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Source file src/runtime/traceback.go

Documentation: runtime

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package runtime
  
  import (
  	"runtime/internal/atomic"
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  // The code in this file implements stack trace walking for all architectures.
  // The most important fact about a given architecture is whether it uses a link register.
  // On systems with link registers, the prologue for a non-leaf function stores the
  // incoming value of LR at the bottom of the newly allocated stack frame.
  // On systems without link registers, the architecture pushes a return PC during
  // the call instruction, so the return PC ends up above the stack frame.
  // In this file, the return PC is always called LR, no matter how it was found.
  //
  // To date, the opposite of a link register architecture is an x86 architecture.
  // This code may need to change if some other kind of non-link-register
  // architecture comes along.
  //
  // The other important fact is the size of a pointer: on 32-bit systems the LR
  // takes up only 4 bytes on the stack, while on 64-bit systems it takes up 8 bytes.
  // Typically this is ptrSize.
  //
  // As an exception, amd64p32 has ptrSize == 4 but the CALL instruction still
  // stores an 8-byte return PC onto the stack. To accommodate this, we use regSize
  // as the size of the architecture-pushed return PC.
  //
  // usesLR is defined below in terms of minFrameSize, which is defined in
  // arch_$GOARCH.go. ptrSize and regSize are defined in stubs.go.
  
  const usesLR = sys.MinFrameSize > 0
  
  var (
  	// initialized in tracebackinit
  	goexitPC             uintptr
  	jmpdeferPC           uintptr
  	mcallPC              uintptr
  	morestackPC          uintptr
  	mstartPC             uintptr
  	rt0_goPC             uintptr
  	sigpanicPC           uintptr
  	runfinqPC            uintptr
  	bgsweepPC            uintptr
  	forcegchelperPC      uintptr
  	timerprocPC          uintptr
  	gcBgMarkWorkerPC     uintptr
  	systemstack_switchPC uintptr
  	systemstackPC        uintptr
  	cgocallback_gofuncPC uintptr
  	skipPC               uintptr
  
  	gogoPC uintptr
  
  	externalthreadhandlerp uintptr // initialized elsewhere
  )
  
  func tracebackinit() {
  	// Go variable initialization happens late during runtime startup.
  	// Instead of initializing the variables above in the declarations,
  	// schedinit calls this function so that the variables are
  	// initialized and available earlier in the startup sequence.
  	goexitPC = funcPC(goexit)
  	jmpdeferPC = funcPC(jmpdefer)
  	mcallPC = funcPC(mcall)
  	morestackPC = funcPC(morestack)
  	mstartPC = funcPC(mstart)
  	rt0_goPC = funcPC(rt0_go)
  	sigpanicPC = funcPC(sigpanic)
  	runfinqPC = funcPC(runfinq)
  	bgsweepPC = funcPC(bgsweep)
  	forcegchelperPC = funcPC(forcegchelper)
  	timerprocPC = funcPC(timerproc)
  	gcBgMarkWorkerPC = funcPC(gcBgMarkWorker)
  	systemstack_switchPC = funcPC(systemstack_switch)
  	systemstackPC = funcPC(systemstack)
  	cgocallback_gofuncPC = funcPC(cgocallback_gofunc)
  	skipPC = funcPC(skipPleaseUseCallersFrames)
  
  	// used by sigprof handler
  	gogoPC = funcPC(gogo)
  }
  
  // Traceback over the deferred function calls.
  // Report them like calls that have been invoked but not started executing yet.
  func tracebackdefers(gp *g, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer) {
  	var frame stkframe
  	for d := gp._defer; d != nil; d = d.link {
  		fn := d.fn
  		if fn == nil {
  			// Defer of nil function. Args don't matter.
  			frame.pc = 0
  			frame.fn = funcInfo{}
  			frame.argp = 0
  			frame.arglen = 0
  			frame.argmap = nil
  		} else {
  			frame.pc = fn.fn
  			f := findfunc(frame.pc)
  			if !f.valid() {
  				print("runtime: unknown pc in defer ", hex(frame.pc), "\n")
  				throw("unknown pc")
  			}
  			frame.fn = f
  			frame.argp = uintptr(deferArgs(d))
  			frame.arglen, frame.argmap = getArgInfo(&frame, f, true, fn)
  		}
  		frame.continpc = frame.pc
  		if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
  			return
  		}
  	}
  }
  
  const sizeofSkipFunction = 256
  
  // This function is defined in asm.s to be sizeofSkipFunction bytes long.
  func skipPleaseUseCallersFrames()
  
  // Generic traceback. Handles runtime stack prints (pcbuf == nil),
  // the runtime.Callers function (pcbuf != nil), as well as the garbage
  // collector (callback != nil).  A little clunky to merge these, but avoids
  // duplicating the code and all its subtlety.
  //
  // The skip argument is only valid with pcbuf != nil and counts the number
  // of logical frames to skip rather than physical frames (with inlining, a
  // PC in pcbuf can represent multiple calls). If a PC is partially skipped
  // and max > 1, pcbuf[1] will be runtime.skipPleaseUseCallersFrames+N where
  // N indicates the number of logical frames to skip in pcbuf[0].
  func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max int, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer, flags uint) int {
  	if skip > 0 && callback != nil {
  		throw("gentraceback callback cannot be used with non-zero skip")
  	}
  	if goexitPC == 0 {
  		throw("gentraceback before goexitPC initialization")
  	}
  	g := getg()
  	if g == gp && g == g.m.curg {
  		// The starting sp has been passed in as a uintptr, and the caller may
  		// have other uintptr-typed stack references as well.
  		// If during one of the calls that got us here or during one of the
  		// callbacks below the stack must be grown, all these uintptr references
  		// to the stack will not be updated, and gentraceback will continue
  		// to inspect the old stack memory, which may no longer be valid.
  		// Even if all the variables were updated correctly, it is not clear that
  		// we want to expose a traceback that begins on one stack and ends
  		// on another stack. That could confuse callers quite a bit.
  		// Instead, we require that gentraceback and any other function that
  		// accepts an sp for the current goroutine (typically obtained by
  		// calling getcallersp) must not run on that goroutine's stack but
  		// instead on the g0 stack.
  		throw("gentraceback cannot trace user goroutine on its own stack")
  	}
  	level, _, _ := gotraceback()
  
  	if pc0 == ^uintptr(0) && sp0 == ^uintptr(0) { // Signal to fetch saved values from gp.
  		if gp.syscallsp != 0 {
  			pc0 = gp.syscallpc
  			sp0 = gp.syscallsp
  			if usesLR {
  				lr0 = 0
  			}
  		} else {
  			pc0 = gp.sched.pc
  			sp0 = gp.sched.sp
  			if usesLR {
  				lr0 = gp.sched.lr
  			}
  		}
  	}
  
  	nprint := 0
  	var frame stkframe
  	frame.pc = pc0
  	frame.sp = sp0
  	if usesLR {
  		frame.lr = lr0
  	}
  	waspanic := false
  	cgoCtxt := gp.cgoCtxt
  	printing := pcbuf == nil && callback == nil
  	_defer := gp._defer
  
  	for _defer != nil && _defer.sp == _NoArgs {
  		_defer = _defer.link
  	}
  
  	// If the PC is zero, it's likely a nil function call.
  	// Start in the caller's frame.
  	if frame.pc == 0 {
  		if usesLR {
  			frame.pc = *(*uintptr)(unsafe.Pointer(frame.sp))
  			frame.lr = 0
  		} else {
  			frame.pc = uintptr(*(*sys.Uintreg)(unsafe.Pointer(frame.sp)))
  			frame.sp += sys.RegSize
  		}
  	}
  
  	f := findfunc(frame.pc)
  	if !f.valid() {
  		if callback != nil {
  			print("runtime: unknown pc ", hex(frame.pc), "\n")
  			throw("unknown pc")
  		}
  		return 0
  	}
  	frame.fn = f
  
  	var cache pcvalueCache
  
  	n := 0
  	for n < max {
  		// Typically:
  		//	pc is the PC of the running function.
  		//	sp is the stack pointer at that program counter.
  		//	fp is the frame pointer (caller's stack pointer) at that program counter, or nil if unknown.
  		//	stk is the stack containing sp.
  		//	The caller's program counter is lr, unless lr is zero, in which case it is *(uintptr*)sp.
  		f = frame.fn
  		if f.pcsp == 0 {
  			// No frame information, must be external function, like race support.
  			// See golang.org/issue/13568.
  			break
  		}
  
  		// Found an actual function.
  		// Derive frame pointer and link register.
  		if frame.fp == 0 {
  			// We want to jump over the systemstack switch. If we're running on the
  			// g0, this systemstack is at the top of the stack.
  			// if we're not on g0 or there's a no curg, then this is a regular call.
  			sp := frame.sp
  			if flags&_TraceJumpStack != 0 && f.entry == systemstackPC && gp == g.m.g0 && gp.m.curg != nil {
  				sp = gp.m.curg.sched.sp
  				frame.sp = sp
  				cgoCtxt = gp.m.curg.cgoCtxt
  			}
  			frame.fp = sp + uintptr(funcspdelta(f, frame.pc, &cache))
  			if !usesLR {
  				// On x86, call instruction pushes return PC before entering new function.
  				frame.fp += sys.RegSize
  			}
  		}
  		var flr funcInfo
  		if topofstack(f) {
  			frame.lr = 0
  			flr = funcInfo{}
  		} else if usesLR && f.entry == jmpdeferPC {
  			// jmpdefer modifies SP/LR/PC non-atomically.
  			// If a profiling interrupt arrives during jmpdefer,
  			// the stack unwind may see a mismatched register set
  			// and get confused. Stop if we see PC within jmpdefer
  			// to avoid that confusion.
  			// See golang.org/issue/8153.
  			if callback != nil {
  				throw("traceback_arm: found jmpdefer when tracing with callback")
  			}
  			frame.lr = 0
  		} else {
  			var lrPtr uintptr
  			if usesLR {
  				if n == 0 && frame.sp < frame.fp || frame.lr == 0 {
  					lrPtr = frame.sp
  					frame.lr = *(*uintptr)(unsafe.Pointer(lrPtr))
  				}
  			} else {
  				if frame.lr == 0 {
  					lrPtr = frame.fp - sys.RegSize
  					frame.lr = uintptr(*(*sys.Uintreg)(unsafe.Pointer(lrPtr)))
  				}
  			}
  			flr = findfunc(frame.lr)
  			if !flr.valid() {
  				// This happens if you get a profiling interrupt at just the wrong time.
  				// In that context it is okay to stop early.
  				// But if callback is set, we're doing a garbage collection and must
  				// get everything, so crash loudly.
  				if callback != nil {
  					print("runtime: unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
  					throw("unknown caller pc")
  				}
  			}
  		}
  
  		frame.varp = frame.fp
  		if !usesLR {
  			// On x86, call instruction pushes return PC before entering new function.
  			frame.varp -= sys.RegSize
  		}
  
  		// If framepointer_enabled and there's a frame, then
  		// there's a saved bp here.
  		if framepointer_enabled && GOARCH == "amd64" && frame.varp > frame.sp {
  			frame.varp -= sys.RegSize
  		}
  
  		// Derive size of arguments.
  		// Most functions have a fixed-size argument block,
  		// so we can use metadata about the function f.
  		// Not all, though: there are some variadic functions
  		// in package runtime and reflect, and for those we use call-specific
  		// metadata recorded by f's caller.
  		if callback != nil || printing {
  			frame.argp = frame.fp + sys.MinFrameSize
  			frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil, nil)
  		}
  
  		// Determine frame's 'continuation PC', where it can continue.
  		// Normally this is the return address on the stack, but if sigpanic
  		// is immediately below this function on the stack, then the frame
  		// stopped executing due to a trap, and frame.pc is probably not
  		// a safe point for looking up liveness information. In this panicking case,
  		// the function either doesn't return at all (if it has no defers or if the
  		// defers do not recover) or it returns from one of the calls to
  		// deferproc a second time (if the corresponding deferred func recovers).
  		// It suffices to assume that the most recent deferproc is the one that
  		// returns; everything live at earlier deferprocs is still live at that one.
  		frame.continpc = frame.pc
  		if waspanic {
  			if _defer != nil && _defer.sp == frame.sp {
  				frame.continpc = _defer.pc
  			} else {
  				frame.continpc = 0
  			}
  		}
  
  		// Unwind our local defer stack past this frame.
  		for _defer != nil && (_defer.sp == frame.sp || _defer.sp == _NoArgs) {
  			_defer = _defer.link
  		}
  
  		if callback != nil {
  			if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
  				return n
  			}
  		}
  
  		if pcbuf != nil {
  			if skip == 0 {
  				(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = frame.pc
  			} else {
  				// backup to CALL instruction to read inlining info (same logic as below)
  				tracepc := frame.pc
  				if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
  					tracepc--
  				}
  				inldata := funcdata(f, _FUNCDATA_InlTree)
  
  				// no inlining info, skip the physical frame
  				if inldata == nil {
  					skip--
  					goto skipped
  				}
  
  				ix := pcdatavalue(f, _PCDATA_InlTreeIndex, tracepc, &cache)
  				inltree := (*[1 << 20]inlinedCall)(inldata)
  				// skip the logical (inlined) frames
  				logicalSkipped := 0
  				for ix >= 0 && skip > 0 {
  					skip--
  					logicalSkipped++
  					ix = inltree[ix].parent
  				}
  
  				// skip the physical frame if there's more to skip
  				if skip > 0 {
  					skip--
  					goto skipped
  				}
  
  				// now we have a partially skipped frame
  				(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = frame.pc
  
  				// if there's room, pcbuf[1] is a skip PC that encodes the number of skipped frames in pcbuf[0]
  				if n+1 < max {
  					n++
  					skipPC := funcPC(skipPleaseUseCallersFrames) + uintptr(logicalSkipped)
  					(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = skipPC
  				}
  			}
  		}
  
  		if printing {
  			// assume skip=0 for printing
  			if (flags&_TraceRuntimeFrames) != 0 || showframe(f, gp, nprint == 0) {
  				// Print during crash.
  				//	main(0x1, 0x2, 0x3)
  				//		/home/rsc/go/src/runtime/x.go:23 +0xf
  				//
  				tracepc := frame.pc // back up to CALL instruction for funcline.
  				if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
  					tracepc--
  				}
  				file, line := funcline(f, tracepc)
  				inldata := funcdata(f, _FUNCDATA_InlTree)
  				if inldata != nil {
  					inltree := (*[1 << 20]inlinedCall)(inldata)
  					ix := pcdatavalue(f, _PCDATA_InlTreeIndex, tracepc, nil)
  					for ix != -1 {
  						name := funcnameFromNameoff(f, inltree[ix].func_)
  						print(name, "(...)\n")
  						print("\t", file, ":", line, "\n")
  
  						file = funcfile(f, inltree[ix].file)
  						line = inltree[ix].line
  						ix = inltree[ix].parent
  					}
  				}
  				name := funcname(f)
  				if name == "runtime.gopanic" {
  					name = "panic"
  				}
  				print(name, "(")
  				argp := (*[100]uintptr)(unsafe.Pointer(frame.argp))
  				for i := uintptr(0); i < frame.arglen/sys.PtrSize; i++ {
  					if i >= 10 {
  						print(", ...")
  						break
  					}
  					if i != 0 {
  						print(", ")
  					}
  					print(hex(argp[i]))
  				}
  				print(")\n")
  				print("\t", file, ":", line)
  				if frame.pc > f.entry {
  					print(" +", hex(frame.pc-f.entry))
  				}
  				if g.m.throwing > 0 && gp == g.m.curg || level >= 2 {
  					print(" fp=", hex(frame.fp), " sp=", hex(frame.sp), " pc=", hex(frame.pc))
  				}
  				print("\n")
  				nprint++
  			}
  		}
  		n++
  
  	skipped:
  		if f.entry == cgocallback_gofuncPC && len(cgoCtxt) > 0 {
  			ctxt := cgoCtxt[len(cgoCtxt)-1]
  			cgoCtxt = cgoCtxt[:len(cgoCtxt)-1]
  
  			// skip only applies to Go frames.
  			// callback != nil only used when we only care
  			// about Go frames.
  			if skip == 0 && callback == nil {
  				n = tracebackCgoContext(pcbuf, printing, ctxt, n, max)
  			}
  		}
  
  		waspanic = f.entry == sigpanicPC
  
  		// Do not unwind past the bottom of the stack.
  		if !flr.valid() {
  			break
  		}
  
  		// Unwind to next frame.
  		frame.fn = flr
  		frame.pc = frame.lr
  		frame.lr = 0
  		frame.sp = frame.fp
  		frame.fp = 0
  		frame.argmap = nil
  
  		// On link register architectures, sighandler saves the LR on stack
  		// before faking a call to sigpanic.
  		if usesLR && waspanic {
  			x := *(*uintptr)(unsafe.Pointer(frame.sp))
  			frame.sp += sys.MinFrameSize
  			if GOARCH == "arm64" {
  				// arm64 needs 16-byte aligned SP, always
  				frame.sp += sys.PtrSize
  			}
  			f = findfunc(frame.pc)
  			frame.fn = f
  			if !f.valid() {
  				frame.pc = x
  			} else if funcspdelta(f, frame.pc, &cache) == 0 {
  				frame.lr = x
  			}
  		}
  	}
  
  	if printing {
  		n = nprint
  	}
  
  	// If callback != nil, we're being called to gather stack information during
  	// garbage collection or stack growth. In that context, require that we used
  	// up the entire defer stack. If not, then there is a bug somewhere and the
  	// garbage collection or stack growth may not have seen the correct picture
  	// of the stack. Crash now instead of silently executing the garbage collection
  	// or stack copy incorrectly and setting up for a mysterious crash later.
  	//
  	// Note that panic != nil is okay here: there can be leftover panics,
  	// because the defers on the panic stack do not nest in frame order as
  	// they do on the defer stack. If you have:
  	//
  	//	frame 1 defers d1
  	//	frame 2 defers d2
  	//	frame 3 defers d3
  	//	frame 4 panics
  	//	frame 4's panic starts running defers
  	//	frame 5, running d3, defers d4
  	//	frame 5 panics
  	//	frame 5's panic starts running defers
  	//	frame 6, running d4, garbage collects
  	//	frame 6, running d2, garbage collects
  	//
  	// During the execution of d4, the panic stack is d4 -> d3, which
  	// is nested properly, and we'll treat frame 3 as resumable, because we
  	// can find d3. (And in fact frame 3 is resumable. If d4 recovers
  	// and frame 5 continues running, d3, d3 can recover and we'll
  	// resume execution in (returning from) frame 3.)
  	//
  	// During the execution of d2, however, the panic stack is d2 -> d3,
  	// which is inverted. The scan will match d2 to frame 2 but having
  	// d2 on the stack until then means it will not match d3 to frame 3.
  	// This is okay: if we're running d2, then all the defers after d2 have
  	// completed and their corresponding frames are dead. Not finding d3
  	// for frame 3 means we'll set frame 3's continpc == 0, which is correct
  	// (frame 3 is dead). At the end of the walk the panic stack can thus
  	// contain defers (d3 in this case) for dead frames. The inversion here
  	// always indicates a dead frame, and the effect of the inversion on the
  	// scan is to hide those dead frames, so the scan is still okay:
  	// what's left on the panic stack are exactly (and only) the dead frames.
  	//
  	// We require callback != nil here because only when callback != nil
  	// do we know that gentraceback is being called in a "must be correct"
  	// context as opposed to a "best effort" context. The tracebacks with
  	// callbacks only happen when everything is stopped nicely.
  	// At other times, such as when gathering a stack for a profiling signal
  	// or when printing a traceback during a crash, everything may not be
  	// stopped nicely, and the stack walk may not be able to complete.
  	// It's okay in those situations not to use up the entire defer stack:
  	// incomplete information then is still better than nothing.
  	if callback != nil && n < max && _defer != nil {
  		if _defer != nil {
  			print("runtime: g", gp.goid, ": leftover defer sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
  		}
  		for _defer = gp._defer; _defer != nil; _defer = _defer.link {
  			print("\tdefer ", _defer, " sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
  		}
  		throw("traceback has leftover defers")
  	}
  
  	if callback != nil && n < max && frame.sp != gp.stktopsp {
  		print("runtime: g", gp.goid, ": frame.sp=", hex(frame.sp), " top=", hex(gp.stktopsp), "\n")
  		print("\tstack=[", hex(gp.stack.lo), "-", hex(gp.stack.hi), "] n=", n, " max=", max, "\n")
  		throw("traceback did not unwind completely")
  	}
  
  	return n
  }
  
  // reflectMethodValue is a partial duplicate of reflect.makeFuncImpl
  // and reflect.methodValue.
  type reflectMethodValue struct {
  	fn    uintptr
  	stack *bitvector // args bitmap
  }
  
  // getArgInfo returns the argument frame information for a call to f
  // with call frame frame.
  //
  // This is used for both actual calls with active stack frames and for
  // deferred calls that are not yet executing. If this is an actual
  // call, ctxt must be nil (getArgInfo will retrieve what it needs from
  // the active stack frame). If this is a deferred call, ctxt must be
  // the function object that was deferred.
  func getArgInfo(frame *stkframe, f funcInfo, needArgMap bool, ctxt *funcval) (arglen uintptr, argmap *bitvector) {
  	arglen = uintptr(f.args)
  	if needArgMap && f.args == _ArgsSizeUnknown {
  		// Extract argument bitmaps for reflect stubs from the calls they made to reflect.
  		switch funcname(f) {
  		case "reflect.makeFuncStub", "reflect.methodValueCall":
  			// These take a *reflect.methodValue as their
  			// context register.
  			var mv *reflectMethodValue
  			if ctxt != nil {
  				// This is not an actual call, but a
  				// deferred call. The function value
  				// is itself the *reflect.methodValue.
  				mv = (*reflectMethodValue)(unsafe.Pointer(ctxt))
  			} else {
  				// This is a real call that took the
  				// *reflect.methodValue as its context
  				// register and immediately saved it
  				// to 0(SP). Get the methodValue from
  				// 0(SP).
  				arg0 := frame.sp + sys.MinFrameSize
  				mv = *(**reflectMethodValue)(unsafe.Pointer(arg0))
  			}
  			if mv.fn != f.entry {
  				print("runtime: confused by ", funcname(f), "\n")
  				throw("reflect mismatch")
  			}
  			bv := mv.stack
  			arglen = uintptr(bv.n * sys.PtrSize)
  			argmap = bv
  		}
  	}
  	return
  }
  
  // tracebackCgoContext handles tracing back a cgo context value, from
  // the context argument to setCgoTraceback, for the gentraceback
  // function. It returns the new value of n.
  func tracebackCgoContext(pcbuf *uintptr, printing bool, ctxt uintptr, n, max int) int {
  	var cgoPCs [32]uintptr
  	cgoContextPCs(ctxt, cgoPCs[:])
  	var arg cgoSymbolizerArg
  	anySymbolized := false
  	for _, pc := range cgoPCs {
  		if pc == 0 || n >= max {
  			break
  		}
  		if pcbuf != nil {
  			(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
  		}
  		if printing {
  			if cgoSymbolizer == nil {
  				print("non-Go function at pc=", hex(pc), "\n")
  			} else {
  				c := printOneCgoTraceback(pc, max-n, &arg)
  				n += c - 1 // +1 a few lines down
  				anySymbolized = true
  			}
  		}
  		n++
  	}
  	if anySymbolized {
  		arg.pc = 0
  		callCgoSymbolizer(&arg)
  	}
  	return n
  }
  
  func printcreatedby(gp *g) {
  	// Show what created goroutine, except main goroutine (goid 1).
  	pc := gp.gopc
  	f := findfunc(pc)
  	if f.valid() && showframe(f, gp, false) && gp.goid != 1 {
  		print("created by ", funcname(f), "\n")
  		tracepc := pc // back up to CALL instruction for funcline.
  		if pc > f.entry {
  			tracepc -= sys.PCQuantum
  		}
  		file, line := funcline(f, tracepc)
  		print("\t", file, ":", line)
  		if pc > f.entry {
  			print(" +", hex(pc-f.entry))
  		}
  		print("\n")
  	}
  }
  
  func traceback(pc, sp, lr uintptr, gp *g) {
  	traceback1(pc, sp, lr, gp, 0)
  }
  
  // tracebacktrap is like traceback but expects that the PC and SP were obtained
  // from a trap, not from gp->sched or gp->syscallpc/gp->syscallsp or getcallerpc/getcallersp.
  // Because they are from a trap instead of from a saved pair,
  // the initial PC must not be rewound to the previous instruction.
  // (All the saved pairs record a PC that is a return address, so we
  // rewind it into the CALL instruction.)
  func tracebacktrap(pc, sp, lr uintptr, gp *g) {
  	traceback1(pc, sp, lr, gp, _TraceTrap)
  }
  
  func traceback1(pc, sp, lr uintptr, gp *g, flags uint) {
  	// If the goroutine is in cgo, and we have a cgo traceback, print that.
  	if iscgo && gp.m != nil && gp.m.ncgo > 0 && gp.syscallsp != 0 && gp.m.cgoCallers != nil && gp.m.cgoCallers[0] != 0 {
  		// Lock cgoCallers so that a signal handler won't
  		// change it, copy the array, reset it, unlock it.
  		// We are locked to the thread and are not running
  		// concurrently with a signal handler.
  		// We just have to stop a signal handler from interrupting
  		// in the middle of our copy.
  		atomic.Store(&gp.m.cgoCallersUse, 1)
  		cgoCallers := *gp.m.cgoCallers
  		gp.m.cgoCallers[0] = 0
  		atomic.Store(&gp.m.cgoCallersUse, 0)
  
  		printCgoTraceback(&cgoCallers)
  	}
  
  	var n int
  	if readgstatus(gp)&^_Gscan == _Gsyscall {
  		// Override registers if blocked in system call.
  		pc = gp.syscallpc
  		sp = gp.syscallsp
  		flags &^= _TraceTrap
  	}
  	// Print traceback. By default, omits runtime frames.
  	// If that means we print nothing at all, repeat forcing all frames printed.
  	n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags)
  	if n == 0 && (flags&_TraceRuntimeFrames) == 0 {
  		n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, flags|_TraceRuntimeFrames)
  	}
  	if n == _TracebackMaxFrames {
  		print("...additional frames elided...\n")
  	}
  	printcreatedby(gp)
  }
  
  func callers(skip int, pcbuf []uintptr) int {
  	sp := getcallersp(unsafe.Pointer(&skip))
  	pc := getcallerpc(unsafe.Pointer(&skip))
  	gp := getg()
  	var n int
  	systemstack(func() {
  		n = gentraceback(pc, sp, 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
  	})
  	return n
  }
  
  func gcallers(gp *g, skip int, pcbuf []uintptr) int {
  	return gentraceback(^uintptr(0), ^uintptr(0), 0, gp, skip, &pcbuf[0], len(pcbuf), nil, nil, 0)
  }
  
  func showframe(f funcInfo, gp *g, firstFrame bool) bool {
  	g := getg()
  	if g.m.throwing > 0 && gp != nil && (gp == g.m.curg || gp == g.m.caughtsig.ptr()) {
  		return true
  	}
  	level, _, _ := gotraceback()
  	name := funcname(f)
  
  	// Special case: always show runtime.gopanic frame
  	// in the middle of a stack trace, so that we can
  	// see the boundary between ordinary code and
  	// panic-induced deferred code.
  	// See golang.org/issue/5832.
  	if name == "runtime.gopanic" && !firstFrame {
  		return true
  	}
  
  	return level > 1 || f.valid() && contains(name, ".") && (!hasprefix(name, "runtime.") || isExportedRuntime(name))
  }
  
  // isExportedRuntime reports whether name is an exported runtime function.
  // It is only for runtime functions, so ASCII A-Z is fine.
  func isExportedRuntime(name string) bool {
  	const n = len("runtime.")
  	return len(name) > n && name[:n] == "runtime." && 'A' <= name[n] && name[n] <= 'Z'
  }
  
  var gStatusStrings = [...]string{
  	_Gidle:      "idle",
  	_Grunnable:  "runnable",
  	_Grunning:   "running",
  	_Gsyscall:   "syscall",
  	_Gwaiting:   "waiting",
  	_Gdead:      "dead",
  	_Gcopystack: "copystack",
  }
  
  func goroutineheader(gp *g) {
  	gpstatus := readgstatus(gp)
  
  	isScan := gpstatus&_Gscan != 0
  	gpstatus &^= _Gscan // drop the scan bit
  
  	// Basic string status
  	var status string
  	if 0 <= gpstatus && gpstatus < uint32(len(gStatusStrings)) {
  		status = gStatusStrings[gpstatus]
  	} else {
  		status = "???"
  	}
  
  	// Override.
  	if gpstatus == _Gwaiting && gp.waitreason != "" {
  		status = gp.waitreason
  	}
  
  	// approx time the G is blocked, in minutes
  	var waitfor int64
  	if (gpstatus == _Gwaiting || gpstatus == _Gsyscall) && gp.waitsince != 0 {
  		waitfor = (nanotime() - gp.waitsince) / 60e9
  	}
  	print("goroutine ", gp.goid, " [", status)
  	if isScan {
  		print(" (scan)")
  	}
  	if waitfor >= 1 {
  		print(", ", waitfor, " minutes")
  	}
  	if gp.lockedm != nil {
  		print(", locked to thread")
  	}
  	print("]:\n")
  }
  
  func tracebackothers(me *g) {
  	level, _, _ := gotraceback()
  
  	// Show the current goroutine first, if we haven't already.
  	g := getg()
  	gp := g.m.curg
  	if gp != nil && gp != me {
  		print("\n")
  		goroutineheader(gp)
  		traceback(^uintptr(0), ^uintptr(0), 0, gp)
  	}
  
  	lock(&allglock)
  	for _, gp := range allgs {
  		if gp == me || gp == g.m.curg || readgstatus(gp) == _Gdead || isSystemGoroutine(gp) && level < 2 {
  			continue
  		}
  		print("\n")
  		goroutineheader(gp)
  		// Note: gp.m == g.m occurs when tracebackothers is
  		// called from a signal handler initiated during a
  		// systemstack call. The original G is still in the
  		// running state, and we want to print its stack.
  		if gp.m != g.m && readgstatus(gp)&^_Gscan == _Grunning {
  			print("\tgoroutine running on other thread; stack unavailable\n")
  			printcreatedby(gp)
  		} else {
  			traceback(^uintptr(0), ^uintptr(0), 0, gp)
  		}
  	}
  	unlock(&allglock)
  }
  
  // Does f mark the top of a goroutine stack?
  func topofstack(f funcInfo) bool {
  	pc := f.entry
  	return pc == goexitPC ||
  		pc == mstartPC ||
  		pc == mcallPC ||
  		pc == morestackPC ||
  		pc == rt0_goPC ||
  		externalthreadhandlerp != 0 && pc == externalthreadhandlerp
  }
  
  // isSystemGoroutine reports whether the goroutine g must be omitted in
  // stack dumps and deadlock detector.
  func isSystemGoroutine(gp *g) bool {
  	pc := gp.startpc
  	return pc == runfinqPC && !fingRunning ||
  		pc == bgsweepPC ||
  		pc == forcegchelperPC ||
  		pc == timerprocPC ||
  		pc == gcBgMarkWorkerPC
  }
  
  // SetCgoTraceback records three C functions to use to gather
  // traceback information from C code and to convert that traceback
  // information into symbolic information. These are used when printing
  // stack traces for a program that uses cgo.
  //
  // The traceback and context functions may be called from a signal
  // handler, and must therefore use only async-signal safe functions.
  // The symbolizer function may be called while the program is
  // crashing, and so must be cautious about using memory.  None of the
  // functions may call back into Go.
  //
  // The context function will be called with a single argument, a
  // pointer to a struct:
  //
  //	struct {
  //		Context uintptr
  //	}
  //
  // In C syntax, this struct will be
  //
  //	struct {
  //		uintptr_t Context;
  //	};
  //
  // If the Context field is 0, the context function is being called to
  // record the current traceback context. It should record in the
  // Context field whatever information is needed about the current
  // point of execution to later produce a stack trace, probably the
  // stack pointer and PC. In this case the context function will be
  // called from C code.
  //
  // If the Context field is not 0, then it is a value returned by a
  // previous call to the context function. This case is called when the
  // context is no longer needed; that is, when the Go code is returning
  // to its C code caller. This permits the context function to release
  // any associated resources.
  //
  // While it would be correct for the context function to record a
  // complete a stack trace whenever it is called, and simply copy that
  // out in the traceback function, in a typical program the context
  // function will be called many times without ever recording a
  // traceback for that context. Recording a complete stack trace in a
  // call to the context function is likely to be inefficient.
  //
  // The traceback function will be called with a single argument, a
  // pointer to a struct:
  //
  //	struct {
  //		Context    uintptr
  //		SigContext uintptr
  //		Buf        *uintptr
  //		Max        uintptr
  //	}
  //
  // In C syntax, this struct will be
  //
  //	struct {
  //		uintptr_t  Context;
  //		uintptr_t  SigContext;
  //		uintptr_t* Buf;
  //		uintptr_t  Max;
  //	};
  //
  // The Context field will be zero to gather a traceback from the
  // current program execution point. In this case, the traceback
  // function will be called from C code.
  //
  // Otherwise Context will be a value previously returned by a call to
  // the context function. The traceback function should gather a stack
  // trace from that saved point in the program execution. The traceback
  // function may be called from an execution thread other than the one
  // that recorded the context, but only when the context is known to be
  // valid and unchanging. The traceback function may also be called
  // deeper in the call stack on the same thread that recorded the
  // context. The traceback function may be called multiple times with
  // the same Context value; it will usually be appropriate to cache the
  // result, if possible, the first time this is called for a specific
  // context value.
  //
  // If the traceback function is called from a signal handler on a Unix
  // system, SigContext will be the signal context argument passed to
  // the signal handler (a C ucontext_t* cast to uintptr_t). This may be
  // used to start tracing at the point where the signal occurred. If
  // the traceback function is not called from a signal handler,
  // SigContext will be zero.
  //
  // Buf is where the traceback information should be stored. It should
  // be PC values, such that Buf[0] is the PC of the caller, Buf[1] is
  // the PC of that function's caller, and so on.  Max is the maximum
  // number of entries to store.  The function should store a zero to
  // indicate the top of the stack, or that the caller is on a different
  // stack, presumably a Go stack.
  //
  // Unlike runtime.Callers, the PC values returned should, when passed
  // to the symbolizer function, return the file/line of the call
  // instruction.  No additional subtraction is required or appropriate.
  //
  // The symbolizer function will be called with a single argument, a
  // pointer to a struct:
  //
  //	struct {
  //		PC      uintptr // program counter to fetch information for
  //		File    *byte   // file name (NUL terminated)
  //		Lineno  uintptr // line number
  //		Func    *byte   // function name (NUL terminated)
  //		Entry   uintptr // function entry point
  //		More    uintptr // set non-zero if more info for this PC
  //		Data    uintptr // unused by runtime, available for function
  //	}
  //
  // In C syntax, this struct will be
  //
  //	struct {
  //		uintptr_t PC;
  //		char*     File;
  //		uintptr_t Lineno;
  //		char*     Func;
  //		uintptr_t Entry;
  //		uintptr_t More;
  //		uintptr_t Data;
  //	};
  //
  // The PC field will be a value returned by a call to the traceback
  // function.
  //
  // The first time the function is called for a particular traceback,
  // all the fields except PC will be 0. The function should fill in the
  // other fields if possible, setting them to 0/nil if the information
  // is not available. The Data field may be used to store any useful
  // information across calls. The More field should be set to non-zero
  // if there is more information for this PC, zero otherwise. If More
  // is set non-zero, the function will be called again with the same
  // PC, and may return different information (this is intended for use
  // with inlined functions). If More is zero, the function will be
  // called with the next PC value in the traceback. When the traceback
  // is complete, the function will be called once more with PC set to
  // zero; this may be used to free any information. Each call will
  // leave the fields of the struct set to the same values they had upon
  // return, except for the PC field when the More field is zero. The
  // function must not keep a copy of the struct pointer between calls.
  //
  // When calling SetCgoTraceback, the version argument is the version
  // number of the structs that the functions expect to receive.
  // Currently this must be zero.
  //
  // The symbolizer function may be nil, in which case the results of
  // the traceback function will be displayed as numbers. If the
  // traceback function is nil, the symbolizer function will never be
  // called. The context function may be nil, in which case the
  // traceback function will only be called with the context field set
  // to zero.  If the context function is nil, then calls from Go to C
  // to Go will not show a traceback for the C portion of the call stack.
  //
  // SetCgoTraceback should be called only once, ideally from an init function.
  func SetCgoTraceback(version int, traceback, context, symbolizer unsafe.Pointer) {
  	if version != 0 {
  		panic("unsupported version")
  	}
  
  	if cgoTraceback != nil && cgoTraceback != traceback ||
  		cgoContext != nil && cgoContext != context ||
  		cgoSymbolizer != nil && cgoSymbolizer != symbolizer {
  		panic("call SetCgoTraceback only once")
  	}
  
  	cgoTraceback = traceback
  	cgoContext = context
  	cgoSymbolizer = symbolizer
  
  	// The context function is called when a C function calls a Go
  	// function. As such it is only called by C code in runtime/cgo.
  	if _cgo_set_context_function != nil {
  		cgocall(_cgo_set_context_function, context)
  	}
  }
  
  var cgoTraceback unsafe.Pointer
  var cgoContext unsafe.Pointer
  var cgoSymbolizer unsafe.Pointer
  
  // cgoTracebackArg is the type passed to cgoTraceback.
  type cgoTracebackArg struct {
  	context    uintptr
  	sigContext uintptr
  	buf        *uintptr
  	max        uintptr
  }
  
  // cgoContextArg is the type passed to the context function.
  type cgoContextArg struct {
  	context uintptr
  }
  
  // cgoSymbolizerArg is the type passed to cgoSymbolizer.
  type cgoSymbolizerArg struct {
  	pc       uintptr
  	file     *byte
  	lineno   uintptr
  	funcName *byte
  	entry    uintptr
  	more     uintptr
  	data     uintptr
  }
  
  // cgoTraceback prints a traceback of callers.
  func printCgoTraceback(callers *cgoCallers) {
  	if cgoSymbolizer == nil {
  		for _, c := range callers {
  			if c == 0 {
  				break
  			}
  			print("non-Go function at pc=", hex(c), "\n")
  		}
  		return
  	}
  
  	var arg cgoSymbolizerArg
  	for _, c := range callers {
  		if c == 0 {
  			break
  		}
  		printOneCgoTraceback(c, 0x7fffffff, &arg)
  	}
  	arg.pc = 0
  	callCgoSymbolizer(&arg)
  }
  
  // printOneCgoTraceback prints the traceback of a single cgo caller.
  // This can print more than one line because of inlining.
  // Returns the number of frames printed.
  func printOneCgoTraceback(pc uintptr, max int, arg *cgoSymbolizerArg) int {
  	c := 0
  	arg.pc = pc
  	for {
  		if c > max {
  			break
  		}
  		callCgoSymbolizer(arg)
  		if arg.funcName != nil {
  			// Note that we don't print any argument
  			// information here, not even parentheses.
  			// The symbolizer must add that if appropriate.
  			println(gostringnocopy(arg.funcName))
  		} else {
  			println("non-Go function")
  		}
  		print("\t")
  		if arg.file != nil {
  			print(gostringnocopy(arg.file), ":", arg.lineno, " ")
  		}
  		print("pc=", hex(pc), "\n")
  		c++
  		if arg.more == 0 {
  			break
  		}
  	}
  	return c
  }
  
  // callCgoSymbolizer calls the cgoSymbolizer function.
  func callCgoSymbolizer(arg *cgoSymbolizerArg) {
  	call := cgocall
  	if panicking > 0 || getg().m.curg != getg() {
  		// We do not want to call into the scheduler when panicking
  		// or when on the system stack.
  		call = asmcgocall
  	}
  	if msanenabled {
  		msanwrite(unsafe.Pointer(arg), unsafe.Sizeof(cgoSymbolizerArg{}))
  	}
  	call(cgoSymbolizer, noescape(unsafe.Pointer(arg)))
  }
  
  // cgoContextPCs gets the PC values from a cgo traceback.
  func cgoContextPCs(ctxt uintptr, buf []uintptr) {
  	if cgoTraceback == nil {
  		return
  	}
  	call := cgocall
  	if panicking > 0 || getg().m.curg != getg() {
  		// We do not want to call into the scheduler when panicking
  		// or when on the system stack.
  		call = asmcgocall
  	}
  	arg := cgoTracebackArg{
  		context: ctxt,
  		buf:     (*uintptr)(noescape(unsafe.Pointer(&buf[0]))),
  		max:     uintptr(len(buf)),
  	}
  	if msanenabled {
  		msanwrite(unsafe.Pointer(&arg), unsafe.Sizeof(arg))
  	}
  	call(cgoTraceback, noescape(unsafe.Pointer(&arg)))
  }
  

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