Resource Acquisition Patterns

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// C++: Stack variables have scope-based lifetimes
 
void demonstrateScopeLifetime() {
    DatabaseConnection global_conn("postgres://...");  // Lives to end of function
    
    {  // New scope
        FileHandle inner_file("data.txt");  // Lives to end of this block
        
        if (condition) {
            Lock guard(mutex);  // Lives to end of if-block
            // Do work while holding lock
        }  // guard destroyed here - lock released
        
        // inner_file still valid here
        
    }  // inner_file destroyed here - file closed
    
    // global_conn still valid here
    
}  // global_conn destroyed here - connection closed
 
 
// Nested scopes for fine-grained control
void finegrainedLifetimes() {
    std::unique_ptr<ExpensiveResource> resource;
    
    {  // Limit scope of setup work
        std::unique_lock<std::mutex> lock(setupMutex);
        resource = createResource();
    }  // Lock released before using resource
    
    // Use resource without holding lock
    resource->process();
    
}  // resource destroyed here
 
 
// Loop scopes - each iteration is a new scope
void processFiles(const std::vector<std::string>& paths) {
    for (const auto& path : paths) {
        FileHandle file(path.c_str(), "r");  // Opened
        processContent(file);
        // file destroyed at end of EACH iteration
    }  // All files have been closed
}
 
 
// Scope-based transaction management
void scopedTransaction(Database& db) {
    Transaction txn(db);  // Transaction starts
    
    auto user = txn.insertUser(userData);
    auto account = txn.createAccount(user.id);
    
    if (!validateAccount(account)) {
        return;  // txn destructor rolls back automatically
    }
    
    txn.commit();  // Explicit commit
    
}  // If not committed, destructor rolls back

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// Pattern 1: Guard Object Pattern
// User gets a guard; releasing the resource is automatic
 
class ConnectionPool {
private:
    std::queue<Connection*> available_;
    std::mutex mutex_;
    
public:
    // Guard that returns connection to pool on destruction
    class ConnectionGuard {
        ConnectionPool& pool_;
        Connection* conn_;
        
    public:
        ConnectionGuard(ConnectionPool& pool, Connection* conn)
            : pool_(pool), conn_(conn) {}
        
        ~ConnectionGuard() {
            pool_.release(conn_);  // Automatic return to pool
        }
        
        // Non-copyable, movable
        ConnectionGuard(const ConnectionGuard&) = delete;
        ConnectionGuard& operator=(const ConnectionGuard&) = delete;
        
        ConnectionGuard(ConnectionGuard&& other) noexcept
            : pool_(other.pool_), conn_(other.conn_) {
            other.conn_ = nullptr;
        }
        
        Connection* operator->() { return conn_; }
        Connection& operator*() { return *conn_; }
    };
    
    ConnectionGuard acquire() {
        std::lock_guard lock(mutex_);
        Connection* conn = available_.front();
        available_.pop();
        return ConnectionGuard(*this, conn);
    }
    
private:
    void release(Connection* conn) {
        if (conn) {
            std::lock_guard lock(mutex_);
            available_.push(conn);
        }
    }
};
 
// Usage - connection automatically returned to pool
void usePool(ConnectionPool& pool) {
    auto conn = pool.acquire();  // Get guard
    conn->query("SELECT 1");
}  // Guard destroyed, connection returned
 
 
// Pattern 2: Callback/Loan Pattern
// User never directly handles the resource
 
class SecureKey {
private:
    std::vector<uint8_t> key_material_;
    
public:
    SecureKey(size_t keySize) : key_material_(keySize) {
        generateRandomBytes(key_material_.data(), keySize);
    }
    
    ~SecureKey() {
        // Secure cleanup - zero memory before freeing
        std::memset(key_material_.data(), 0, key_material_.size());
    }
    
    // Callback pattern - key never escapes this scope
    template<typename F>
    auto useKey(F&& operation) const -> decltype(operation(key_material_.data())) {
        return operation(key_material_.data());
    }
    
    // No methods that return the key data directly!
    // User cannot accidentally keep a reference
};
 
// Usage
void encryptData(SecureKey& key, const std::vector<uint8_t>& data) {
    key.useKey([&](const uint8_t* keyBytes) {
        performEncryption(keyBytes, data);
    });
    // keyBytes pointer is invalid outside callback
}
 
 
// Pattern 3: Builder with Scope Completion
class HttpRequestBuilder {
private:
    HttpClient& client_;
    Request request_;
    
public:
    HttpRequestBuilder(HttpClient& client, std::string url)
        : client_(client) {
        request_.url = std::move(url);
    }
    
    HttpRequestBuilder& header(std::string key, std::string value) {
        request_.headers[key] = value;
        return *this;
    }
    
    // execute() takes ownership and performs the request
    // Builder cannot be used after this
    Response execute() && {  // && means only callable on rvalue
        return client_.send(std::move(request_));
    }
};
 
// Usage - builder consumed by execute
Response response = client.request("https://api.example.com")
    .header("Authorization", token)
    .header("Content-Type", "application/json")
    .execute();  // Builder consumed here

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from contextlib import ExitStack, contextmanager
from typing import List, Callable, TypeVar
 
T = TypeVar('T')
 
# Basic composition with ExitStack
def process_all_files(paths: List[str]) -> List[str]:
    """Process multiple files with guaranteed cleanup of all."""
    with ExitStack() as stack:
        # Open all files, tracking for cleanup
        files = [stack.enter_context(open(p)) for p in paths]
        
        # Process all files
        return [f.read() for f in files]
    # All files closed, even if one fails
 
 
# Building complex transactions with ExitStack
def complex_operation():
    with ExitStack() as stack:
        # Each resource is tracked as we acquire it
        db = stack.enter_context(get_db_connection())
        
        # Conditional resource acquisition
        if needs_cache:
            cache = stack.enter_context(get_cache_connection())
        else:
            cache = None
        
        # Cleanup callbacks for non-context-manager resources
        temp_file = create_temp_file()
        stack.callback(os.unlink, temp_file)  # Delete on exit
        
        # Operations...
        result = process(db, cache, temp_file)
        
        return result
    # All resources cleaned up in reverse order
 
 
# Composing context managers into new context managers
@contextmanager
def full_transaction(db_url: str, cache_url: str):
    """Create a composed context manager for complete transaction."""
    with ExitStack() as stack:
        db = stack.enter_context(DatabaseConnection(db_url))
        cache = stack.enter_context(CacheConnection(cache_url))
        tx = stack.enter_context(db.begin_transaction())
        
        yield TransactionContext(db=db, cache=cache, tx=tx)
        
        # If we reach here without exception, commit
        tx.commit()
    # On exception: tx.rollback() via its __exit__, then cache & db close
 
 
# Usage of composed context manager
with full_transaction(db_url, cache_url) as ctx:
    user = ctx.db.find_user(user_id)
    ctx.cache.invalidate(f"user:{user_id}")
    ctx.tx.update_user(user, new_data)
# Transaction committed, all connections closed
 
 
# Nested scope manager for hierarchical resources
class ScopedResourceTree:
    """Manage hierarchical resources with proper cleanup order."""
    
    def __init__(self):
        self._stack = ExitStack()
        self._children: List['ScopedResourceTree'] = []
    
    def __enter__(self):
        self._stack.__enter__()
        return self
    
    def __exit__(self, *exc_info):
        # First close all children
        for child in reversed(self._children):
            child.__exit__(*exc_info)
        # Then close own resources
        return self._stack.__exit__(*exc_info)
    
    def add_resource(self, cm):
        """Add a resource to this scope."""
        return self._stack.enter_context(cm)
    
    def create_child_scope(self) -> 'ScopedResourceTree':
        """Create a child scope that closes before parent."""
        child = ScopedResourceTree()
        child.__enter__()
        self._children.append(child)
        return child
 
 
# Hierarchical scope usage
with ScopedResourceTree() as root:
    db = root.add_resource(DatabaseConnection(url))
    
    for shard in shards:
        child_scope = root.create_child_scope()
        shard_conn = child_scope.add_resource(db.connect_shard(shard))
        process_shard(shard_conn)
    
    # All shard connections closed before main db connection

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// Rust's ownership system explicitly tracks scope extension
 
// Returning ownership - resource escapes factory scope
fn create_resource() -> FileHandle {
    let file = FileHandle::open("data.txt")?;
    // file ownership is MOVED to caller
    file  // Not 'return', just the expression
}
 
fn use_created_resource() {
    let my_file = create_resource();  // Ownership transferred here
    my_file.read();
}  // my_file dropped here, not in create_resource
 
 
// Moving into collections
fn store_resources() -> Vec<FileHandle> {
    let mut files = Vec::new();
    
    for path in paths {
        let file = FileHandle::open(path)?;  // Created here
        files.push(file);  // Ownership moved to vector
    }
    
    files  // Ownership of entire vector (and contents) returned
}  // If we returned files, nothing is dropped here
 
 
// Async contexts - ownership across await points
async fn async_file_operation() -> Result<String, Error> {
    let file = File::open("data.txt").await?;  // Opened
    
    // file ownership held across await
    let content = read_file(&file).await?;
    
    Ok(content)
}  // file dropped when future completes
 
 
// Explicit scope extension with Rc/Arc
use std::rc::Rc;
 
fn shared_resource() {
    let resource = Rc::new(ExpensiveData::load());
    
    // Multiple owners
    let consumer1 = Consumer::new(Rc::clone(&resource));
    let consumer2 = Consumer::new(Rc::clone(&resource));
    
    // resource lives until ALL Rc references are dropped
}
 
 
// ManuallyDrop for controlled non-RAII scenarios
use std::mem::ManuallyDrop;
 
struct OptionallyOwned<T> {
    data: ManuallyDrop<T>,
    owned: bool,
}
 
impl<T> OptionallyOwned<T> {
    fn owned(data: T) -> Self {
        Self { data: ManuallyDrop::new(data), owned: true }
    }
    
    fn borrowed(data: T) -> Self {
        Self { data: ManuallyDrop::new(data), owned: false }
    }
}
 
impl<T> Drop for OptionallyOwned<T> {
    fn drop(&mut self) {
        if self.owned {
            // SAFETY: We only drop if we own it, and only once
            unsafe { ManuallyDrop::drop(&mut self.data); }
        }
        // If borrowed, data is NOT dropped - original owner handles it
    }
}

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import asyncio
from contextlib import asynccontextmanager
from typing import AsyncIterator
 
# Async context managers for async resources
class AsyncDatabaseConnection:
    async def __aenter__(self):
        self.conn = await asyncpg.connect(self.url)
        return self.conn
    
    async def __aexit__(self, exc_type, exc_val, exc_tb):
        await self.conn.close()
 
 
# Using asynccontextmanager decorator
@asynccontextmanager
async def managed_http_session() -> AsyncIterator[aiohttp.ClientSession]:
    """Create an HTTP session with proper async cleanup."""
    async with aiohttp.ClientSession() as session:
        yield session
    # Session closed when exiting async with
 
 
# Resource valid across await points
async def fetch_all_pages(urls: list[str]) -> list[str]:
    async with managed_http_session() as session:
        results = []
        for url in urls:
            # Session remains valid across await
            async with session.get(url) as response:
                content = await response.text()
                results.append(content)
        return results
    # Session closed after all awaits complete
 
 
# Handling cancellation
async def cancellation_safe_operation():
    """Ensure cleanup happens even on cancellation."""
    resource = await acquire_async_resource()
    try:
        while True:
            await do_work(resource)
            await asyncio.sleep(1)
    except asyncio.CancelledError:
        # Task was cancelled - cleanup explicitly
        await resource.cleanup()
        raise  # Re-raise to propagate cancellation
    finally:
        # Or use finally for guaranteed cleanup
        await resource.close()
 
 
# Structured concurrency with TaskGroup (Python 3.11+)
async def structured_concurrent_work():
    """Resources are scoped to the TaskGroup."""
    async with AsyncDatabaseConnection() as db:
        async with asyncio.TaskGroup() as tg:
            # All tasks share the db connection
            # db is guaranteed to outlive all tasks
            for item in items:
                tg.create_task(process_item(db, item))
        # TaskGroup exits only when ALL tasks complete
        # Then db connection is closed
    # This structure guarantees: tasks complete -> db closes
 
 
# AsyncExitStack for dynamic async resource management
async def dynamic_async_resources(urls: list[str]):
    async with AsyncExitStack() as stack:
        # Dynamically acquire resources as needed
        sessions = []
        for url in urls:
            session = await stack.enter_async_context(
                aiohttp.ClientSession(url)
            )
            sessions.append(session)
        
        # All sessions valid here
        results = await asyncio.gather(*[
            fetch_from(s) for s in sessions
        ])
        
        return results
    # All sessions closed in reverse order

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import org.junit.jupiter.api.*;
import static org.assertj.core.api.Assertions.*;
import java.util.concurrent.atomic.*;
 
class ResourceManagementTest {
    
    // Track cleanup state
    static class TrackingResource implements AutoCloseable {
        final int id;
        static final List<Integer> closeOrder = new ArrayList<>();
        boolean closed = false;
        
        TrackingResource(int id) {
            this.id = id;
        }
        
        @Override
        public void close() {
            closed = true;
            closeOrder.add(id);
        }
        
        static void reset() {
            closeOrder.clear();
        }
    }
    
    @BeforeEach
    void setup() {
        TrackingResource.reset();
    }
    
    @Test
    void resourcesClosedInReverseOrder() {
        try (
            var r1 = new TrackingResource(1);
            var r2 = new TrackingResource(2);
            var r3 = new TrackingResource(3)
        ) {
            // Use resources
        }
        
        // Verify reverse order: 3, 2, 1
        assertThat(TrackingResource.closeOrder)
            .containsExactly(3, 2, 1);
    }
    
    @Test
    void resourcesClosedOnException() {
        try {
            try (var r = new TrackingResource(1)) {
                throw new RuntimeException("Simulated failure");
            }
        } catch (RuntimeException e) {
            // Expected
        }
        
        // Verify resource was still closed
        assertThat(TrackingResource.closeOrder).containsExactly(1);
    }
    
    @Test
    void closeExceptionSuppressedNotLost() {
        class FailingResource implements AutoCloseable {
            @Override
            public void close() {
                throw new RuntimeException("Close failed");
            }
        }
        
        try (var r = new FailingResource()) {
            throw new IllegalStateException("Primary exception");
        } catch (IllegalStateException e) {
            // Primary exception is thrown
            assertThat(e.getMessage()).isEqualTo("Primary exception");
            
            // Close exception is suppressed
            assertThat(e.getSuppressed())
                .hasSize(1)
                .extracting(Throwable::getMessage)
                .containsExactly("Close failed");
        }
    }
    
    @Test
    void nestedResourcesCleanedUpOnInnerException() {
        try {
            try (var outer = new TrackingResource(1)) {
                try (var inner = new TrackingResource(2)) {
                    throw new RuntimeException("Inner failure");
                }
            }
        } catch (RuntimeException e) {
            // Expected
        }
        
        // Both resources closed, inner first
        assertThat(TrackingResource.closeOrder).containsExactly(2, 1);
    }
    
    // Testing async cleanup
    @Test
    void asyncResourceCleanup() throws Exception {
        var cleanedUp = new CompletableFuture<Boolean>();
        
        AsyncResource resource = new AsyncResource(cleanedUp);
        
        CompletableFuture<Void> operation = CompletableFuture.runAsync(() -> {
            try (resource) {
                doAsyncWork();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        });
        
        operation.get(5, TimeUnit.SECONDS);
        
        // Verify async cleanup happened
        assertThat(cleanedUp.get(1, TimeUnit.SECONDS)).isTrue();
    }
}
 
// Testing mock/spy approach
class PooledResourceTest {
    @Test
    void connectionReturnedToPoolOnException() {
        ConnectionPool pool = mock(ConnectionPool.class);
        Connection conn = mock(Connection.class);
        when(pool.acquire()).thenReturn(new ConnectionGuard(pool, conn));
        
        try {
            pool.withConnection(c -> {
                throw new SQLException("Query failed");
            });
        } catch (SQLException e) {
            // Expected
        }
        
        // Verify connection was returned to pool
        verify(pool).release(conn);
    }
}