internal/bigquery: ignore PreconditionFailed error when updating table

It arises from a benign race condition.
See the comment in the CL for details.

Change-Id: Ica5fce00f21a72b1a3bcf080db528c26f1663393
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/493456
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>

internal/bigquery/bigquery.go

diff --git a/internal/bigquery/bigquery.go b/internal/bigquery/bigquery.go
index 95971af..35d42b8 100644
--- a/internal/bigquery/bigquery.go
+++ b/internal/bigquery/bigquery.go
@@ -155,10 +155,14 @@
 		return false, fmt.Errorf("no schema registered for table %q", tableID)
 	}
 	_, err = c.Table(tableID).Update(ctx, bq.TableMetadataToUpdate{Schema: schema}, meta.ETag)
-	if err != nil && !isAlreadyExistsError(err) {
-		return false, err
+	// There is a race condition if multiple threads of control call this function concurrently:
+	// The table may have changed since Metadata was called above, making the Etag invalid and
+	// resulting in a PreconditionFailed error. This error is harmless: it just means that someone
+	// else updated the table before us. Ignore it.
+	if isAlreadyExistsError(err) || hasCode(err, http.StatusPreconditionFailed) {
+		return false, nil
 	}
-	return false, nil
+	return false, err
 }
 
 // A Row is something that can be uploaded to BigQuery.