Refuse an /events aggregate that also asks for distinct_resources

distinct_resources sends /events to openvoxdb's legacy compiler, which has no
function or group_by, so every backend failed and the client saw a generic 502
instead of the reason.

- Refuse an aggregate carrying a truthy distinct_resources with 400, before any fan-out
- Read the param the way openvoxdb does, so any capitalisation of "true" counts
- Leave non-aggregate distinct_resources queries and every other route alone
- Document the refusal

aggregate.go

@@ -2,7 +2,9 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+	"net/url"
 	"sort"
 	"strconv"
 	"strings"
@@ -167,6 +169,25 @@ func groupByClash(name string) error {
 	return fmt.Errorf("group_by names the aggregate column %q: pdbmux folds that column across backends, so it cannot also be a grouping key", name)
 }
 
+const distinctResourcesParam = "distinct_resources"
+
+// errDistinctResourcesAggregate refuses an aggregate asking for the
+// distinct-resources form of /events. openvoxdb answers that form from its
+// legacy events compiler, whose operator map carries neither function nor
+// group_by (src/puppetlabs/puppetdb/query_eng.clj:189-190 and
+// src/puppetlabs/puppetdb/query/events.clj:183-187), so every backend fails and
+// a fan-out could only report a client mistake as an outage.
+var errDistinctResourcesAggregate = errors.New(
+	"distinct_resources cannot be combined with an extract function column: openvoxdb answers a distinct_resources /events query from its legacy compiler, which supports neither function nor group_by")
+
+// distinctResources reports whether a request asks for the distinct-resources
+// form. openvoxdb coerces the param with Boolean/parseBoolean
+// (src/puppetlabs/puppetdb/http/query.clj:245-250, applied at query.clj:296),
+// so any capitalisation of "true" turns it on and everything else reads false.
+func distinctResources(params url.Values) bool {
+	return strings.EqualFold(params.Get(distinctResourcesParam), "true")
+}
+
 // rewriteAvg replaces the client's avg column with the sum and count of the same
 // expression, so the true weighted average can be computed from the shards:
 // Postgres avg(x) is sum(x)/count(x), and both of those do combine.