proc: fix GoroutinesInfo cache

The allg cache was corrupted when the count parameter was actually
reached.

Fix the bug and add a test for this.

pkg/proc/proc.go

@@ -578,7 +578,9 @@ func GoroutinesInfo(dbp Process, start, count int) ([]*G, int, error) {
 			allg = append(allg, g)
 		}
 	}
-	dbp.Common().allGCache = allg
+	if start == 0 {
+		dbp.Common().allGCache = allg
+	}
 
 	return allg, -1, nil
 }

pkg/proc/proc_test.go

@@ -4135,3 +4135,32 @@ func TestIssue1432(t *testing.T) {
 		assertNoError(err, t, "SetVariable")
 	})
 }
+
+func TestGoroutinesInfoLimit(t *testing.T) {
+	withTestProcess("teststepconcurrent", t, func(p proc.Process, fixture protest.Fixture) {
+		setFileBreakpoint(p, t, fixture, 37)
+		assertNoError(proc.Continue(p), t, "Continue()")
+
+		gcount := 0
+		nextg := 0
+		const goroutinesInfoLimit = 10
+		for nextg >= 0 {
+			oldnextg := nextg
+			var gs []*proc.G
+			var err error
+			gs, nextg, err = proc.GoroutinesInfo(p, nextg, goroutinesInfoLimit)
+			assertNoError(err, t, fmt.Sprintf("GoroutinesInfo(%d, %d)", oldnextg, goroutinesInfoLimit))
+			gcount += len(gs)
+			t.Logf("got %d goroutines\n", len(gs))
+		}
+
+		t.Logf("number of goroutines: %d\n", gcount)
+
+		gs, _, err := proc.GoroutinesInfo(p, 0, 0)
+		assertNoError(err, t, "GoroutinesInfo(0, 0)")
+		t.Logf("number of goroutines (full scan): %d\n", gcount)
+		if len(gs) != gcount {
+			t.Fatalf("mismatch in the number of goroutines %d %d\n", gcount, len(gs))
+		}
+	})
+}