Generate crash report txt and webpage

The last 100 lines of logs are now cached, and when a crash occurs, they
are saved to a file in the temp directory ("/tmp" on *nix), and pretty
HTML version is also created and opened in the browser.
* Currently only handles panics, will be included in more places soon
* Copy button and button to generate a GH issue will be added

linecache/go.mod

@@ -0,0 +1,3 @@
+module github.com/hrfee/jfa-go/linecache
+
+go 1.16

linecache/linecache.go

@@ -0,0 +1,66 @@
+// Package linecache provides a writer that stores n lines of text at once, overwriting old content as it reaches its capacity. Its contents can be read from with a String() method.
+package linecache
+
+import (
+	"strings"
+	"sync"
+)
+
+// LineCache provides an io.Writer that stores a fixed number of lines of text.
+type LineCache struct {
+	count   int
+	lines   [][]byte
+	current int
+	lock    *sync.Mutex
+}
+
+// NewLineCache returns a new line cache of capacity (n) lines.
+func NewLineCache(n int) *LineCache {
+	return &LineCache{
+		current: 0,
+		count:   n,
+		lines:   make([][]byte, n),
+		lock:    &sync.Mutex{},
+	}
+}
+
+// Write writes a given byte array to the cache.
+func (l *LineCache) Write(p []byte) (n int, err error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+	lines := strings.Split(string(p), "\n")
+	for _, line := range lines {
+		if string(line) == "" {
+			continue
+		}
+		if l.current == l.count {
+			l.current = 0
+		}
+		l.lines[l.current] = []byte(line)
+		l.current++
+	}
+	n = len(p)
+	return
+}
+
+// String returns a string representation of the cache contents.
+func (l *LineCache) String() string {
+	i := 0
+	if l.lines[l.count-1] != nil && l.current != l.count {
+		i = l.current
+	}
+	out := ""
+	for {
+		if l.lines[i] == nil {
+			return out
+		}
+		out += string(l.lines[i]) + "\n"
+		i++
+		if i == l.current {
+			return out
+		}
+		if i == l.count {
+			i = 0
+		}
+	}
+}

linecache/linecache_test.go

@@ -0,0 +1,17 @@
+package linecache
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+func Test(t *testing.T) {
+	wr := NewLineCache(10)
+	for i := 10; i < 50; i++ {
+		fmt.Fprintln(wr, i)
+		fmt.Print(strings.ReplaceAll(wr.String(), "\n", " "), "\n")
+		time.Sleep(time.Second)
+	}
+}