Fix reading interrupted messages

When an message is received and a timeout occurs after reading the
header but before reading the entire body the connection state could
be corrupted due to the header being consumed. The next read would
consider the body of the previous message as the header for the next.

fixes #348

backend_test.go

@@ -0,0 +1,37 @@
+package pgproto3_test
+
+import (
+	"testing"
+
+	"github.com/jackc/pgx/pgproto3"
+)
+
+func TestBackendReceiveInterrupted(t *testing.T) {
+	t.Parallel()
+
+	server := &interruptReader{}
+	server.push([]byte{'Q', 0, 0, 0, 6})
+
+	backend, err := pgproto3.NewBackend(server, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	msg, err := backend.Receive()
+	if err == nil {
+		t.Fatal("expected err")
+	}
+	if msg != nil {
+		t.Fatalf("did not expect msg, but %v", msg)
+	}
+
+	server.push([]byte{'I', 0})
+
+	msg, err = backend.Receive()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if msg, ok := msg.(*pgproto3.Query); !ok || msg.String != "I" {
+		t.Fatalf("unexpected msg: %v", msg)
+	}
+}