Retry once on stale-connection ConnectionError in Pool#with

lib/clickhouse_native/pool.rb

@@ -29,11 +29,26 @@ def initialize(host:, port:, database: "default", user: "default", password: "",
     # session settings), producing misleading log lines and re-raises in
     # unrelated code. A fresh socket + handshake is cheap relative to
     # debugging that.
+    #
+    # ConnectionError gets one automatic retry: pooled connections that
+    # have been idle long enough for the server / an LB to FIN them
+    # surface as "closed" on the very next recv (errno 0, message
+    # "closed: Success"). Discarding and re-checking out lands a fresh
+    # socket and the operation succeeds. The retry only triggers when
+    # the dead-connection error fired before any data was sent, so
+    # write operations don't risk double-execution from this path.
     def with
-      @pool.with do |client|
-        yield client
-      rescue
-        @pool.discard_current_connection(&:close)
+      attempts = 0
+      begin
+        @pool.with do |client|
+          yield client
+        rescue
+          @pool.discard_current_connection(&:close)
+          raise
+        end
+      rescue ConnectionError
+        attempts += 1
+        retry if attempts == 1
         raise
       end
     end

lib/clickhouse_native/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module ClickhouseNative
-  VERSION = "0.7.0"
+  VERSION = "0.8.0"
 end

spec/clickhouse_native_spec.rb

@@ -477,6 +477,41 @@
     expect(pool.database).to eq("default")
   end
 
+  describe "ConnectionError retry" do
+    it "retries once after a stale-connection ConnectionError" do
+      attempts = 0
+      result = pool.with do |client|
+        attempts += 1
+        raise ClickhouseNative::ConnectionError, "closed: Success" if attempts == 1
+        client.query_value("SELECT 1")
+      end
+      expect(result).to eq(1)
+      expect(attempts).to eq(2)
+    end
+
+    it "raises after the second ConnectionError" do
+      attempts = 0
+      expect do
+        pool.with do
+          attempts += 1
+          raise ClickhouseNative::ConnectionError, "closed: Success"
+        end
+      end.to raise_error(ClickhouseNative::ConnectionError)
+      expect(attempts).to eq(2)
+    end
+
+    it "does not retry non-connection errors" do
+      attempts = 0
+      expect do
+        pool.with do
+          attempts += 1
+          raise ClickhouseNative::ServerError, "boom"
+        end
+      end.to raise_error(ClickhouseNative::ServerError)
+      expect(attempts).to eq(1)
+    end
+  end
+
   describe "settings:" do
     it "applies Integer settings to every client on checkout" do
       p = described_class.new(**CH_KWARGS, pool_size: 2, settings: { max_threads: 7 })