runtime: make TestMemStats failure messages useful

Currently most TestMemStats failures dump the whole MemStats object if
anything is amiss without telling you what is amiss, or even which
field is wrong. This makes it hard to figure out what the actual
problem is.

Replace this with a reflection walk over MemStats and a map of
predicates to check. If one fails, we can construct a detailed and
descriptive error message. The predicates are a direct translation of
the current tests.

Change-Id: I5a7cafb8e6a1eeab653d2e18bb74e2245eaa5444
Reviewed-on: https://go-review.googlesource.com/37512
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/malloc_test.go

@@ -6,6 +6,8 @@ package runtime_test
 
 import (
 	"flag"
+	"fmt"
+	"reflect"
 	. "runtime"
 	"testing"
 	"time"
@@ -20,24 +22,50 @@ func TestMemStats(t *testing.T) {
 	st := new(MemStats)
 	ReadMemStats(st)
 
-	// Everything except HeapReleased, HeapIdle, and NumGC,
-	// because they indeed can be 0.
-	if st.Alloc == 0 || st.TotalAlloc == 0 || st.Sys == 0 || st.Lookups == 0 ||
-		st.Mallocs == 0 || st.Frees == 0 || st.HeapAlloc == 0 || st.HeapSys == 0 ||
-		st.HeapInuse == 0 || st.HeapObjects == 0 || st.StackInuse == 0 ||
-		st.StackSys == 0 || st.MSpanInuse == 0 || st.MSpanSys == 0 || st.MCacheInuse == 0 ||
-		st.MCacheSys == 0 || st.BuckHashSys == 0 || st.GCSys == 0 || st.OtherSys == 0 ||
-		st.NextGC == 0 || st.NumForcedGC == 0 {
-		t.Fatalf("Zero value: %+v", *st)
+	nz := func(x interface{}) error {
+		if x != reflect.Zero(reflect.TypeOf(x)).Interface() {
+			return nil
+		}
+		return fmt.Errorf("zero value")
+	}
+	le := func(thresh uint64) func(interface{}) error {
+		return func(x interface{}) error {
+			if reflect.ValueOf(x).Uint() < thresh {
+				return nil
+			}
+			return fmt.Errorf("insanely high value (overflow?); want <= %d", thresh)
+		}
+	}
+	// Of the uint fields, HeapReleased, HeapIdle, PauseTotalNs, and NumGC can be 0.
+	fields := map[string][]func(interface{}) error{
+		"Alloc": {nz, le(1e10)}, "TotalAlloc": {nz, le(1e11)}, "Sys": {nz, le(1e10)},
+		"Lookups": {nz, le(1e10)}, "Mallocs": {nz, le(1e10)}, "Frees": {nz, le(1e10)},
+		"HeapAlloc": {nz, le(1e10)}, "HeapSys": {nz, le(1e10)}, "HeapIdle": {le(1e10)},
+		"HeapInuse": {nz, le(1e10)}, "HeapReleased": nil, "HeapObjects": {nz, le(1e10)},
+		"StackInuse": {nz, le(1e10)}, "StackSys": {nz, le(1e10)},
+		"MSpanInuse": {nz, le(1e10)}, "MSpanSys": {nz, le(1e10)},
+		"MCacheInuse": {nz, le(1e10)}, "MCacheSys": {nz, le(1e10)},
+		"BuckHashSys": {nz, le(1e10)}, "GCSys": {nz, le(1e10)}, "OtherSys": {nz, le(1e10)},
+		"NextGC": {nz, le(1e10)}, "LastGC": nil,
+		"PauseTotalNs": {le(1e11)}, "PauseNs": nil, "PauseEnd": nil,
+		"NumGC": {le(1e9)}, "NumForcedGC": {nz, le(1e9)},
+		"GCCPUFraction": nil, "EnableGC": nil, "DebugGC": nil,
+		"BySize": nil,
 	}
 
-	if st.Alloc > 1e10 || st.TotalAlloc > 1e11 || st.Sys > 1e10 || st.Lookups > 1e10 ||
-		st.Mallocs > 1e10 || st.Frees > 1e10 || st.HeapAlloc > 1e10 || st.HeapSys > 1e10 ||
-		st.HeapIdle > 1e10 || st.HeapInuse > 1e10 || st.HeapObjects > 1e10 || st.StackInuse > 1e10 ||
-		st.StackSys > 1e10 || st.MSpanInuse > 1e10 || st.MSpanSys > 1e10 || st.MCacheInuse > 1e10 ||
-		st.MCacheSys > 1e10 || st.BuckHashSys > 1e10 || st.GCSys > 1e10 || st.OtherSys > 1e10 ||
-		st.NextGC > 1e10 || st.NumGC > 1e9 || st.NumForcedGC > 1e9 || st.PauseTotalNs > 1e11 {
-		t.Fatalf("Insanely high value (overflow?): %+v", *st)
+	rst := reflect.ValueOf(st).Elem()
+	for i := 0; i < rst.Type().NumField(); i++ {
+		name, val := rst.Type().Field(i).Name, rst.Field(i).Interface()
+		checks, ok := fields[name]
+		if !ok {
+			t.Errorf("unknown MemStats field %s", name)
+			continue
+		}
+		for _, check := range checks {
+			if err := check(val); err != nil {
+				t.Errorf("%s = %v: %s", name, val, err)
+			}
+		}
 	}
 
 	if st.Sys != st.HeapSys+st.StackSys+st.MSpanSys+st.MCacheSys+