fix zip archives paths and add zip files test

- create_zip from a subdirectory wrote entry names with a leading
  '/', so the mounted archive looked empty
- memory_ostream did not support tellp(), so create_zip into a memory
  device wrote invalid offsets (-1) to the archive
- ZipFileDevice added missing parent directories for the first entry
  only
- ZipFileDevice::exists returned false for the archive root
- MemoryDevice::mkdirs recreated existing directories, dropping their
  content (mkdir("a/b") cleared "a"); overwriting a file duplicated its
  name in the directory listing

Added dev/tests/zip_files.lua: builds archives with file.create_zip
from memory device root, subdirectories and the real filesystem,
mounts them and checks reading, listing and read-only access.
memory_filesystem.lua got checks for the MemoryDevice issues.

Closes #544

src/io/devices/MemoryDevice.cpp

@@ -180,6 +180,10 @@ std::unique_ptr<io::PathsGenerator> io::MemoryDevice::list(std::string_view path
 }
 
 io::MemoryDevice::Dir* io::MemoryDevice::createDir(std::string path) {
+    const auto& found = nodes.find(path);
+    if (found != nodes.end()) {
+        return found->second.get_if<Dir>();
+    }
     io::path filePath = path;
     io::path parent = filePath.parent();
     auto parentDir = getDir(parent.string());
@@ -195,6 +199,15 @@ io::MemoryDevice::Dir* io::MemoryDevice::createDir(std::string path) {
 io::MemoryDevice::Node* io::MemoryDevice::createFile(
     std::string path, util::Buffer<char>&& content
 ) {
+    const auto& found = nodes.find(path);
+    if (found != nodes.end()) {
+        auto& node = found->second;
+        if (!node.holds_alternative<File>()) {
+            return nullptr;
+        }
+        node.data = File {std::move(content)};
+        return &node;
+    }
     io::path filePath = path;
     io::path parent = filePath.parent();
     auto dir = getDir(parent.string());

src/io/devices/ZipFileDevice.cpp

@@ -173,7 +173,13 @@ ZipFileDevice::ZipFileDevice(
         entries[entry.fileName] = std::move(entry);
     }
 
-    for (auto& [name, _] : entries) {
+    // Add missing parent directories entries
+    std::vector<std::string> names;
+    names.reserve(entries.size());
+    for (const auto& [name, _] : entries) {
+        names.push_back(name);
+    }
+    for (const auto& name : names) {
         io::path path = name;
 
         while (!(path = path.parent()).pathPart().empty()) {
@@ -184,7 +190,6 @@ ZipFileDevice::ZipFileDevice(
             entry.isDirectory = true;
             entries[path.pathPart()] = entry;
         }
-        break;
     }
 
     for (auto& [_, entry] : entries) {
@@ -253,6 +258,9 @@ file_time_type ZipFileDevice::lastWriteTime(std::string_view path) {
 }
 
 bool ZipFileDevice::exists(std::string_view path) {
+    if (path.empty()) {
+        return true;
+    }
     return entries.find(std::string(path)) != entries.end();
 }
 
@@ -441,7 +449,11 @@ static size_t write_zip(
 void io::write_zip(const path& folder, const path& file) {
     ByteBuilder central_dir;
     auto out = io::write(file);
-    size_t entries = write_zip(folder.pathPart(), folder, *out, central_dir);
+    auto root = folder.pathPart();
+    if (!root.empty() && root.back() != '/') {
+        root += '/';
+    }
+    size_t entries = write_zip(root, folder, *out, central_dir);
 
     size_t central_dir_offset = out->tellp();
     out->write(reinterpret_cast<const char*>(central_dir.data()), central_dir.size());

src/io/memory_ostream.hpp

@@ -50,6 +50,19 @@ protected:
         return c;
     }
 
+    pos_type seekoff(
+        off_type off,
+        std::ios_base::seekdir way,
+        std::ios_base::openmode mode = std::ios_base::out
+    ) override {
+        // only position query (tellp) is supported
+        if (off == 0 && way == std::ios_base::cur &&
+            (mode & std::ios_base::out)) {
+            return pos_type(off_type(size()));
+        }
+        return pos_type(off_type(-1));
+    }
+
     std::streamsize xsputn(const char* s, std::streamsize count) override {
         const std::streamsize avail = epptr() - pptr();