job control: implement jobpid() to get PID of job

runtime/doc/eval.txt

@@ -1894,6 +1894,7 @@ isdirectory( {directory})	Number	TRUE if {directory} is a directory
 islocked( {expr})		Number	TRUE if {expr} is locked
 items( {dict})			List	key-value pairs in {dict}
 jobclose({job}[, {stream}])	Number	Closes a job stream(s)
+jobpid({job})			Number	Returns pid of a job.
 jobresize({job}, {width}, {height})
 				Number	Resize {job}'s pseudo terminal window
 jobsend({job}, {data})		Number	Writes {data} to {job}'s stdin
@@ -4157,6 +4158,9 @@ jobclose({job}[, {stream}])				{Nvim} *jobclose()*
 		Close {job}'s {stream}, which can be one "stdin", "stdout" or
 		"stderr". If {stream} is omitted, all streams are closed.
 
+jobpid({job})						{Nvim} *jobpid()*
+		Return the pid (process id) of {job}.
+
 jobresize({job}, {width}, {height})			{Nvim} *jobresize()*
 		Resize {job}'s pseudo terminal window to {width} and {height}.
 		This function will fail if used on jobs started without the

src/nvim/eval.c

@@ -7236,6 +7236,7 @@ static struct fst {
   { "islocked",          1, 1, f_islocked },
   { "items",             1, 1, f_items },
   { "jobclose",          1, 2, f_jobclose },
+  { "jobpid",            1, 1, f_jobpid },
   { "jobresize",         3, 3, f_jobresize },
   { "jobsend",           2, 2, f_jobsend },
   { "jobstart",          1, 2, f_jobstart },
@@ -11611,6 +11612,31 @@ static void f_jobclose(typval_T *argvars, typval_T *rettv)
   }
 }
 
+// "jobpid(id)" function
+static void f_jobpid(typval_T *argvars, typval_T *rettv)
+{
+  rettv->v_type = VAR_NUMBER;
+  rettv->vval.v_number = 0;
+
+  if (check_restricted() || check_secure()) {
+    return;
+  }
+
+  if (argvars[0].v_type != VAR_NUMBER) {
+    EMSG(_(e_invarg));
+    return;
+  }
+
+  TerminalJobData *data = find_job(argvars[0].vval.v_number);
+  if (!data) {
+    EMSG(_(e_invjob));
+    return;
+  }
+
+  Process *proc = (Process *)&data->proc;
+  rettv->vval.v_number = proc->pid;
+}
+
 // "jobsend()" function
 static void f_jobsend(typval_T *argvars, typval_T *rettv)
 {