// Copyright (c) 2020-2023 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package webapi

import (
	"bytes"
	"errors"
	"fmt"
	"net"
	"net/http"
	"net/http/httputil"
	"os"
	"runtime"
	"strings"

	"github.com/decred/slog"
	"github.com/gin-gonic/gin"
)

var (
	dunno     = []byte("???")
	centerDot = []byte("·")
	dot       = []byte(".")
	slash     = []byte("/")
)

// recovery returns a middleware that recovers from any panics which occur in
// request handlers. It logs the panic, a stack trace, and the full request
// details. It also ensure the client receives a 500 response rather than no
// response at all.
func recovery(log slog.Logger) gin.HandlerFunc {
	return func(c *gin.Context) {
		defer func() {
			if err := recover(); err != nil {
				// Check for a broken connection, as it is not really a
				// condition that warrants a panic stack trace.
				var brokenPipe bool
				if ne, ok := err.(*net.OpError); ok {
					var se *os.SyscallError
					if errors.As(ne, &se) {
						seStr := strings.ToLower(se.Error())
						if strings.Contains(seStr, "broken pipe") ||
							strings.Contains(seStr, "connection reset by peer") {
							brokenPipe = true
						}
					}
				}

				httpRequest, _ := httputil.DumpRequest(c.Request, true)
				if brokenPipe {
					log.Errorf("%s\n%s", err, httpRequest)
				} else {
					log.Errorf("panic recovered: %s\n\n%s\n\n%s",
						err, formattedStack(), httpRequest)
				}

				if brokenPipe {
					// If the connection is dead, we can't write a status to it.
					_ = c.Error(err.(error))
					c.Abort()
				} else {
					c.AbortWithStatus(http.StatusInternalServerError)
				}
			}
		}()
		c.Next()
	}
}

// formattedStack returns a nicely formatted stack frame.
func formattedStack() []byte {
	buf := new(bytes.Buffer)
	// As we loop, we open files and read them. These variables record the
	// currently loaded file.
	var lines [][]byte
	var lastFile string
	for i := 3; ; i++ {
		pc, file, line, ok := runtime.Caller(i)
		if !ok {
			break
		}
		// Print this much at least. If we can't find the source, it won't show.
		fmt.Fprintf(buf, "%s:%d (0x%x)\n", file, line, pc)
		if file != lastFile {
			data, err := os.ReadFile(file)
			if err != nil {
				continue
			}
			lines = bytes.Split(data, []byte{'\n'})
			lastFile = file
		}
		fmt.Fprintf(buf, "\t%s: %s\n", function(pc), source(lines, line))
	}
	return buf.Bytes()
}

// source returns a space-trimmed slice of the n'th line.
func source(lines [][]byte, n int) []byte {
	n-- // in stack trace, lines are 1-indexed but our array is 0-indexed
	if n < 0 || n >= len(lines) {
		return dunno
	}
	return bytes.TrimSpace(lines[n])
}

// function returns, if possible, the name of the function containing the PC.
func function(pc uintptr) []byte {
	fn := runtime.FuncForPC(pc)
	if fn == nil {
		return dunno
	}
	name := []byte(fn.Name())
	// The name includes the path name to the package, which is unnecessary
	// since the file name is already included. Plus, it has center dots.
	// That is, we see
	//	runtime/debug.*T·ptrmethod
	// and want
	//	*T.ptrmethod
	// Also the package path might contain dot (e.g. code.google.com/...),
	// so first eliminate the path prefix
	if lastSlash := bytes.LastIndex(name, slash); lastSlash >= 0 {
		name = name[lastSlash+1:]
	}
	if period := bytes.Index(name, dot); period >= 0 {
		name = name[period+1:]
	}
	name = bytes.ReplaceAll(name, centerDot, dot)
	return name
}