Handle the case where Purge is called while upload is in progress.

components/ukm/ukm_service.cc

Index: components/ukm/ukm_service.cc
diff --git a/components/ukm/ukm_service.cc b/components/ukm/ukm_service.cc
index c58eef8482c8c17680a4359290e250e67d3e914c..0c7829077d2da046cb5633b4d8e71ef66620dc04 100644
--- a/components/ukm/ukm_service.cc
+++ b/components/ukm/ukm_service.cc
@@ -298,21 +298,27 @@ void UkmService::OnLogUploadComplete(int response_code) {
   UMA_HISTOGRAM_SPARSE_SLOWLY("UKM.Upload.ResponseCode", response_code);
 
   bool upload_succeeded = response_code == 200;
-
-  // Provide boolean for error recovery (allow us to ignore response_code).
-  bool discard_log = false;
-  const size_t log_size_bytes = persisted_logs_.staged_log().length();
-  if (upload_succeeded) {
-    UMA_HISTOGRAM_COUNTS_10000("UKM.LogSize.OnSuccess", log_size_bytes / 1024);
-  } else if (response_code == 400) {
-    // Bad syntax.  Retransmission won't work.
-    discard_log = true;
-  }
-
-  if (upload_succeeded || discard_log) {
-    persisted_logs_.DiscardStagedLog();

[Alexei Svitkine (slow), 2017/03/07 22:02:10]

You'll need to rebase on my change.

[Steven Holte, 2017/03/07 22:26:18]

Done.

-    // Store the updated list to disk now that the removed log is uploaded.
-    persisted_logs_.PersistUnsentLogs();
+  if (persisted_logs_.has_staged_log()) {
+    // Provide boolean for error recovery (allow us to ignore response_code).
+    bool discard_log = false;
+    const size_t log_size_bytes = persisted_logs_.staged_log().length();
+    if (upload_succeeded) {
+      UMA_HISTOGRAM_COUNTS_10000("UKM.LogSize.OnSuccess",
+                                 log_size_bytes / 1024);
+    } else if (response_code == 400) {
+      // Bad syntax.  Retransmission won't work.
+      discard_log = true;
+    }
+
+    if (upload_succeeded || discard_log) {
+      persisted_logs_.DiscardStagedLog();
+      // Store the updated list to disk now that the removed log is uploaded.
+      persisted_logs_.PersistUnsentLogs();
+    }
+  } else {
+    // Staged log may have been deleted by Purge already.
+    UMA_HISTOGRAM_SPARSE_SLOWLY("UKM.Upload.Purged.ResponseCode",

[Alexei Svitkine (slow), 2017/03/07 22:02:10]

I don't understand why we should do a separate code path and separate histogram
when a log is purged.

If it's a valid thing to happen, why not just update my TODO comment to explain
the case and not do anything special?

[Steven Holte, 2017/03/07 22:26:18]

Was just adding this to record how often it actually happened.  Removed it for
now.

+                                response_code);
   }
 
   // Error 400 indicates a problem with the log, not with the server, so