cmd/htpacker: cope with zero-length input files

Sometimes we might be asked to serve up a zero-length input file,
typically from some machine-generated CSS etc. We make some very
rudimentary guess about the content-type the caller wanted and skip the
mmap(2) call.

cmd/htpacker/packer/packer.go

@@ -325,11 +325,14 @@ func (p *packer) packFile(path string, fileToPack FileToPack) {
 		return
 	}
 
-	data, err := unix.Mmap(int(f.Fd()), 0, int(fi.Size()),
-		unix.PROT_READ, unix.MAP_SHARED)
-	if err != nil {
-		p.Abort(fmt.Errorf("mmap %s: %v", fileToPack.Filename, err))
-		return
+	var data []byte
+	if fi.Size() > 0 {
+		data, err = unix.Mmap(int(f.Fd()), 0, int(fi.Size()),
+			unix.PROT_READ, unix.MAP_SHARED)
+		if err != nil {
+			p.Abort(fmt.Errorf("mmap %s: %v", fileToPack.Filename, err))
+			return
+		}
 	}
 
 	// prepare initial directory entry

cmd/htpacker/yaml.go

@@ -3,6 +3,7 @@ package main
 import (
 	"errors"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"net/http"
 	"os"
@@ -125,13 +126,22 @@ func filesFromListR(prefix, arg string, ftp packer.FilesToPack) error {
 
 	case fi.Mode().IsRegular():
 		// sniff content type
+		var ctype string
 		buf := make([]byte, 512)
 		n, err := f.Read(buf)
-		if err != nil {
+		switch err {
+		case nil:
+			buf = buf[:n]
+			ctype = http.DetectContentType(buf)
+
+		case io.EOF:
+			// Empty file; this is typically due to things like
+			// npm webpack producing empty .css files.
+			ctype = "text/plain; charset=UTF-8"
+
+		default:
 			return fmt.Errorf("failed to read %s: %v", arg, err)
 		}
-		buf = buf[:n]
-		ctype := http.DetectContentType(buf)
 
 		// augmented rules for JS / CSS / etc.
 		switch {