libutil: make RunningProgram more useful

make it moveable, make it killable, and add a stdout fd accessor.

Change-Id: I2387cbe8ac67b899a322cd6c7d306ef9ea7abcd0

lix/libcmd/repl.cc

@@ -250,7 +250,7 @@ void runNix(Path program, const Strings & args)
         .program = settings.nixBinDir+ "/" + program,
         .args = args,
         .environment = subprocessEnv,
-    }).wait();
+    }).waitAndCheck();
 
     return;
 }
@@ -672,7 +672,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
 
         // runProgram redirects stdout to a StringSink,
         // using runProgram2 to allow editors to display their UI
-        runProgram2(RunOptions { .program = editor, .searchPath = true, .args = args }).wait();
+        runProgram2(RunOptions { .program = editor, .searchPath = true, .args = args }).waitAndCheck();
 
         // Reload right after exiting the editor if path is not in store
         // Store is immutable, so there could be no changes, so there's no need to reload

lix/libfetchers/git.cc

@@ -759,7 +759,7 @@ struct GitInputScheme : InputScheme
                 .args = { "-C", repoDir, "--git-dir", gitDir, "archive", input.getRev()->gitRev() },
                 .captureStdout = true,
             });
-            Finally const _wait([&] { proc.wait(); });
+            Finally const _wait([&] { proc.waitAndCheck(); });
 
             unpackTarfile(*proc.getStdout(), tmpDir);
         }

lix/libstore/build/derivation-goal.cc

@@ -990,7 +990,7 @@ void runPostBuildHook(
         .captureStdout = true,
         .mergeStderrToStdout = true,
     });
-    Finally const _wait([&] { proc.wait(); });
+    Finally const _wait([&] { proc.waitAndCheck(); });
 
     // FIXME just process the data, without a wrapper sink class
     proc.getStdout()->drainInto(sink);

lix/libutil/processes.cc

@@ -249,7 +249,7 @@ std::pair<int, std::string> runProgram(RunOptions && options)
 
     try {
         auto proc = runProgram2(options);
-        Finally const _wait([&] { proc.wait(); });
+        Finally const _wait([&] { proc.waitAndCheck(); });
         stdout = proc.getStdout()->drain();
     } catch (ExecError & e) {
         status = e.status;
@@ -277,7 +277,22 @@ RunningProgram::~RunningProgram()
     }
 }
 
-void RunningProgram::wait()
+std::tuple<pid_t, std::unique_ptr<Source>, int> RunningProgram::release()
+{
+    return {pid.release(), std::move(stdoutSource), stdout_.release()};
+}
+
+int RunningProgram::kill()
+{
+    return pid.kill();
+}
+
+int RunningProgram::wait()
+{
+    return pid.wait();
+}
+
+void RunningProgram::waitAndCheck()
 {
     if (std::uncaught_exceptions() == 0) {
         int status = pid.wait();

lix/libutil/processes.hh

@@ -102,9 +102,23 @@ private:
 
 public:
     RunningProgram() = default;
+    RunningProgram(RunningProgram &&) = default;
+    RunningProgram & operator=(RunningProgram &&) = default;
     ~RunningProgram();
 
-    void wait();
+    explicit operator bool() const { return bool(pid); }
+
+    std::tuple<pid_t, std::unique_ptr<Source>, int> release();
+
+    int kill();
+    [[nodiscard]]
+    int wait();
+    void waitAndCheck();
+
+    std::optional<int> getStdoutFD() const
+    {
+        return stdout_ ? std::optional(stdout_.get()) : std::nullopt;
+    }
 
     Source * getStdout() const { return stdoutSource.get(); };
 };