libutil,nix: fix clashes with stdio stream identifiers

The stdio stream identifiers (stdin, stdout, stderr) are allowed to be macros.
In musl libc they are, for example doing `#define stdout (stdout)`, breaking
compilation with an error when one of the clashing variables is attempted to be
initialized the "wrong" way:

    ../lix/libutil/processes.cc:272:7: error: expected class member or base class name
      272 |     , stdout(stdout ? std::make_unique<AsyncFdIoStream>(std::move(stdout)) : nullptr)
          |       ^
    /nix/store/ziw42d7rvgnf3vkbfc8kry07kipwf1xm-musl-static-x86_64-unknown-linux-musl-1.2.5-dev/include/stdio.h:67:16: note: expanded from macro 'stdout'
       67 | #define stdout (stdout)
          |                ^

Other places only cause warnings on musl:

    ../lix/libutil/processes.cc:254:17: warning: parentheses were disambiguated as redundant parentheses around declaration of variable named 'stdout' [-Wvexing-parse]
      254 |     std::string stdout;
          |                 ^~~~~~
    /nix/store/ziw42d7rvgnf3vkbfc8kry07kipwf1xm-musl-static-x86_64-unknown-linux-musl-1.2.5-dev/include/stdio.h:67:16: note: expanded from macro 'stdout'
       67 | #define stdout (stdout)
          |                ^~~~~~~~
    ../lix/libutil/processes.cc:254:17: note: add a variable name to declare a 'std::string' (aka 'basic_string<char>') initialized with 'stdout'
      254 |     std::string stdout;
          |                 ^
          |                  varname
    /nix/store/ziw42d7rvgnf3vkbfc8kry07kipwf1xm-musl-static-x86_64-unknown-linux-musl-1.2.5-dev/include/stdio.h:67:16: note: expanded from macro 'stdout'
       67 | #define stdout (stdout)
          |                ^
    ../lix/libutil/processes.cc:254:5: note: add enclosing parentheses to perform a function-style cast
      254 |     std::string stdout;
          |     ^                 
          |     (                 )
    ../lix/libutil/processes.cc:254:17: note: remove parentheses to silence this warning
      254 |     std::string stdout;
          |                 ^
    /nix/store/ziw42d7rvgnf3vkbfc8kry07kipwf1xm-musl-static-x86_64-unknown-linux-musl-1.2.5-dev/include/stdio.h:67:16: note: expanded from macro 'stdout'
       67 | #define stdout (stdout)
          |                ^

However they are still wrong, since the macro could be more complicated. Fix
them as well.

Change-Id: I6a6a6964a50ef7dec8f05f0bd8fc8f13f3036d51

lix/libutil/processes.cc

@@ -251,25 +251,25 @@ try {
     options.captureStdout = true;
 
     int status = 0;
-    std::string stdout;
+    std::string childStdout;
 
     try {
         auto proc = runProgram2(options);
         Finally const _wait([&] { proc.waitAndCheck(); });
-        stdout = TRY_AWAIT(proc.getStdout()->drain());
+        childStdout = TRY_AWAIT(proc.getStdout()->drain());
     } catch (ExecError & e) {
         status = e.status;
     }
 
-    co_return {status, std::move(stdout)};
+    co_return {status, std::move(childStdout)};
 } catch (...) {
     co_return result::current_exception();
 }
 
-RunningProgram::RunningProgram(PathView program, Pid pid, AutoCloseFD stdout)
+RunningProgram::RunningProgram(PathView program, Pid pid, AutoCloseFD childStdout)
     : program(program)
     , pid(std::move(pid))
-    , stdout(stdout ? std::make_unique<AsyncFdIoStream>(std::move(stdout)) : nullptr)
+    , childStdout(childStdout ? std::make_unique<AsyncFdIoStream>(std::move(childStdout)) : nullptr)
 {
 }
 
@@ -286,7 +286,7 @@ RunningProgram::~RunningProgram()
 
 std::tuple<pid_t, std::unique_ptr<AsyncFdIoStream>> RunningProgram::release()
 {
-    return {pid.release(), std::move(stdout)};
+    return {pid.release(), std::move(childStdout)};
 }
 
 int RunningProgram::kill()

lix/libutil/processes.hh

@@ -109,9 +109,9 @@ struct [[nodiscard("you must call RunningProgram::wait()")]] RunningProgram
 private:
     Path program;
     Pid pid;
-    std::unique_ptr<AsyncFdIoStream> stdout;
+    std::unique_ptr<AsyncFdIoStream> childStdout;
 
-    RunningProgram(PathView program, Pid pid, AutoCloseFD stdout);
+    RunningProgram(PathView program, Pid pid, AutoCloseFD childStdout);
 
 public:
     RunningProgram() = default;
@@ -130,12 +130,12 @@ public:
 
     std::optional<int> getStdoutFD() const
     {
-        return stdout ? std::optional(stdout->getFD()) : std::nullopt;
+        return childStdout ? std::optional(childStdout->getFD()) : std::nullopt;
     }
 
     AsyncFdIoStream * getStdout() const
     {
-        return stdout.get();
+        return childStdout.get();
     };
 };
 

lix/nix/daemon.cc

@@ -458,16 +458,16 @@ static void forwardStdioConnection(AsyncIoRoot & aio, RemoteStore & store)
 {
     auto conn = store.openConnectionWrapper();
     auto connSocket = AIO().lowLevelProvider.wrapSocketFd(conn->getFD());
-    auto stdin = AIO().lowLevelProvider.wrapInputFd(STDIN_FILENO);
-    auto stdout = AIO().lowLevelProvider.wrapOutputFd(STDOUT_FILENO);
+    auto asyncStdin = AIO().lowLevelProvider.wrapInputFd(STDIN_FILENO);
+    auto asyncStdout = AIO().lowLevelProvider.wrapOutputFd(STDOUT_FILENO);
 
-    aio.blockOn(connSocket->pumpTo(*stdout)
+    aio.blockOn(connSocket->pumpTo(*asyncStdout)
                     .then([](auto) -> Result<void> {
                         return {
                             std::make_exception_ptr(EndOfFile("unexpected EOF from daemon socket"))
                         };
                     })
-                    .exclusiveJoin(stdin->pumpTo(*connSocket).then([](auto) -> Result<void> {
+                    .exclusiveJoin(asyncStdin->pumpTo(*connSocket).then([](auto) -> Result<void> {
                         return result::success();
                     })));
 }