telemetry/internal/counter: fix debugging Read

Read needs to expand the names of stack pointers,
and return an error when a counter is not found.

Test writers now need to be prepared for errors if they ask to
Read counters that haven't been used.

Change-Id: I8f3e10935289bff81cbb2751f91d097a82c03e73
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/564617
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Peter Weinberger <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: pjweinbgo

counter/countertest/countertest_test.go

@@ -32,8 +32,12 @@ func TestReadCounter(t *testing.T) {
 	testenv.SkipIfUnsupportedPlatform(t)
 	c := counter.New("foobar")
 
-	if got, err := ReadCounter(c); err != nil || got != 0 {
-		t.Errorf("ReadCounter = (%v, %v), want (%v, nil)", got, err, 0)
+	got, err := ReadCounter(c)
+	if got != 0 {
+		t.Errorf("ReadCounter = %d, want 0", got)
+	}
+	if err == nil {
+		t.Errorf("ReadCounter = (%v, nil), want (%v, error)", got, 0)
 	}
 
 	var wg sync.WaitGroup

internal/counter/counter.go

@@ -300,8 +300,11 @@ func Read(c *Counter) (uint64, error) {
 	if err != nil {
 		return 0, err
 	}
-	// counter doesn't write the entry to file until the value becomes non-zero.
-	return pf.Count[c.name], nil
+	v, ok := pf.Count[DecodeStack(c.Name())]
+	if !ok {
+		return v, fmt.Errorf("not found:%q", DecodeStack(c.Name()))
+	}
+	return v, nil
 }
 
 func readFile(f *file) (*File, error) {

internal/counter/counter_test.go

@@ -443,6 +443,10 @@ func TestStack(t *testing.T) {
 		if ct == nil {
 			t.Fatal("nil counter")
 		}
+		_, err := Read(ct)
+		if err != nil {
+			t.Errorf("failed to read known counter %v", err)
+		}
 		if ct.ptr.count == nil {
 			t.Errorf("%q has nil ptr.count", ct.Name())
 			continue
@@ -491,7 +495,7 @@ func TestStack(t *testing.T) {
 	}
 }
 
-// fn calls itself n times recursively as incrementing the stack counter.
+// fn calls itself n times recursively while incrementing the stack counter.
 func fn(t *testing.T, n int, c *StackCounter) {
 	c.Inc()
 	if n > 0 {