internal/postgres: sort paths before insertion to avoid deadlock

Deadlock is rare but we have observed it when multiple versions of the
same module are inserted concurrently.

Change-Id: Idd28bd558009ff053ac198bdd310fa780af66f83
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/320750
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

internal/postgres/path.go

@@ -9,6 +9,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"sort"
 	"strings"
 
 	"github.com/lib/pq"
@@ -138,6 +139,8 @@ func upsertPaths(ctx context.Context, db *database.DB, paths []string) (pathToID
 		}
 	}
 	if len(values) > 0 {
+		// Sort to avoid deadlock.
+		sort.Slice(values, func(i, j int) bool { return values[i].(string) < values[j].(string) })
 		// Insert data into the paths table.
 		pathCols := []string{"path"}
 		returningPathCols := []string{"id", "path"}