sysx/xattr: fix getxattrAll

kolyshkin · kolyshkin · commit 9e256e61eee8 · 2020-02-26T18:47:47.000-08:00
The current implementation is suboptimal for two reasons:

 * the initial buffer size of 5 bytes makes no sense
   (my guess someone forgot to change it back to a sane
   value after the retry logic testing);

 * it never gets the real buffer size, resulting in extra
   iterations (5 -&gt; 10 -&gt; 20 -&gt; ...).

This commit
 * sets the initial size to 128;
 * uses the logic to get the real size in case we get ERANGE

Signed-off-by: Kir Kolyshkin &lt;kolyshkin@gmail.com&gt;
diff --git a/sysx/xattr.go b/sysx/xattr.go
@@ -20,7 +20,6 @@ package sysx
 
 import (
 	"bytes"
-	"syscall"
 
 	"golang.org/x/sys/unix"
 )
@@ -66,7 +65,7 @@ func LGetxattr(path, attr string) ([]byte, error) {
 	return getxattrAll(path, attr, unix.Lgetxattr)
 }
 
-const defaultXattrBufferSize = 5
+const defaultXattrBufferSize = 128
 
 type listxattrFunc func(path string, dest []byte) (int, error)
 
@@ -102,24 +101,19 @@ func listxattrAll(path string, listFunc listxattrFunc) ([]string, error) {
 type getxattrFunc func(string, string, []byte) (int, error)
 
 func getxattrAll(path, attr string, getFunc getxattrFunc) ([]byte, error) {
-	p := make([]byte, defaultXattrBufferSize)
-	for {
-		n, err := getFunc(path, attr, p)
+	buf := make([]byte, defaultXattrBufferSize)
+	n, err := getFunc(path, attr, buf)
+	for err == unix.ERANGE {
+		// Buffer too small, use zero-sized buffer to get the actual size
+		n, err = getFunc(path, attr, []byte{})
 		if err != nil {
-			if errno, ok := err.(syscall.Errno); ok && errno == syscall.ERANGE {
-				p = make([]byte, len(p)*2) // this can't be ideal.
-				continue                   // try again!
-			}
-
 			return nil, err
 		}
-
-		// realloc to correct size and repeat
-		if n > len(p) {
-			p = make([]byte, n)
-			continue
-		}
-
-		return p[:n], nil
+		buf = make([]byte, n)
+		n, err = getFunc(path, attr, buf)
+	}
+	if err != nil {
+		return nil, err
 	}
+	return buf[:n], nil
 }
