);
   }
 
   // Like hasArgument, but checks for "-[Flag]=...".
-  bool hasFlag(const std::string &Flag) {
+  bool hasFlag(const std::string &Flag) const {
     std::string Arg("-" + Flag + "=");
     auto IsMatch = [&](const std::string &Other) {
       return Arg.compare(0, std::string::npos, Other, 0, Arg.length()) == 0;
     };
     return std::any_of(Args.begin(), endMutableArgs(), IsMatch);
   }
 
   // Returns the value of the first instance of a given flag, or an empty string
   // if the flag isn't present.  Ignores any occurrences after
   // "-ignore_remaining_args=1", if present.
-  std::string getFlagValue(const std::string &Flag) {
+  std::string getFlagValue(const std::string &Flag) const {
     std::string Arg("-" + Flag + "=");
     auto IsMatch = [&](const std::string &Other) {
       return Arg.compare(0, std::string::npos, Other, 0, Arg.length()) == 0;
     };
     auto i = endMutableArgs();
     auto j = std::find_if(Args.begin(), i, IsMatch);
     std::string result;
     if (j != i) {

compiler-rt/trunk/lib/fuzzer/FuzzerUtilFuchsia.cpp

int ExecuteCommand(const Command &Cmd) {
   auto Args = Cmd.getArguments();
   size_t Argc = Args.size();
   assert(Argc != 0);
   std::unique_ptr<const char *[]> Argv(new const char *[Argc + 1]);
   for (size_t i = 0; i < Argc; ++i)
     Argv[i] = Args[i].c_str();
   Argv[Argc] = nullptr;
 
-  // Determine stdout
+  // 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.
   int FdOut = STDOUT_FILENO;
-
   if (Cmd.hasOutputFile()) {
-    auto Filename = Cmd.getOutputFile();
-    FdOut = open(Filename.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
+    std::string Path;
+    if (Cmd.hasFlag("artifact_prefix"))
+      Path = Cmd.getFlagValue("artifact_prefix") + "/" + Cmd.getOutputFile();
+    else
+      Path = Cmd.getOutputFile();
+    FdOut = open(Path.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
     if (FdOut == -1) {
-      Printf("libFuzzer: failed to open %s: %s\n", Filename.c_str(),
+      Printf("libFuzzer: failed to open %s: %s\n", Path.c_str(),
              strerror(errno));
       return ZX_ERR_IO;
     }
   }
-  auto CloseFdOut = at_scope_exit([&]() { close(FdOut); } );
+  auto CloseFdOut = at_scope_exit([FdOut]() {
+    if (FdOut != STDOUT_FILENO)
+      close(FdOut);
+  });
 
   // Determine stderr
   int FdErr = STDERR_FILENO;
   if (Cmd.isOutAndErrCombined())
     FdErr = FdOut;
 
   // Clone the file descriptors into the new process
   fdio_spawn_action_t SpawnAction[] = {