From 3abb10611804f0550192a9f2d50fba64925c8473 Mon Sep 17 00:00:00 2001
From: Josh Bleecher Snyder <josh@tailscale.com>
Date: Tue, 9 Feb 2021 11:24:32 -0800
Subject: [PATCH] WIP

---
 block/block.go      | 112 ++++++++++++++++++++++++++++++++++++++++++++
 block/block_test.go |  32 +++++++++++++
 2 files changed, 144 insertions(+)
 create mode 100644 block/block.go
 create mode 100644 block/block_test.go

diff --git a/block/block.go b/block/block.go
new file mode 100644
index 000000000..4b610579d
--- /dev/null
+++ b/block/block.go
@@ -0,0 +1,112 @@
+// Copyright (c) 2021 Tailscale Inc & 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 block TODO TODO TODO.
+package block
+
+// Next steps:
+//  * refactor out chunk selection, with tests.
+//  * support AllowNextLine
+//  * add docs
+
+import (
+	"bytes"
+	"runtime"
+	"strconv"
+	"time"
+
+	"tailscale.com/types/logger"
+)
+
+func Watch(maxMinutes int, logf logger.Logf) {
+	buf := make([]byte, 4096)
+	for {
+		time.Sleep(time.Duration(maxMinutes) * time.Minute)
+
+		// Read all goroutine stacks.
+		// It'd be nicer to use pprof.Lookup("goroutine"),
+		// but it doesn't have the per-goroutine header that includes
+		// how long that goroutine has been blocked.
+		for {
+			n := runtime.Stack(buf, true)
+			if n < len(buf) {
+				buf = buf[:n]
+				break
+			}
+			buf = buf[:cap(buf)]
+			buf = append(buf, 0)
+		}
+
+		// Parse the goroutine stacks, looking for goroutines that have been blocked for a long time.
+		// This is best-effort; the formatting that the runtime uses can change.
+		// See runtime.goroutineheader for the code that writes the header.
+
+		// Stacks come in goroutine chunks separated by blank lines.
+		chunks := bytes.Split(buf, doubleNewline)
+
+		// Check each goroutine to see whether it is over the time limit.
+		for _, chunk := range chunks {
+			minutes, ok := goroutineMinutesBlocked(chunk)
+			if !ok {
+				continue
+			}
+			if minutes > maxMinutes {
+				// Dump all stacks.
+				logf("detected goroutines blocked > %d minutes\n%q", maxMinutes, buf)
+				break
+			}
+		}
+	}
+}
+
+func AllowNextLine() {
+
+}
+
+func AllowLine() {
+
+}
+
+var (
+	doubleNewline = []byte("\n\n")
+	goroutine     = []byte("goroutine ")
+	commaSpace    = []byte(", ")
+	spaceMinutes  = []byte(" minutes")
+)
+
+// goroutineMinutesBlocked reports the number of minutes the goroutine
+// whose stack is in buf was blocked for (and whether the parse succeeded).
+func goroutineMinutesBlocked(stack []byte) (minutes int, ok bool) {
+	// Each chunk begins like
+	//   goroutine 0 [idle]:
+	// or
+	//   goroutine 1 [chan receive, 9 minutes]:
+	// We only care about lines that have a minutes count.
+	if !bytes.HasPrefix(stack, goroutine) {
+		return 0, false
+	}
+	// Extract first line.
+	i := bytes.IndexByte(stack, '\n')
+	if i < 0 {
+		return 0, false
+	}
+	stack = stack[:i]
+	// Find the part between the comma and the m.
+	i = bytes.Index(stack, commaSpace)
+	if i < 0 {
+		return 0, false
+	}
+	stack = stack[i+len(commaSpace):]
+	i = bytes.Index(stack, spaceMinutes)
+	if i < 0 {
+		return 0, false
+	}
+	stack = stack[:i]
+	// Attempt to decode the number of minutes.
+	minutes, err := strconv.Atoi(string(stack))
+	if err != nil {
+		return 0, false
+	}
+	return minutes, true
+}
diff --git a/block/block_test.go b/block/block_test.go
new file mode 100644
index 000000000..1a214c501
--- /dev/null
+++ b/block/block_test.go
@@ -0,0 +1,32 @@
+// Copyright (c) 2021 Tailscale Inc & 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 block
+
+import (
+	"testing"
+)
+
+func TestWatch(t *testing.T) {
+	Watch(3, t.Logf)
+}
+
+func TestGoroutineMinutesBlocked(t *testing.T) {
+	tests := []struct {
+		stack   string
+		wantMin int
+		wantOK  bool
+	}{
+		{stack: "some junk"},
+		{stack: "goroutine 0 [idle]:\nstack traces..."},
+		{stack: "goroutine 1 [chan receive, 9 minutes]:\nstack traces...", wantMin: 9, wantOK: true},
+	}
+
+	for _, tt := range tests {
+		gotMin, gotOK := goroutineMinutesBlocked([]byte(tt.stack))
+		if tt.wantMin != gotMin || tt.wantOK != gotOK {
+			t.Errorf("goroutineMinutesBlocked(%q) = (%v, %v) want (%v, %v)", tt.stack, gotMin, gotOK, tt.wantMin, tt.wantOK)
+		}
+	}
+}