MetricsService: Send a hash of the UMA log in a header.

chrome/browser/metrics/metrics_service.cc

Index: chrome/browser/metrics/metrics_service.cc
===================================================================
--- chrome/browser/metrics/metrics_service.cc	(revision 228712)
+++ chrome/browser/metrics/metrics_service.cc	(working copy)
@@ -1463,6 +1463,10 @@
       current_fetch_->SetUploadData(kMimeType, compressed_log_text);
       // Tell the server that we're uploading gzipped protobufs.
       current_fetch_->SetExtraRequestHeaders("content-encoding: gzip");
+      const std::string hash =
+          base::HexEncode(log_manager_.staged_log_hash().data(),
+                          log_manager_.staged_log_hash().size());

[Ilya Sherman, 2013/10/16 18:26:14]

nit: Why does the hash need to be hex-encoded?  If it needs to be hex-encoded,
why do that here, rather than when first setting the hash?  Is this to satisfy
constraints on what sorts of values can be passed along via a header?

[Ilya Sherman, 2013/10/16 18:26:14]

Please consider adding a DCHECK to verify that the hash is non-empty.

[Alexei Svitkine (slow), 2013/10/16 19:31:46]

Done.

[Alexei Svitkine (slow), 2013/10/16 19:31:46]

The SHA1 hash returned by the API we're using the the 20 byte raw hash
(non-text).

Whereas HTTP headers are expected to be in human readable text - so conversion
to hex is needed (40 bytes - this is how sha1 hashes are normally shown when
human readable).

I prefer the log to keep the raw sha1 bytes and convert to hex where needed
(e.g. here) - though willing to budge if you have a strong opinion on this.

+      current_fetch_->AddExtraRequestHeader("X-Chrome-UMA-Log-SHA1: " + hash);
       UMA_HISTOGRAM_PERCENTAGE(
           "UMA.ProtoCompressionRatio",
           100 * compressed_log_text.size() / log_text.size());