libutil: add useful functions to Pid

src/libutil/include/nix/util/processes.hh

@@ -35,6 +35,10 @@ class Pid
 #endif
 public:
     Pid();
+    Pid(const Pid &) = delete;
+    Pid(Pid && other) noexcept;
+    Pid & operator=(const Pid &) = delete;
+    Pid & operator=(Pid && other) noexcept;
 #ifndef _WIN32
     Pid(pid_t pid);
     void operator=(pid_t pid);
@@ -53,6 +57,18 @@ public:
     void setKillSignal(int signal);
     pid_t release();
 #endif
+
+    friend void swap(Pid & lhs, Pid & rhs) noexcept
+    {
+        using std::swap;
+#ifndef _WIN32
+        swap(lhs.pid, rhs.pid);
+        swap(lhs.separatePG, rhs.separatePG);
+        swap(lhs.killSignal, rhs.killSignal);
+#else
+        swap(lhs.pid, rhs.pid);
+#endif
+    }
 };
 
 #ifndef _WIN32

src/libutil/meson.build

@@ -156,6 +156,7 @@ sources = [ config_priv_h ] + files(
   'pos-table.cc',
   'position.cc',
   'posix-source-accessor.cc',
+  'processes.cc',
   'serialise.cc',
   'signature/local-keys.cc',
   'signature/signer.cc',

src/libutil/processes.cc

@@ -0,0 +1,11 @@
+#include "nix/util/processes.hh"
+
+namespace nix {
+
+Pid & Pid::operator=(Pid && other) noexcept
+{
+    swap(*this, other);
+    return *this;
+}
+
+} // namespace nix

src/libutil/unix/processes.cc

@@ -35,6 +35,14 @@ namespace nix {
 
 Pid::Pid() {}
 
+Pid::Pid(Pid && other) noexcept
+    : pid(other.pid)
+    , separatePG(other.separatePG)
+    , killSignal(other.killSignal)
+{
+    other.release();
+}
+
 Pid::Pid(pid_t pid)
     : pid(pid)
 {
@@ -113,7 +121,12 @@ void Pid::setKillSignal(int signal)
 pid_t Pid::release()
 {
     pid_t p = pid;
+    /* We use the move assignment operator rather than setting the individual fields so we aren't duplicating the
+       default values from the header, which would be hard to keep in sync. If we just used the assignment operator
+       without manually resetting pid first it would kill that process, however, so we do manually reset that one field.
+     */
     pid = -1;
+    *this = Pid();
     return p;
 }
 

src/libutil/windows/processes.cc

@@ -33,6 +33,11 @@ using namespace nix::windows;
 
 Pid::Pid() {}
 
+Pid::Pid(Pid && other) noexcept
+    : pid(std::move(other.pid))
+{
+}
+
 Pid::Pid(AutoCloseFD pid)
     : pid(std::move(pid))
 {