Fix subsequent use of external tables in client

Before this patch it fails on the second and all other invocations:

    $ chc --external --file=/tmp/test-no-lf.tsv --name=t --structure='x String'
    azat.local$ select * from t limit 1
       ┌─x───┐
    1. │ foo │
       └─────┘

    azat.local$ select * from t limit 1
    Exception on client:
    Code: 27. DB::Exception: Cannot parse input: expected '\t' at end of stream.: (at row 1)
    : Buffer has gone, cannot extract information about what has been parsed.: (in file/uri /tmp/test-no-lf.tsv): While executing TabSeparatedRowInputFormat. (CANNOT_PARSE_INPUT_ASSERTION_FAILED)

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>

src/Core/ExternalTable.cpp

@@ -106,6 +106,9 @@ void BaseExternalTable::parseStructureFromTypesField(const std::string & argumen
 
 void BaseExternalTable::initSampleBlock()
 {
+    if (sample_block)
+        return;
+
     const DataTypeFactory & data_type_factory = DataTypeFactory::instance();
 
     for (const auto & elem : structure)

tests/queries/0_stateless/03140_client_subsequent_external_tables.reference

@@ -0,0 +1,2 @@
+foo
+foo

tests/queries/0_stateless/03140_client_subsequent_external_tables.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+INPUT_FILE=$CUR_DIR/$CLICKHOUSE_DATABASE.tsv
+echo "foo" > "$INPUT_FILE"
+
+$CLICKHOUSE_CLIENT --external --file="$INPUT_FILE" --name=t --structure='x String' -nm -q "
+select * from t;
+select * from t;
+"
+
+rm "${INPUT_FILE:?}"