perf: Build a log sink only for a new partition

Logs::get called sinks_.emplace(name, makeSink(name, thresh_)), which
evaluates makeSink() and so allocates a sink on every call, then drops
it when the partition already exists. Every getJournal() call paid for
that allocation under the mutex. Look the partition up first and build
a sink only when it is missing.

src/libxrpl/basics/Log.cpp

@@ -123,8 +123,12 @@ beast::Journal::Sink&
 Logs::get(std::string const& name)
 {
     std::scoped_lock const lock(mutex_);
-    auto const result = sinks_.emplace(name, makeSink(name, thresh_));
-    return *result.first->second;
+    // Look up first: emplace() would evaluate makeSink(), and so allocate a
+    // sink, on every call even when the partition already has one.
+    auto it = sinks_.find(name);
+    if (it == sinks_.end())
+        it = sinks_.emplace(name, makeSink(name, thresh_)).first;
+    return *it->second;
 }
 
 beast::Journal::Sink&

src/tests/libxrpl/basics/Log.cpp

@@ -0,0 +1,46 @@
+#include <xrpl/basics/Log.h>
+
+#include <xrpl/beast/utility/Journal.h>
+
+#include <gtest/gtest.h>
+
+#include <memory>
+#include <string>
+
+using namespace xrpl;
+
+namespace {
+
+class CountingLogs : public Logs
+{
+public:
+    int made = 0;
+
+    CountingLogs() : Logs(beast::Severity::Warning)
+    {
+    }
+
+    std::unique_ptr<beast::Journal::Sink>
+    makeSink(std::string const& partition, beast::Severity threshold) override
+    {
+        ++made;
+        return Logs::makeSink(partition, threshold);
+    }
+};
+
+}  // namespace
+
+TEST(LogsTest, get_makes_one_sink_per_partition)
+{
+    CountingLogs logs;
+    beast::Journal::Sink const& first = logs.get("View");
+    EXPECT_EQ(logs.made, 1);
+
+    // Same partition, any case: same sink, and no new one is built.
+    EXPECT_EQ(&logs.get("View"), &first);
+    EXPECT_EQ(&logs.get("view"), &first);
+    EXPECT_EQ(logs.made, 1);
+
+    EXPECT_NE(&logs.get("Ledger"), &first);
+    EXPECT_EQ(logs.made, 2);
+}