Always write to *error from PerfCounters::stop() if error is no…

…n-null

src/PerfCounters.cc

@@ -956,6 +956,9 @@ void PerfCounters::close() {
 
 Ticks PerfCounters::stop(Task* t, Error* error) {
   if (!counting) {
+    if (error) {
+      *error = Error::None;
+    }
     return 0;
   }
 
@@ -996,6 +999,10 @@ Ticks PerfCounters::ticks_for_direct_call(Task*) {
 }
 
 Ticks PerfCounters::read_ticks(Task* t, Error* error) {
+  if (error) {
+    *error = Error::None;
+  }
+
   ASSERT(t, opened);
   ASSERT(t, counting);
   ASSERT(t, counting_period > 0);
@@ -1077,10 +1084,6 @@ Ticks PerfCounters::read_ticks(Task* t, Error* error) {
       ASSERT(t, false) << "Detected " << ret
           << " ticks, expected no more than " << adjusted_counting_period;
     }
-  } else {
-    if (error) {
-      *error = Error::None;
-    }
   }
   return ret;
 }

src/PerfCounters.h

@@ -105,9 +105,10 @@ class PerfCounters {
    * Suspend counting until the next start.
    * Returns the current value of the ticks counter.
    * `t` is used for debugging purposes.
-   * If `error` is non-null and a transient error is detected,
-   * `*error` will be set to `Error::Transient`. If `error` is null
-   * and a transient error is detected, it will be treated as fatal.
+   * If `error` is non-null,`*error` will be set to `Error::Transient`
+   * if a transient error is detected, otherwise `Error::None`.
+   * If `error` is null and a transient error is detected, it will be
+   * treated as fatal.
    */
   Ticks stop(Task* t, Error* error = nullptr);
 
@@ -184,6 +185,12 @@ class PerfCounters {
    */
   uint32_t recording_skid_size() { return skid_size() * 5; }
 
+  /**
+   * If `error` is non-null,`*error` will be set to `Error::Transient`
+   * if a transient error is detected, otherwise `Error::None`.
+   * If `error` is null and a transient error is detected, it will be
+   * treated as fatal.
+   */
   Ticks read_ticks(Task* t, Error* error);
 
   // Only valid while 'counting' is true