How to log HTTP server errors in golang?
I am trying to catch and log an http error 400 that occurs when url parameters are not encoded properly.
My server implementation looks like this:
router := http.NewServeMux()
router.HandleFunc("/", requestHandler)
s := &http.Server{
Addr: ":8080",
Handler: router,
ErrorLog: myLogger,
}
log.Fatal(s.ListenAndServe())
The request never reaches requestHandler
and ErrorLog: myLogger
doesn't matter.
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2 answers
You need to create a custom Wrapper around the requestHandler that records the StatusCode and validates it after the request has been processed.
Notice how we wrap the main router. fromWrapHandler(router)
package main
import (
"log"
"net/http"
)
type LogRecord struct {
http.ResponseWriter
status int
}
func (r *LogRecord) Write(p []byte) (int, error) {
return r.ResponseWriter.Write(p)
}
func (r *LogRecord) WriteHeader(status int) {
r.status = status
r.ResponseWriter.WriteHeader(status)
}
func WrapHandler(f http.Handler) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
record := &LogRecord{
ResponseWriter: w,
}
f.ServeHTTP(record, r)
log.Println("Bad Request ", record.status)
if record.status == http.StatusBadRequest {
log.Println("Bad Request ", r)
}
}
}
func main() {
router := http.NewServeMux()
s := &http.Server{
Addr: ":8080",
Handler: WrapHandler(router),
}
log.Fatal(s.ListenAndServe())
}
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Just for anyone else who finds it, you can't catch the 400 error with net / http in Go. See this answer for more details:
How do I customize Golang HTTP 400 responses for parsing errors?
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