1 // Copyright 2011 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4 
5 // Package debug contains facilities for programs to debug themselves while
6 // they are running.
7 package debug
8 
9 import (
10 	"internal/poll"
11 	"os"
12 	"runtime"
13 	_ "unsafe" // for linkname
14 )
15 
16 // PrintStack prints to standard error the stack trace returned by runtime.Stack.
17 func PrintStack() {
18 	os.Stderr.Write(Stack())
19 }
20 
21 // Stack returns a formatted stack trace of the goroutine that calls it.
22 // It calls [runtime.Stack] with a large enough buffer to capture the entire trace.
23 func Stack() []byte {
24 	buf := make([]byte, 1024)
25 	for {
26 		n := runtime.Stack(buf, false)
27 		if n < len(buf) {
28 			return buf[:n]
29 		}
30 		buf = make([]byte, 2*len(buf))
31 	}
32 }
33 
34 // CrashOptions provides options that control the formatting of the
35 // fatal crash message.
36 type CrashOptions struct {
37 	/* for future expansion */
38 }
39 
40 // SetCrashOutput configures a single additional file where unhandled
41 // panics and other fatal errors are printed, in addition to standard error.
42 // There is only one additional file: calling SetCrashOutput again overrides
43 // any earlier call.
44 // SetCrashOutput duplicates f's file descriptor, so the caller may safely
45 // close f as soon as SetCrashOutput returns.
46 // To disable this additional crash output, call SetCrashOutput(nil).
47 // If called concurrently with a crash, some in-progress output may be written
48 // to the old file even after an overriding SetCrashOutput returns.
49 func SetCrashOutput(f *os.File, opts CrashOptions) error {
50 	fd := ^uintptr(0)
51 	if f != nil {
52 		// The runtime will write to this file descriptor from
53 		// low-level routines during a panic, possibly without
54 		// a G, so we must call f.Fd() eagerly. This creates a
55 		// danger that that the file descriptor is no longer
56 		// valid at the time of the write, because the caller
57 		// (incorrectly) called f.Close() and the kernel
58 		// reissued the fd in a later call to open(2), leading
59 		// to crashes being written to the wrong file.
60 		//
61 		// So, we duplicate the fd to obtain a private one
62 		// that cannot be closed by the user.
63 		// This also alleviates us from concerns about the
64 		// lifetime and finalization of f.
65 		// (DupCloseOnExec returns an fd, not a *File, so
66 		// there is no finalizer, and we are responsible for
67 		// closing it.)
68 		//
69 		// The new fd must be close-on-exec, otherwise if the
70 		// crash monitor is a child process, it may inherit
71 		// it, so it will never see EOF from the pipe even
72 		// when this process crashes.
73 		//
74 		// A side effect of Fd() is that it calls SetBlocking,
75 		// which is important so that writes of a crash report
76 		// to a full pipe buffer don't get lost.
77 		fd2, _, err := poll.DupCloseOnExec(int(f.Fd()))
78 		if err != nil {
79 			return err
80 		}
81 		runtime.KeepAlive(f) // prevent finalization before dup
82 		fd = uintptr(fd2)
83 	}
84 	if prev := runtime_setCrashFD(fd); prev != ^uintptr(0) {
85 		// We use NewFile+Close because it is portable
86 		// unlike syscall.Close, whose parameter type varies.
87 		os.NewFile(prev, "").Close() // ignore error
88 	}
89 	return nil
90 }
91 
92 //go:linkname runtime_setCrashFD runtime.setCrashFD
93 func runtime_setCrashFD(uintptr) uintptr
94