diff -r 56409232e1a1 -r c64e1588f428 src/StreamletAttributeFinder.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/StreamletAttributeFinder.h	Sat Jan 11 18:13:30 2020 +0100
@@ -0,0 +1,139 @@
+/**
+ * Relational pipes
+ * Copyright © 2019 František Kučera (Frantovo.cz, GlobalCode.info)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <vector>
+#include <filesystem>
+#include <regex>
+#include <memory>
+
+#include <relpipe/writer/typedefs.h>
+#include <relpipe/writer/AttributeMetadata.h>
+#include <relpipe/writer/RelationalWriter.h>
+
+#include "RequestedField.h"
+#include "SubProcess.h"
+#include "AttributeFinder.h"
+#include "StreamletMsg.h"
+
+namespace relpipe {
+namespace in {
+namespace filesystem {
+
+namespace fs = std::filesystem;
+using namespace relpipe::writer;
+
+class StreamletAttributeFinder : public AttributeFinder {
+private:
+	std::wstring_convert<codecvt_utf8<wchar_t>> convertor; // TODO: support also other encodings.
+	std::map<int, std::shared_ptr<SubProcess>> subProcesses;
+	std::map<int, std::vector<AttributeMetadata>> cachedMetadata;
+
+	string_t getExecCommand(const RequestedField& field) {
+		// TODO: move to another directory, exec, not script + use custom $PATH with no prefix
+		return SCRIPT_PREFIX + field.name;
+	}
+
+protected:
+
+	virtual void writeFieldOfExistingFile(RelationalWriter* writer, const RequestedField& field) override {
+		// TODO: paralelize also over records → fork multiple processes and distribute records across them; then collect results (with a lock)
+		if (field.group == RequestedField::GROUP_STREAMLET) {
+
+			subProcesses[field.id]->write({StreamletMsg::INPUT_ATTRIBUTE, L"0", convertor.from_bytes(currentFileRaw), L"false"}); // index, value, isNull
+			subProcesses[field.id]->write({StreamletMsg::WAITING_FOR_OUTPUT_ATTRIBUTES});
+
+			for (auto metadata : cachedMetadata[field.id]) {
+				SubProcess::Message m = subProcesses[field.id]->read();
+				if (m.code == StreamletMsg::OUTPUT_ATTRIBUTE) writer->writeAttribute(m.parameters[0]);
+				else throw RelpipeWriterException(L"Protocol violation from exec sub-process while reading: „" + metadata.attributeName + L"“. Expected OUTPUT_ATTRIBUTE but got: " + m.toString());
+			}
+
+			SubProcess::Message m = subProcesses[field.id]->read();
+			if (m.code != StreamletMsg::WAITING_FOR_INPUT_ATTRIBUTES) throw RelpipeWriterException(L"Protocol violation from exec sub-process. Expected WAITING_FOR_INPUT_ATTRIBUTES but got: " + m.toString());
+			// TODO: generic protocol violation error messages / method for checking responses
+		}
+	}
+
+public:
+
+	static const string_t SCRIPT_PREFIX;
+
+	virtual vector<AttributeMetadata> toMetadata(RelationalWriter* writer, const RequestedField& field) override {
+		if (field.group == RequestedField::GROUP_STREAMLET) {
+
+			if (cachedMetadata.count(field.id)) {
+				return cachedMetadata[field.id];
+			} else {
+
+				std::vector<string_t> commandLine = {getExecCommand(field)};
+				std::map<string_t, string_t> environment;
+
+				for (auto mn : StreamletMsg::getMessageNames()) {
+					environment[L"EXEC_MSG_" + mn.second] = std::to_wstring(mn.first);
+					environment[L"EXEC_MSG_" + std::to_wstring(mn.first)] = mn.second;
+				}
+
+				shared_ptr<SubProcess> subProcess(SubProcess::create(commandLine, environment));
+				subProcesses[field.id] = subProcess;
+
+				string_t version = L"1";
+				subProcess->write({StreamletMsg::VERSION_SUPPORTED, version});
+				subProcess->write({StreamletMsg::WAITING_FOR_VERSION});
+				SubProcess::Message versionMessage = subProcess->read();
+				if (versionMessage.code == StreamletMsg::VERSION_ACCEPTED && versionMessage.parameters[0] == version) {
+					subProcess->write({StreamletMsg::RELATION_START});
+					subProcess->write({StreamletMsg::INPUT_ATTRIBUTE_METADATA, L"path", L"string"});
+					for (string_t alias : field.getAliases()) subProcess->write({StreamletMsg::OUTPUT_ATTRIBUTE_ALIAS, alias});
+					for (int i = 0; i < field.options.size();) subProcess->write({StreamletMsg::OPTION, field.options[i++], field.options[i++]});
+					subProcess->write({StreamletMsg::WAITING_FOR_OUTPUT_ATTRIBUTES_METADATA});
+
+					vector<AttributeMetadata> metadata;
+					while (true) {
+						SubProcess::Message m = subProcess->read();
+						if (m.code == StreamletMsg::OUTPUT_ATTRIBUTE_METADATA) metadata.push_back({m.parameters[0], writer->toTypeId(m.parameters[1])});
+						else if (m.code == StreamletMsg::WAITING_FOR_INPUT_ATTRIBUTES) break;
+					}
+
+					cachedMetadata[field.id] = metadata;
+					return metadata;
+				} else {
+					throw RelpipeWriterException(L"Incompatible exec sub-process version or message: " + versionMessage.toString());
+				}
+			}
+		} else {
+			return {};
+		}
+	}
+
+	virtual ~StreamletAttributeFinder() override {
+		for (auto s : subProcesses) {
+			try {
+				s.second->write({StreamletMsg::RELATION_END});
+				s.second->wait();
+			} catch (...) {
+				std::wcerr << L"Exception caught during closing sub-process #" + std::to_wstring(s.first) + L" and waiting for its end." << std::endl;
+			}
+		}
+	}
+};
+
+const relpipe::writer::string_t StreamletAttributeFinder::SCRIPT_PREFIX = L"__relpipe_in_filesystem_script_";
+
+}
+}
+}