From 621cc0b055ce34015526b0a577ffa4b6d2c09a3c Mon Sep 17 00:00:00 2001
From: Felicitas Pojtinger <felicitas@pojtinger.com>
Date: Fri, 19 Nov 2021 00:10:13 +0100
Subject: [PATCH] feat: Make `seek` command universal

---
 cmd/stfs-seek-tape/main.go | 77 --------------------------------------
 cmd/stfs-seek/main.go      | 72 ++++++++++-------------------------
 2 files changed, 20 insertions(+), 129 deletions(-)
 delete mode 100644 cmd/stfs-seek-tape/main.go

diff --git a/cmd/stfs-seek-tape/main.go b/cmd/stfs-seek-tape/main.go
deleted file mode 100644
index 5b4658f..0000000
--- a/cmd/stfs-seek-tape/main.go
+++ /dev/null
@@ -1,77 +0,0 @@
-package main
-
-import (
-	"archive/tar"
-	"bufio"
-	"flag"
-	"log"
-	"os"
-	"syscall"
-	"unsafe"
-)
-
-// See https://github.com/benmcclelland/mtio
-const (
-	MTIOCTOP = 0x40086d01 // Do magnetic tape operation
-	MTSEEK   = 22         // Seek to block
-
-	blockSize = 512
-)
-
-// Operation is struct for MTIOCTOP
-type Operation struct {
-	Op    int16 // Operation ID
-	Pad   int16 // Padding to match C structures
-	Count int32 // Operation count
-}
-
-func main() {
-	file := flag.String("file", "/dev/nst0", "File (tape drive or tar file) to open")
-	recordSize := flag.Int("recordSize", 20, "Amount of 512-bit blocks per record")
-	record := flag.Int("record", 0, "Record to seek too")
-	block := flag.Int("block", 0, "Block in record to seek too")
-
-	flag.Parse()
-
-	f, err := os.OpenFile(*file, os.O_RDONLY, os.ModeCharDevice)
-	if err != nil {
-		panic(err)
-	}
-	defer f.Close()
-
-	if err := seekToRecordOnTape(f, int32(*record)); err != nil {
-		panic(err)
-	}
-
-	br := bufio.NewReaderSize(f, blockSize**recordSize)
-	if _, err := br.Read(make([]byte, *block*blockSize)); err != nil {
-		panic(err)
-	}
-
-	tr := tar.NewReader(br)
-
-	hdr, err := tr.Next()
-	if err != nil {
-		panic(err)
-	}
-
-	log.Println(hdr)
-}
-
-func seekToRecordOnTape(f *os.File, record int32) error {
-	if _, _, err := syscall.Syscall(
-		syscall.SYS_IOCTL,
-		f.Fd(),
-		MTIOCTOP,
-		uintptr(unsafe.Pointer(
-			&Operation{
-				Op:    MTSEEK,
-				Count: record,
-			},
-		)),
-	); err != 0 {
-		return err
-	}
-
-	return nil
-}
diff --git a/cmd/stfs-seek/main.go b/cmd/stfs-seek/main.go
index 29eb810..e1be816 100644
--- a/cmd/stfs-seek/main.go
+++ b/cmd/stfs-seek/main.go
@@ -4,7 +4,6 @@ import (
 	"archive/tar"
 	"bufio"
 	"flag"
-	"io"
 	"log"
 	"os"
 	"syscall"
@@ -51,60 +50,43 @@ func main() {
 		if err != nil {
 			panic(err)
 		}
-
-		// Seek to record & block
-		if _, err := f.Seek(int64((*recordSize*blockSize**record)+*block*blockSize), 0); err != nil {
-			panic(err)
-		}
 	} else {
 		f, err = os.OpenFile(*file, os.O_RDONLY, os.ModeCharDevice)
 		if err != nil {
 			panic(err)
 		}
-
-		// Seek to record (we can't seek to block on tape)
-		// TODO: Seek to next header here too; currently this only works for the start of each archive/after a filemark
-		if err := seekToRecordOnTape(f, int32(*record)); err != nil {
-			panic(err)
-		}
 	}
 	defer f.Close()
 
 	var tr *tar.Reader
 	if fileDescription.Mode().IsRegular() {
-		tr = tar.NewReader(f)
-	} else {
-		br := bufio.NewReaderSize(f, blockSize**recordSize)
-		tr = tar.NewReader(br)
-	}
-
-	for {
-		hdr, err := tr.Next()
-		if err != nil {
+		// Seek to record and block
+		if _, err := f.Seek(int64((*recordSize*blockSize**record)+*block*blockSize), 0); err != nil {
 			panic(err)
 		}
 
-		log.Println(hdr)
-
-		currentRecord := int64(0)
-		if fileDescription.Mode().IsRegular() {
-			curr, err := f.Seek(0, io.SeekCurrent)
-			if err != nil {
-				panic(err)
-			}
-
-			currentRecord = ((curr + hdr.Size) / blockSize) / int64(*recordSize)
-		} else {
-			currentRecord, err = getCurrentRecordFromTape(f)
-			if err != nil {
-				panic(err)
-			}
+		tr = tar.NewReader(f)
+	} else {
+		// Seek to record
+		if err := seekToRecordOnTape(f, int32(*record)); err != nil {
+			panic(err)
 		}
 
-		if currentRecord > int64(*record) {
-			break
+		// Seek to block
+		br := bufio.NewReaderSize(f, blockSize**recordSize)
+		if _, err := br.Read(make([]byte, *block*blockSize)); err != nil {
+			panic(err)
 		}
+
+		tr = tar.NewReader(br)
 	}
+
+	hdr, err := tr.Next()
+	if err != nil {
+		panic(err)
+	}
+
+	log.Println(hdr)
 }
 
 func seekToRecordOnTape(f *os.File, record int32) error {
@@ -124,17 +106,3 @@ func seekToRecordOnTape(f *os.File, record int32) error {
 
 	return nil
 }
-
-func getCurrentRecordFromTape(f *os.File) (int64, error) {
-	pos := &Position{}
-	if _, _, err := syscall.Syscall(
-		syscall.SYS_IOCTL,
-		f.Fd(),
-		MTIOCPOS,
-		uintptr(unsafe.Pointer(pos)),
-	); err != 0 {
-		return 0, err
-	}
-
-	return pos.BlkNo, nil
-}