windows: revert process listing logic to that of v1.6.10 (#24494)

* windows: revert process listing logic to that of v1.6.10

In Nomad 1.7 much of the process management code was refactored, including
a rewrite of how the process tree of an executor was determined on Windows
machines. Unfortunately that rewrite has been cursed with performance issues
and bugs. Instead, revert to the logic used in v1.6.10.

* changelog

.changelog/24494.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+windows: Restore process accounting logic from Nomad 1.6.x
+```

drivers/shared/executor/procstats/list_test.go

@@ -63,31 +63,26 @@ func Test_list(t *testing.T) {
 		name     string
 		needles  int
 		haystack int
-		expect   int
 	}{
 		{
 			name:     "minimal",
 			needles:  2,
 			haystack: 10,
-			expect:   16,
 		},
 		{
 			name:     "small needles small haystack",
 			needles:  5,
 			haystack: 200,
-			expect:   212,
 		},
 		{
 			name:     "small needles large haystack",
 			needles:  10,
 			haystack: 1000,
-			expect:   1022,
 		},
 		{
 			name:     "moderate needles giant haystack",
 			needles:  20,
 			haystack: 2000,
-			expect:   2042,
 		},
 	}
 
@@ -100,11 +95,10 @@ func Test_list(t *testing.T) {
 				return procs, nil
 			}
 
-			result, examined := list(executorPID, lister)
+			result := list(executorPID, lister)
 			must.SliceContainsAll(t, expect, result.Slice(),
 				must.Sprintf("exp: %v; got: %v", expect, result),
 			)
-			must.Eq(t, tc.expect, examined)
 		})
 	}
 }

drivers/shared/executor/procstats/list_windows.go

@@ -26,6 +26,6 @@ import (
 // happens when you use syscalls to work your way from the root down to its
 // descendants.
 func List(executorPID int) set.Collection[ProcessID] {
-	procs, _ := list(executorPID, ps.Processes)
+	procs := list(executorPID, ps.Processes)
 	return procs
 }

drivers/shared/executor/procstats/procstats.go

@@ -82,51 +82,40 @@ func Aggregate(systemStats *cpustats.Tracker, procStats ProcUsages) *drivers.Tas
 	}
 }
 
-func list(executorPID int, processes func() ([]ps.Process, error)) (set.Collection[ProcessID], int) {
-	family := set.From([]int{executorPID})
+func list(executorPID int, processes func() ([]ps.Process, error)) set.Collection[ProcessID] {
+	processFamily := set.From([]ProcessID{executorPID})
 
-	all, err := processes()
+	allPids, err := processes()
 	if err != nil {
-		return family, 0
+		return processFamily
 	}
 
-	parents, examined := mapping(all)
-	examined += gather(family, parents, executorPID)
-
-	return family, examined
-}
-
-func gather(family set.Collection[int], parents map[int]set.Collection[int], parent int) int {
-	examined := 0
-	candidates, ok := parents[parent]
-	if !ok {
-		return examined
-	}
-	for _, candidate := range candidates.Slice() {
-		examined++
-		family.Insert(candidate)
-		examined += gather(family, parents, candidate)
+	// A mapping of pids to their parent pids. It is used to build the process
+	// tree of the executing task
+	pidsRemaining := make(map[int]int, len(allPids))
+	for _, pid := range allPids {
+		pidsRemaining[pid.Pid()] = pid.PPid()
 	}
 
-	return examined
-}
-
-// mapping builds a reverse map of parent to children
-func mapping(all []ps.Process) (map[int]set.Collection[int], int) {
+	for {
+		// flag to indicate if we have found a match
+		foundNewPid := false
 
-	parents := map[int]set.Collection[int]{}
-	examined := 0
+		for pid, ppid := range pidsRemaining {
+			childPid := processFamily.Contains(ppid)
 
-	for _, candidate := range all {
-		if candidate != nil {
-			examined++
-			if children, ok := parents[candidate.PPid()]; ok {
-				children.Insert(candidate.Pid())
-			} else {
-				parents[candidate.PPid()] = set.From([]int{candidate.Pid()})
+			// checking if the pid is a child of any of the parents
+			if childPid {
+				processFamily.Insert(pid)
+				delete(pidsRemaining, pid)
+				foundNewPid = true
 			}
 		}
+
+		if !foundNewPid {
+			break
+		}
 	}
 
-	return parents, examined
+	return processFamily
 }