Go

Write reliable Go code avoiding goroutine leaks, interface traps, and common concurrency bugs.

Quick Reference

Topic	File
Concurrency patterns	concurrency.md
Interface and type system	interfaces.md
Slices, maps, strings	collections.md
Error handling patterns	errors.md

Goroutine Leaks

• Goroutine blocked on channel with no sender = leak forever—always ensure channel closes or use context
• Unbuffered channel send blocks until receive—deadlock if receiver never comes
• for range on channel loops forever until channel closed—sender must close(ch)
• Context cancellation doesn't stop goroutine automatically—must check ctx.Done() in loop
• Leaked goroutines accumulate memory and never garbage collect

Channel Traps

• Sending to nil channel blocks forever—receiving from nil also blocks forever
• Sending to closed channel panics—closing already closed channel panics
• Only sender should close channel—receiver closing causes sender panic
• Buffered channel full = send blocks—size buffer for expected load
• select with multiple ready cases picks randomly—not first listed

Defer Traps

• Defer arguments evaluated immediately, not when deferred function runs—defer log(time.Now()) captures now
• Defer in loop accumulates—defers stack, run at function end not iteration end
• Defer runs even on panic—good for cleanup, but recover only in deferred function
• Named return values modifiable in defer—defer func() { err = wrap(err) }() works
• Defer order is LIFO—last defer runs first

Interface Traps

• Nil concrete value in interface is not nil interface—var p *MyType; var i interface{} = p; i != nil is true
• Type assertion on wrong type panics—use comma-ok: v, ok := i.(Type)
• Empty interface any accepts anything but loses type safety—avoid when possible
• Interface satisfaction is implicit—no compile error if method signature drifts
• Pointer receiver doesn't satisfy interface for value type—only *T has the method

Error Handling

• Errors are values, not exceptions—always check returned error
• err != nil after every call—unchecked errors are silent bugs
• errors.Is for wrapped errors—== doesn't work with fmt.Errorf("%w", err)
• Sentinel errors should be var ErrFoo = errors.New() not recreated
• Panic for programmer errors only—return error for runtime failures

Slice Traps

• Slice is reference to array—modifying slice modifies original
• Append may or may not reallocate—never assume capacity
• Slicing doesn't copy—a[1:3] shares memory with a
• Nil slice and empty slice differ—var s []int vs s := []int{}
• copy() copies min of lengths—doesn't extend destination

Map Traps

• Reading from nil map returns zero value—writing to nil map panics
• Map iteration order is random—don't rely on order
• Maps not safe for concurrent access—use sync.Map or mutex
• Taking address of map element forbidden—&m[key] doesn't compile
• Delete from map during iteration is safe—but add may cause issues

String Traps

• Strings are immutable byte slices—each modification creates new allocation
• range over string iterates runes, not bytes—index jumps for multi-byte chars
• len(s) is bytes, not characters—use utf8.RuneCountInString()
• String comparison is byte-wise—not Unicode normalized
• Substring shares memory with original—large string keeps memory alive

Struct and Memory

• Struct fields padded for alignment—field order affects memory size
• Zero value is valid—var wg sync.WaitGroup works, no constructor needed
• Copying struct with mutex copies unlocked mutex—always pass pointer
• Embedding is not inheritance—promoted methods can be shadowed
• Exported fields start uppercase—lowercase fields invisible outside package

Build Traps

• go build caches aggressively—use -a flag to force rebuild
• Unused imports fail compilation—use _ import for side effects only
• init() runs before main, order by dependency—not file order
• go:embed paths relative to source file—not working directory
• Cross-compile: GOOS=linux GOARCH=amd64 go build—easy but test on target