Go Troubleshooting

Go Troubleshooting

Diagnosis before fixes. Reproduce, observe, localize, then change code. Never "fix" a symptom you haven't explained — the bug will move.

1. Pick the Procedure by Symptom

2. Reading a Panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6bb0e4]

goroutine 43 [running]:
myapp/internal/service.(*UserService).Notify(0x0, {0xc000123456?, ...})
        /app/internal/service/user.go:87 +0x24
myapp/internal/handler.(*Handler).Create(0xc0001a2000, ...)
        /app/internal/handler/user.go:41 +0x1c5

Read it mechanically:

1. First line: what kind of panic. nil pointer dereference + addr=0x0 means a nil receiver, nil field, or nil map/pointer argument.
2. Top frame in YOUR code: user.go:87 — go there.
3. Receiver value in the frame: (*UserService).Notify(0x0, ...) — the 0x0 first argument IS the receiver: the service itself was nil. Trace where it was constructed (or wasn't).
4. goroutine 43 — if it's not goroutine 1, find who spawned it and whether a recover boundary should exist there.

3. Hangs and Deadlocks

Get a goroutine dump from the hanging process:

kill -QUIT <pid>      # dumps all goroutine stacks to stderr, then exits
# or, if net/http/pprof is mounted (see §4):
curl 'localhost:6060/debug/pprof/goroutine?debug=2'

Then classify the stacks:

• [semacquire] on sync.(*Mutex).Lock — find which goroutine HOLDS the mutex: look for another stack inside the critical section. Two goroutines each holding one of two locks = lock-order inversion.
• [chan send] / [chan receive] — the other side is gone. Find who should be receiving/sending and why it exited (or was never started).
• [select] with a ctx.Done() case missing — blocked call that ignores cancellation.
• Hundreds of identical stacks — that's a leak (§5), not a deadlock.

4. Memory Leaks

Mount pprof in long-running services (private port only, never public):

import _ "net/http/pprof"

go func() {
    log.Println(http.ListenAndServe("localhost:6060", nil))
}()

Diff heap profiles over time — a leak is growth that never returns:

curl -s localhost:6060/debug/pprof/heap > heap1.pb.gz
sleep 300   # let the leak accumulate
curl -s localhost:6060/debug/pprof/heap > heap2.pb.gz
go tool pprof -base heap1.pb.gz heap2.pb.gz
(pprof) top          # biggest positive delta = the leak
(pprof) list FuncName

Usual suspects: unbounded caches/maps without eviction, subslices pinning large arrays, time.Ticker never stopped, response bodies not closed, growing global slices, forgotten goroutines holding buffers.

5. Goroutine Leaks

curl -s localhost:6060/debug/pprof/goroutine > g1.pb.gz
sleep 300
curl -s localhost:6060/debug/pprof/goroutine > g2.pb.gz
go tool pprof -base g1.pb.gz g2.pb.gz
(pprof) top    # the growing stack is your leak site

The leaking stack tells you which go statement never terminates. Fix the termination path (context, channel close) — patterns in the concurrency skill. In tests, goleak (uber-go/goleak) fails a test that leaves goroutines behind.

6. Race Detector

go test -race ./...        # in CI, always
go build -race ./cmd/api   # staging binaries under real traffic

A report shows two stacks: the write and the concurrent read/write, each with the goroutine's creation site. The fix is never "add a sleep" — protect the state (mutex), transfer ownership (channel), or make it immutable. -race only reports races that actually executed: a clean run proves nothing about untested paths.

7. Delve

dlv test ./internal/service -- -test.run TestTransfer   # debug a test
dlv attach <pid>                                        # running process
dlv core ./api core.1234                                # post-mortem

(dlv) break user.go:87
(dlv) continue
(dlv) print svc.repo          # inspect exact values
(dlv) goroutines -t           # all goroutines with stacks
(dlv) goroutine 43 bt         # switch and backtrace

Use delve when you need actual values or goroutine states, not just locations. For quick localizations, a focused t.Logf or slog.Debug plus one test run is often faster.

8. Diagnostic Environment Variables

GOTRACEBACK=all ./api        # panic dumps ALL goroutines, not just one
GODEBUG=gctrace=1 ./api      # GC cycles: pacing, heap goal, pause times
GOMEMLIMIT=512MiB ./api      # soft memory limit — mitigates OOM while
                             # you find the real leak

Verification Checklist

1. Symptom reproduced (or captured via dump/profile) before any code change
2. Root cause explained: you can say WHY the failure happened at that site
3. Panic fixes address the nil/bounds source, not a wrapper recover
4. Deadlock fixes establish a single lock order or remove the shared lock
5. Leak fixes verified: goroutine/heap profile flat after the fix
6. go test -race ./... passes after concurrency-related fixes
7. A regression test now fails without the fix
8. pprof endpoints bound to localhost/private interfaces only