fix: OPDS browser OOM (#403)

## Summary

- Rewrite OpdsParser to stream parsing instead of full content
- Fix OOM due to big http xml response

Closes #385 

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

lib/OpdsParser/OpdsParser.cpp

@@ -4,6 +4,14 @@
 
 #include <cstring>
 
+OpdsParser::OpdsParser() {
+  parser = XML_ParserCreate(nullptr);
+  if (!parser) {
+    errorOccured = true;
+    Serial.printf("[%lu] [OPDS] Couldn't allocate memory for parser\n", millis());
+  }
+}
+
 OpdsParser::~OpdsParser() {
   if (parser) {
     XML_StopParser(parser, XML_FALSE);
@@ -14,13 +22,11 @@ OpdsParser::~OpdsParser() {
   }
 }
 
-bool OpdsParser::parse(const char* xmlData, const size_t length) {
-  clear();
+size_t OpdsParser::write(uint8_t c) { return write(&c, 1); }
 
-  parser = XML_ParserCreate(nullptr);
-  if (!parser) {
-    Serial.printf("[%lu] [OPDS] Couldn't allocate memory for parser\n", millis());
-    return false;
+size_t OpdsParser::write(const uint8_t* xmlData, const size_t length) {
+  if (errorOccured) {
+    return length;
   }
 
   XML_SetUserData(parser, this);
@@ -28,43 +34,48 @@ bool OpdsParser::parse(const char* xmlData, const size_t length) {
   XML_SetCharacterDataHandler(parser, characterData);
 
   // Parse in chunks to avoid large buffer allocations
-  const char* currentPos = xmlData;
+  const char* currentPos = reinterpret_cast<const char*>(xmlData);
   size_t remaining = length;
   constexpr size_t chunkSize = 1024;
 
   while (remaining > 0) {
     void* const buf = XML_GetBuffer(parser, chunkSize);
     if (!buf) {
+      errorOccured = true;
       Serial.printf("[%lu] [OPDS] Couldn't allocate memory for buffer\n", millis());
       XML_ParserFree(parser);
       parser = nullptr;
-      return false;
+      return length;
     }
 
     const size_t toRead = remaining < chunkSize ? remaining : chunkSize;
     memcpy(buf, currentPos, toRead);
 
-    const bool isFinal = (remaining == toRead);
-    if (XML_ParseBuffer(parser, static_cast<int>(toRead), isFinal) == XML_STATUS_ERROR) {
+    if (XML_ParseBuffer(parser, static_cast<int>(toRead), 0) == XML_STATUS_ERROR) {
+      errorOccured = true;
       Serial.printf("[%lu] [OPDS] Parse error at line %lu: %s\n", millis(), XML_GetCurrentLineNumber(parser),
                     XML_ErrorString(XML_GetErrorCode(parser)));
       XML_ParserFree(parser);
       parser = nullptr;
-      return false;
+      return length;
     }
 
     currentPos += toRead;
     remaining -= toRead;
   }
-
-  // Clean up parser
-  XML_ParserFree(parser);
-  parser = nullptr;
-
-  Serial.printf("[%lu] [OPDS] Parsed %zu entries\n", millis(), entries.size());
-  return true;
+  return length;
 }
 
+void OpdsParser::flush() {
+  if (XML_Parse(parser, nullptr, 0, XML_TRUE) != XML_STATUS_OK) {
+    errorOccured = true;
+    XML_ParserFree(parser);
+    parser = nullptr;
+  }
+}
+
+bool OpdsParser::error() const { return errorOccured; }
+
 void OpdsParser::clear() {
   entries.clear();
   currentEntry = OpdsEntry{};

lib/OpdsParser/OpdsParser.h

@@ -1,4 +1,5 @@
 #pragma once
+#include <Print.h>
 #include <expat.h>
 
 #include <string>
@@ -42,28 +43,30 @@ using OpdsBook = OpdsEntry;
  *     }
  *   }
  */
-class OpdsParser {
+class OpdsParser final : public Print {
  public:
-  OpdsParser() = default;
+  OpdsParser();
   ~OpdsParser();
 
   // Disable copy
   OpdsParser(const OpdsParser&) = delete;
   OpdsParser& operator=(const OpdsParser&) = delete;
 
-  /**
-   * Parse an OPDS XML feed.
-   * @param xmlData Pointer to the XML data
-   * @param length Length of the XML data
-   * @return true if parsing succeeded, false on error
-   */
-  bool parse(const char* xmlData, size_t length);
+  size_t write(uint8_t) override;
+  size_t write(const uint8_t*, size_t) override;
+
+  void flush() override;
+
+  bool error() const;
+
+  operator bool() { return !error(); }
 
   /**
    * Get the parsed entries (both navigation and book entries).
    * @return Vector of OpdsEntry entries
    */
-  const std::vector<OpdsEntry>& getEntries() const { return entries; }
+  const std::vector<OpdsEntry>& getEntries() const& { return entries; }
+  std::vector<OpdsEntry> getEntries() && { return std::move(entries); }
 
   /**
    * Get only book entries (legacy compatibility).
@@ -96,4 +99,6 @@ class OpdsParser {
   bool inAuthor = false;
   bool inAuthorName = false;
   bool inId = false;
+
+  bool errorOccured = false;
 };

lib/OpdsParser/OpdsStream.cpp

@@ -0,0 +1,15 @@
+#include "OpdsStream.h"
+
+OpdsParserStream::OpdsParserStream(OpdsParser& parser) : parser(parser) {}
+
+int OpdsParserStream::available() { return 0; }
+
+int OpdsParserStream::peek() { abort(); }
+
+int OpdsParserStream::read() { abort(); }
+
+size_t OpdsParserStream::write(uint8_t c) { return parser.write(c); }
+
+size_t OpdsParserStream::write(const uint8_t* buffer, size_t size) { return parser.write(buffer, size); }
+
+OpdsParserStream::~OpdsParserStream() { parser.flush(); }

lib/OpdsParser/OpdsStream.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <Stream.h>
+
+#include "OpdsParser.h"
+
+class OpdsParserStream : public Stream {
+ public:
+  explicit OpdsParserStream(OpdsParser& parser);
+
+  // That functions are not implimented for that stream
+  int available() override;
+  int peek() override;
+  int read() override;
+
+  virtual size_t write(uint8_t c) override;
+  virtual size_t write(const uint8_t* buffer, size_t size) override;
+
+  ~OpdsParserStream() override;
+
+ private:
+  OpdsParser& parser;
+};