1 files changed, 170 insertions, 0 deletions
diff --git a/client/go/script-utils/logfmt/tail_unix.go b/client/go/script-utils/logfmt/tail_unix.go
new file mode 100644
index 00000000000..7703844da48
--- /dev/null
+++ b/client/go/script-utils/logfmt/tail_unix.go
@@ -0,0 +1,170 @@
+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+// vespa logfmt command
+// Author: arnej
+
+//go:build !windows
+
+package logfmt
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+const lastLinesSize = 4 * 1024
+
+// an active "tail -f" like object
+
+type unixTail struct {
+	lines   chan Line
+	lineBuf []byte
+	curFile *os.File
+	fn      string
+	reader  *bufio.Reader
+	curStat unix.Stat_t
+}
+
+func (t *unixTail) Lines() chan Line { return t.lines }
+
+// API for starting to follow a log file
+
+func FollowFile(fn string) (Tail, error) {
+	res := unixTail{}
+	res.fn = fn
+	res.lineBuf = make([]byte, lastLinesSize)
+	res.openTail()
+	res.lines = make(chan Line, 20)
+	res.lineBuf = res.lineBuf[:0]
+	go runTailWith(&res)
+	return &res, nil
+}
+
+func (t *unixTail) setFile(f *os.File) {
+	if t.curFile != nil {
+		t.curFile.Close()
+	}
+	t.curFile = f
+	if f != nil {
+		err := unix.Fstat(int(f.Fd()), &t.curStat)
+		if err != nil {
+			f.Close()
+			fmt.Fprintf(os.Stderr, "unexpected failure: %v\n", err)
+			return
+		}
+		t.reader = bufio.NewReaderSize(f, 1024*1024)
+	} else {
+		t.reader = nil
+	}
+}
+
+// open log file and seek to the start of a line near the end, if possible.
+func (t *unixTail) openTail() {
+	file, err := os.Open(t.fn)
+	if err != nil {
+		return
+	}
+	sz, err := file.Seek(0, os.SEEK_END)
+	if err != nil {
+		return
+	}
+	if sz < lastLinesSize {
+		sz, err = file.Seek(0, os.SEEK_SET)
+		if err == nil {
+			// just read from start of file, all OK
+			t.setFile(file)
+		}
+		return
+	}
+	sz, _ = file.Seek(-lastLinesSize, os.SEEK_END)
+	n, err := file.Read(t.lineBuf)
+	if err != nil {
+		return
+	}
+	for i := 0; i < n; i++ {
+		if t.lineBuf[i] == '\n' {
+			sz, err = file.Seek(sz+int64(i+1), os.SEEK_SET)
+			if err == nil {
+				t.setFile(file)
+			}
+			return
+		}
+	}
+}
+
+func (t *unixTail) reopen(cur *unix.Stat_t) {
+	for cnt := 0; cnt < 100; cnt++ {
+		file, err := os.Open(t.fn)
+		if err != nil {
+			t.setFile(nil)
+			if cnt == 0 {
+				fmt.Fprintf(os.Stderr, "%v (waiting for log file to appear)\n", err)
+			}
+			time.Sleep(1000 * time.Millisecond)
+			continue
+		}
+		var stat unix.Stat_t
+		err = unix.Fstat(int(file.Fd()), &stat)
+		if err != nil {
+			file.Close()
+			fmt.Fprintf(os.Stderr, "unexpected failure: %v\n", err)
+			time.Sleep(5000 * time.Millisecond)
+			continue
+		}
+		if cur != nil && cur.Dev == stat.Dev && cur.Ino == stat.Ino {
+			// same file, continue following it
+			file.Close()
+			return
+		}
+		// new file, start following it
+		t.setFile(file)
+		return
+	}
+}
+
+// runs as a goroutine
+func runTailWith(t *unixTail) {
+	defer t.setFile(nil)
+loop:
+	for {
+		for t.curFile == nil {
+			t.reopen(nil)
+		}
+		bytes, err := t.reader.ReadSlice('\n')
+		t.lineBuf = append(t.lineBuf, bytes...)
+		if err == bufio.ErrBufferFull {
+			continue
+		}
+		if err == nil {
+			ll := len(t.lineBuf) - 1
+			t.lines <- Line{Text: string(t.lineBuf[:ll])}
+			t.lineBuf = t.lineBuf[:0]
+			continue
+		}
+		if err == io.EOF {
+			pos, _ := t.curFile.Seek(0, os.SEEK_CUR)
+			for cnt := 0; cnt < 100; cnt++ {
+				time.Sleep(10 * time.Millisecond)
+				sz, _ := t.curFile.Seek(0, os.SEEK_END)
+				if sz != pos {
+					if sz < pos {
+						// truncation case
+						pos = 0
+					}
+					t.curFile.Seek(pos, os.SEEK_SET)
+					continue loop
+				}
+			}
+			// no change in file size, try reopening
+			t.reopen(&t.curStat)
+		} else {
+			fmt.Fprintf(os.Stderr, "error tailing '%s': %v\n", t.fn, err)
+			close(t.lines)
+			return
+		}
+	}
+}