pprof: create HTTP endpoint for setting MutexProfileFraction

Allows to dynamically change the MutexProfileFraction to enable and
disable mutex profiling. It should be very useful for detecting
deadlocks, lock contention and general concurrency problems.

How to use:
To enable run: curl -X POST -v 'localhost:5001/debug/pprof-mutex/?fraction=10
To disable: curl -X POST -v 'localhost:5001/debug/pprof-mutex/?fraction=0'

Fraction defines which fraction of events will be profiled. Higher it is
the lower performance impact but less reliable the result.

To fetch the result use:
go tool pprof $PATH_TO_IPFS_BIN http://localhost:5001/debug/pprof/mutex

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

cmd/ipfs/daemon.go

@@ -448,6 +448,7 @@ func serveHTTPApi(req *cmds.Request, cctx *oldcmds.Context) (<-chan error, error
 		corehttp.VersionOption(),
 		defaultMux("/debug/vars"),
 		defaultMux("/debug/pprof/"),
+		corehttp.MutexFractionOption("/debug/pprof-mutex/"),
 		corehttp.MetricsScrapingOption("/debug/metrics/prometheus"),
 		corehttp.LogOption(),
 	}

core/corehttp/mutex_profile.go

+package corehttp
+
+import (
+	"net"
+	"net/http"
+	"runtime"
+	"strconv"
+
+	core "github.com/ipfs/go-ipfs/core"
+)
+
+// MutexFractionOption allows to set runtime.SetMutexProfileFraction via HTTP
+// using POST request with parameter 'fraction'.
+func MutexFractionOption(path string) ServeOption {
+	return func(_ *core.IpfsNode, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
+		mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
+			if r.Method != http.MethodPost {
+				w.WriteHeader(http.StatusMethodNotAllowed)
+				return
+			}
+			if err := r.ParseForm(); err != nil {
+				w.WriteHeader(http.StatusBadRequest)
+				w.Write([]byte(err.Error()))
+				return
+			}
+
+			asfr := r.Form.Get("fraction")
+			if len(asfr) == 0 {
+				w.WriteHeader(http.StatusBadRequest)
+				return
+			}
+
+			fr, err := strconv.Atoi(asfr)
+			if err != nil {
+				w.WriteHeader(http.StatusBadRequest)
+				w.Write([]byte(err.Error()))
+				return
+			}
+			log.Infof("Setting MutexProfileFraction to %d", fr)
+			runtime.SetMutexProfileFraction(fr)
+		})
+
+		return mux, nil
+	}
+}