libutil, libexpr: #10542 abstract over getrusage for getting cpuTime stat and implement windows version

Update src/libutil/windows/current-process.cc

Prefer `nullptr` over `NULL`

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

Update src/libutil/unix/current-process.cc

Prefer C++ type casts

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

Update src/libutil/windows/current-process.cc

Prefer C++ type casts

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

Update src/libutil/unix/current-process.cc

Don't allocate exception

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/libutil/include/nix/util/current-process.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include <optional>
+#include <chrono>
 
 #ifndef _WIN32
 #  include <sys/resource.h>
@@ -11,6 +12,11 @@
 
 namespace nix {
 
+/**
+ * Get the current process's user space CPU time.
+ */
+std::chrono::microseconds getCpuUserTime();
+
 /**
  * If cgroups are active, attempt to calculate the number of CPUs available.
  * If cgroups are unavailable or if cpu.max is set to "max", return 0.

src/libutil/unix/current-process.cc

@@ -0,0 +1,23 @@
+#include "nix/util/current-process.hh"
+#include "nix/util/error.hh"
+#include <cmath>
+
+#include <sys/resource.h>
+
+namespace nix {
+
+std::chrono::microseconds getCpuUserTime()
+{
+    struct rusage buf;
+
+    if (getrusage(RUSAGE_SELF, &buf) != 0) {
+        throw SysError("failed to get CPU time");
+    }
+
+    std::chrono::seconds seconds(buf.ru_utime.tv_sec);
+    std::chrono::microseconds microseconds(buf.ru_utime.tv_usec);
+
+    return seconds + microseconds;
+}
+
+} // namespace nix

src/libutil/unix/meson.build

@@ -49,6 +49,7 @@ config_unix_priv_h = configure_file(
 sources += config_unix_priv_h
 
 sources += files(
+  'current-process.cc',
   'environment-variables.cc',
   'file-descriptor.cc',
   'file-path.cc',

src/libutil/windows/current-process.cc

@@ -0,0 +1,35 @@
+#include "nix/util/current-process.hh"
+#include "nix/util/windows-error.hh"
+#include <cmath>
+
+#ifdef _WIN32
+#  define WIN32_LEAN_AND_MEAN
+#  include <windows.h>
+
+namespace nix {
+
+std::chrono::microseconds getCpuUserTime()
+{
+    FILETIME creationTime;
+    FILETIME exitTime;
+    FILETIME kernelTime;
+    FILETIME userTime;
+
+    if (!GetProcessTimes(GetCurrentProcess(), &creationTime, &exitTime, &kernelTime, &userTime)) {
+        auto lastError = GetLastError();
+        throw windows::WinError(lastError, "failed to get CPU time");
+    }
+
+    ULARGE_INTEGER uLargeInt;
+    uLargeInt.LowPart = userTime.dwLowDateTime;
+    uLargeInt.HighPart = userTime.dwHighDateTime;
+
+    // FILETIME stores units of 100 nanoseconds.
+    // Dividing by 10 gives microseconds.
+    std::chrono::microseconds microseconds(uLargeInt.QuadPart / 10);
+
+    return microseconds;
+}
+
+} // namespace nix
+#endif // ifdef _WIN32

src/libutil/windows/meson.build

@@ -1,4 +1,5 @@
 sources += files(
+  'current-process.cc',
   'environment-variables.cc',
   'file-descriptor.cc',
   'file-path.cc',