package symbolyze

import (
	"bufio"
	"debug/elf"
	"fmt"
	"log"
	"os"
	"path/filepath"
	"strconv"
	"strings"
)

// Scanner represents an engine for scanning for a specific symbol in all
// ELF-files matching a certain pattern. The pattern is described in
// fileapth.Match().
//
// Once a Scanner is created with New(), it should be populated with Observer
// functions using OnFound().  Optionally, the scanner can be put into
// debugging mode by a call to DebugOn() prior to a call to Run().
//
// A call to Scanner.Run() then starts the engine and it will scan all pids in
// /proc.  Whenever a match is found, all observers will be called with the
// (pid, offset), concurrently.
type Scanner struct {
	symbol    string
	pathglob  string
	cache     map[string]uint64 // Contains (pathname, offset)
	observers []Observer        // Callbacks

	logger // Embedded logger

	// Instead of using a boolean to indicate debugging, we use function
	// members.  This way we can populate them with noop-functions in the
	// non-debug case and not polute the code with if-statements.
	debugf  func(format string, v ...interface{})
	debugln func(v ...interface{})

	err error // error state of the scanner.
}

// We use a lowercase type alias for *log.Logger so that we can embedd it in
// Scanner without exporting it.
type logger = *log.Logger

// An Observer is a callback that can be registerd with Scanner.OnFound.  It
// will be called with a pid and an offset.  Observers are called concurrently.
// They have to be thread-safe.
type Observer func(pid int, offset uint64) error

// New returns a new Scanner that scans all running processes for the given
// symbol name in all memory-mapped files matching the given pathglob.
// To be useful, one or more Observer functions should be registerd with
// Scanner.OnFound(). The scanning starts with a call of Scanner.Run().
func New(symbol, pathglob string) *Scanner {
	return &Scanner{
		symbol:   symbol,
		pathglob: pathglob,
		cache:    map[string]uint64{},

		logger: log.New(os.Stderr, "[symbolyze] ", log.Ltime|log.Lmicroseconds),

		// debugging is off per default.
		debugf:  func(string, ...interface{}) {},
		debugln: func(...interface{}) {},
	}
}

// Debug sets the scanner into debugging mode.  It must called only once before
// a call to Scanner.Run().
func (S *Scanner) DebugOn() {
	// Use the embedded *log.Logger for debugging.
	S.debugf = S.Printf
	S.debugln = S.Println
	S.debugln("starting in debug-mode")
}

// setErrorf puts the Scanner into an error state with the given error
// statement.  It also logs the error.
func (S *Scanner) setErrorf(format string, a ...interface{}) {
	S.err = fmt.Errorf(format, a...)
	S.Printf(format, a...)
}

// OnFound puts an Observer function into the interal queue.  The functions are
// called in sequence in their own goroutine whenever the scanner finds the
// symbol in the a running program.  That implies that an Observer has to be
// thread-safe.  Errors from the observers will be logged.
//
// Calling OnFound is not thread-safe.
func (S *Scanner) OnFound(fun Observer) {
	S.observers = append(S.observers, fun)
	return
}

// Run starts the scanning process.  It scans the maps file all processes in
// /proc for pathnames that match the provided pathglob and that are ELF
// executables or shared libraries.  It searches for the provided symbol in
// those files and calls the registered Observer functions concurrently with
// the pid and offset of the symbol.
//
// Run will return an error if it couldn't read the proc filesystem.  Otherwise
// it will try to continue to loop over all pids, writing potential errors to
// the console.  Errors from the observer functions are also logged and the
// final such error is the return value of Run().
func (S *Scanner) Run() error {
	if S.err != nil {
		return S.err
	}

	proc, err := os.Open("/proc")
	if err != nil {
		S.setErrorf("Failed to open /proc: %v\n", err)
		return S.err
	}

	infos, err := proc.Readdir(-1)
	if err != nil {
		S.setErrorf("Failed to read /proc: %v\n", err)
		return S.err
	}

	proc.Close()

	for _, pinfo := range infos {
		var pid_s = pinfo.Name()

		// The entry /proc/NNN/ must be a directory with integer name
		if !pinfo.IsDir() {
			continue
		} else if pid, err := strconv.Atoi(pid_s); err != nil {
			continue
		} else if offset, found := S.searchSymbolIn(pid); !found {
			continue
		} else {
			// Notify the observers.
			for n, observer := range S.observers {
				go func() {
					err = observer(pid, offset)
					if err != nil {
						S.setErrorf("S.observer[%d] error: %v", n, err)
						// TODO: propagate errors from all Observers.
					}

				}()
			}
		}
	}

	return S.err
}

// searchSymbolIn loops over the entries in /proc/<pid>/maps and searches for
// the symbol in the mapped files.
//
// The current implementation makes the following assumptions:
//   1. The pathname in an entry does not contain spaces.
//   2. The pathname starts with /.
//   3. The symbol must be in a region that has permission "rw-p".
//   4. The symbol is present at most in one mapped file at the same time.
//
// It returns the offsets in memory of the running program, if found.
func (S *Scanner) searchSymbolIn(pid int) (offset uint64, found bool) {

	path := filepath.Join("/proc", strconv.Itoa(pid), "maps")
	maps, err := os.Open(path)
	if err != nil {
		S.Printf("%v\n", err)
		return 0, false
	}

	defer maps.Close()

	// Read the entries by line
	scanner := bufio.NewScanner(maps)
	for scanner.Scan() {

		// A line of our interest in the maps file has the following
		// structure, see man proc(5).
		//
		// 0                         1     2       3     4           5
		// address                   perms offset  dev   inode       pathname
		// 7fdd8fece000-7fdd8ff74000 rw-p 00423000 fd:01 14156759    /usr/lib/x86_64-linux-gnu/libpython3.7m.so.1.0

		// We assume that the pathname contains no spaces so
		// bytes.Fields splits the line excactly into six fields
		fields := strings.Fields(scanner.Text())
		if len(fields) != 6 {
			continue
		}

		pathname := fields[5]
		if !strings.HasPrefix(pathname, "/") { // Not a pathname
			continue
		}

		// The filename must match the given pattern
		filename := filepath.Base(pathname)
		ok, err := filepath.Match(S.pathglob, filename)
		if err != nil || !ok {
			continue
		}

		// The symbol needs to be writable
		if fields[1] != "rw-p" {
			continue
		}

		// Get the start address of the mapped region in memory
		startAddress, _, err := parseRange(fields[0])
		if err != nil {
			S.Printf("%v\n", err)
			continue
		}

		// Read the offset in the file that this region is mapping
		fileOffset, err := strconv.ParseUint(fields[2], 16, 64)
		if err != nil {
			S.Printf("fields[2] %#q: %v\n", fields[2], err)
			continue
		}

		// Finally, find the symbol in the binary.  If found,
		// findSymbol returns the offset of the symbol in memory,
		// taking alignment into account.
		memOffset, found := S.findSymbol(pathname)
		if !found {
			continue
		}

		// Hurray, we've found an entry!
		return startAddress + memOffset - fileOffset, true
	}

	return 0, false
}

// findSymbol searches for the provided symbol in the given pathname to an
// ELF-file.  If found, it returns the offset of the symbol in the virtual
// memory according to the fomula:
//
// 	vmOffset = alignedOffset(section) + offsetInSection(symbol)
//
// The result will be cached so that subsequent calls to findSymbol with the
// same pathname can quickly return.
func (S *Scanner) findSymbol(pathname string) (offset uint64, found bool) {

	// 0. Return the value from the cache, if found.
	if offset, found = S.cache[pathname]; found {
		return offset, found
	}

	// 1. Open the file with the ELF-parser
	file, err := elf.Open(pathname)
	if err != nil {
		S.Printf("%v", err)
		return 0, false
	}

	defer file.Close()

	// 2. Find the symbol
	symbols, err := file.DynamicSymbols()
	if err != nil {
		S.Printf("%v", err)
		return 0, false
	}

	var sym *elf.Symbol
	for _, s := range symbols {
		if s.Name == S.symbol {
			S.debugf("Found symbol %#v in %s: %#v\n", sym, pathname, s)
			sym = &s
			break
		}
	}
	if sym == nil {
		S.debugf("symbol %q not found in %s\n", sym, pathname)
		return 0, false
	}

	// 3. Extract the information about the section
	if len(file.Sections) < int(sym.Section) {
		S.debugf("len(file.Section) < int(sym.Section) for symbol %q in %s\n", sym, pathname)
		return 0, false
	}

	section := file.Sections[sym.Section]
	if section == nil {
		S.debugf("Section %v not found for ELF-Header %q in %s\n", sym.Section, pathname)
		return 0, false
	}

	// 4. Calculate the offset of the given section, aligned according to
	// the SectionHeader.Addralign entry.
	mask := section.SectionHeader.Addralign - 1
	alignedSectOff := (section.SectionHeader.Offset + mask) & (^mask)

	// 5. The location of the symbol in virtual memory is finally:
	vmOffset := alignedSectOff + (sym.Value - section.SectionHeader.Addr)

	// 6. Store this calculation in our cache so that we don't to touch
	// this file again.
	S.cache[pathname] = vmOffset

	return vmOffset, true
}

// parseRange is a helper function that parses the first field in a line in
// /proc/<pid>/maps:
// 	7fdd8fece000-7fdd8ff74000 rw-p ...
// It returns the start and end addresses of the range and a potential error.
func parseRange(input string) (start, end uint64, e error) {
	// 7fdd8fece000-7fdd8ff74000
	parts := strings.Split(input, "-")

	if len(parts) != 2 {
		e = fmt.Errorf("[parseRange] unrecognized format for region: %#q", input)
		return 0, 0, e
	}

	start, e = strconv.ParseUint(parts[0], 16, 64)
	if e != nil {
		e = fmt.Errorf("[parseRange] couldn't parse start-address %#q in %#q: %w", parts[0], input, e)
		return 0, 0, e
	}

	end, e = strconv.ParseUint(parts[1], 16, 64)
	if e != nil {
		e = fmt.Errorf("[parseRange] couldn't parse end-address %#q in %#q: %w", parts[1], input, e)
		return 0, 0, e
	}

	return start, end, e
}