defer in Loops: The Resource Leak Go Still Lets You Write

Book: The Complete Guide to Go Programming Also by me: Thinking in Go (2-book series) — Complete Guide to Go Programming + Hexagonal Architecture in Go My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub You wrote a function that walks a directory and reads every file. It opened cleanly in review. It passed tests on a fixture folder with three files. Then a customer pointed it at a folder with 20,000 files, and the logs filled with: open /data/batch/img-08431.png: too many open files The code never closed anything early. It deferred every close. And that is exactly the problem. defer runs at function return, not end of iteration defer schedules a call to run when the surrounding function returns. Not when the current block ends. Not when the loop iteration ends. When the function returns. Most of the time that distinction does not matter, because most deferred calls live in short functions that return quickly. Put a defer inside a for loop, though, and every iteration adds one more deferred call to a stack that does not unwind until the whole loop is done and the function exits. Here is the version that ships: func processFiles ( paths [] string ) error { for _ , p := range paths { f , err := os . Open ( p ) if err != nil { return err } defer f . Close () // runs at RETURN, not here if err := handle ( f ); err != nil { return err } } return nil } Read it the way the language reads it. On the first iteration, os.Open returns a file handle and you defer its close. On the second iteration, another handle, another deferred close. By the time the loop has touched 20,000 files, you are holding 20,000 open descriptors, and not one of them closes until processFiles returns. Your process has a file-descriptor limit. On Linux the soft default is often 1024. You blow through it somewhere around the 1024th file, and the error you get back says nothing about defer . It says too many open files , wh