use BufferedInputStream in readTextFile

With FileInputStream, using assumption of bytes read (as
returned by read function) not matching buffer size as EOF is
not correct, we hit this when reading from /proc in some cases.

BufferedInputStream's behavior matches that assumption due to
its implementation.

Bug: 7342759
Change-Id: Ibb06dbe1fbe33f6f880524f7555ff5542889ea50

Author: guang-google

core/java/android/os/FileUtils.java

@@ -16,6 +16,7 @@
 
 package android.os;
 
+import java.io.BufferedInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.FileInputStream;
@@ -91,7 +92,7 @@ public static boolean copyFile(File srcFile, File destFile) {
         }
         return result;
     }
-    
+
     /**
      * Copy data from a source stream to destFile.
      * Return true if succeed, return false if failed.
@@ -143,12 +144,16 @@ public static boolean isFilenameSafe(File file) {
      */
     public static String readTextFile(File file, int max, String ellipsis) throws IOException {
         InputStream input = new FileInputStream(file);
+        // wrapping a BufferedInputStream around it because when reading /proc with unbuffered
+        // input stream, bytes read not equal to buffer size is not necessarily the correct
+        // indication for EOF; but it is true for BufferedInputStream due to its implementation.
+        BufferedInputStream bis = new BufferedInputStream(input);
         try {
             long size = file.length();
             if (max > 0 || (size > 0 && max == 0)) {  // "head" mode: read the first N bytes
                 if (size > 0 && (max == 0 || size < max)) max = (int) size;
                 byte[] data = new byte[max + 1];
-                int length = input.read(data);
+                int length = bis.read(data);
                 if (length <= 0) return "";
                 if (length <= max) return new String(data, 0, length);
                 if (ellipsis == null) return new String(data, 0, max);
@@ -161,7 +166,7 @@ public static String readTextFile(File file, int max, String ellipsis) throws IO
                     if (last != null) rolled = true;
                     byte[] tmp = last; last = data; data = tmp;
                     if (data == null) data = new byte[-max];
-                    len = input.read(data);
+                    len = bis.read(data);
                 } while (len == data.length);
 
                 if (last == null && len <= 0) return "";
@@ -178,12 +183,13 @@ public static String readTextFile(File file, int max, String ellipsis) throws IO
                 int len;
                 byte[] data = new byte[1024];
                 do {
-                    len = input.read(data);
+                    len = bis.read(data);
                     if (len > 0) contents.write(data, 0, len);
                 } while (len == data.length);
                 return contents.toString();
             }
         } finally {
+            bis.close();
             input.close();
         }
     }