Where Should I Place `defer` Statements in a Loop to Properly Release Resources in Go?

How to Release Resources with Defer in a Loop: The Best Approach

In contexts involving database queries within a loop, the question arises as to the optimal placement of defer statements to ensure proper resource release. Consider the following loop:

for rows.Next() {

   fields, err := db.Query(.....)
   if err != nil {
      // ...
   }
   defer fields.Close()

   // do something with `fields`

}

Two options for defer placement emerge:

1. Defer within the loop body:

   for rows.Next() {
   
      fields, err := db.Query(.....)
      if err != nil {
         // ...
      }
   
      // do something with `fields`
   }
   
   defer fields.Close()

2. Defer after the loop body:

   for rows.Next() {
   
      fields, err := db.Query(.....)
      if err != nil {
         // ...
      }
      
      // do something with `fields`
      
      defer fields.Close()
   }

Understanding Defer Execution

To choose the best approach, we must first understand the behavior of defer. Defered functions are not only delayed until the surrounding function returns, but are also executed even if the function terminates abruptly due to an exception (e.g., panic). This serves as an essential mechanism to ensure resource release even in exceptional situations.

Potential Issues with Defer Placement

Placing defer inside the loop can hinder resource release upon loop termination. If the loop exits early due to an error handled within the loop, the deferred fields.Close() call will not be executed.

Conversely, placing defer after the loop body guarantees that resources will be released regardless of how the loop exits. However, this approach delays resource cleanup until after the entire loop completes, which may not be desirable in all scenarios.

Optimal Solution: Anonymous or Named Function Wrapper

To address both issues, the recommended solution is to encapsulate resource allocation and release within an anonymous or named function. By doing so, defer can be used within the function to ensure resource release upon function return.

For instance:

// Anonymous function wrapper
for rows.Next() {
    func() {
        fields, err := db.Query(...)
        if err != nil {
            // Handle error and return
            return
        }
        defer fields.Close()

        // do something with `fields`
    }()
}

// Named function wrapper
func foo(rs *db.Rows) {
    fields, err := db.Query(...)
    if err != nil {
        // Handle error and return
        return
    }
    defer fields.Close()

    // do something with `fields`
}

for rows.Next() {
    foo(rs)
}

This approach allows for resource release as soon as they are no longer needed, even in cases of exceptions. Additionally, if the goal is to terminate the loop on the first error, the error can be returned from the wrapper function and handled accordingly:

func foo(rs *db.Rows) error {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer fields.Close()

    // do something with `fields`
    return nil
}

for rows.Next() {
    if err := foo(rs); err != nil {
        // Handle error and return
        return
    }
}

Error Handling with Rows.Close()

It's important to note that Rows.Close() returns an error. To handle this error, an anonymous function with a deferred call to Rows.Close() can be used:

func foo(rs *db.Rows) (err error) {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer func() {
        if err = fields.Close(); err != nil {
            err = fmt.Errorf("Rows.Close() error: %w", err)
        }
    }()

    // do something with `fields`
    return nil
}

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