[JENKINS-49971] Fix a race condition in Util.loadFile() (#3225)

* Fix a race condition in Util.loadFile()

If the file is deleted in between when its existence is checked and the
file is opened for reading, then the method will fail to return an empty
string.

* Switch to using FileUtils.readFileToString()

(cherry picked from commit aaae71af)

core/src/main/java/hudson/Util.java

@@ -87,6 +87,9 @@ import javax.annotation.Nullable;
 
 import org.apache.commons.codec.digest.DigestUtils;
 
+import org.apache.commons.codec.digest.DigestUtils;
+import org.apache.commons.io.FileUtils;
+
 /**
  * Various utility methods that don't have more proper home.
  *
@@ -179,43 +182,54 @@ public class Util {
     }
 
     /**
-     * Loads the contents of a file into a string.
+     * Reads the entire contents of the text file at <code>logfile</code> into a
+     * string using the {@link Charset#defaultCharset() default charset} for
+     * decoding. If no such file exists, an empty string is returned.
+     * @param logfile The text file to read in its entirety.
+     * @return The entire text content of <code>logfile</code>.
+     * @throws IOException If an error occurs while reading the file.
+     * @deprecated call {@link #loadFile(java.io.File, java.nio.charset.Charset)}
+     * instead to specify the charset to use for decoding (preferably
+     * {@link java.nio.charset.StandardCharsets#UTF_8}).
      */
     @Nonnull
+    @Deprecated
     public static String loadFile(@Nonnull File logfile) throws IOException {
         return loadFile(logfile, Charset.defaultCharset());
     }
 
+    /**
+     * Reads the entire contents of the text file at <code>logfile</code> into a
+     * string using <code>charset</code> for decoding. If no such file exists,
+     * an empty string is returned.
+     * @param logfile The text file to read in its entirety.
+     * @param charset The charset to use for decoding the bytes in <code>logfile</code>.
+     * @return The entire text content of <code>logfile</code>.
+     * @throws IOException If an error occurs while reading the file.
+     */
     @Nonnull
     public static String loadFile(@Nonnull File logfile, @Nonnull Charset charset) throws IOException {
-        if(!logfile.exists())
-            return "";
-
-        StringBuilder str = new StringBuilder((int)logfile.length());
-
-        // We're not using Files.newBufferedReader() here because there is a
-        // difference in how an InputStreamReader constructed from a Charset and
-        // the reader returned by Files.newBufferedReader() handle malformed and
-        // unmappable byte sequences for the specified encoding; the latter is
-        // more picky and will throw a CharacterCodingException. See:
-        // https://issues.jenkins-ci.org/browse/JENKINS-49060?focusedCommentId=325989&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-325989
+        // Note: Until charset handling is resolved (e.g. by implementing
+        // https://issues.jenkins-ci.org/browse/JENKINS-48923 ), this method
+        // must be able to handle character encoding errors. As reported at
+        // https://issues.jenkins-ci.org/browse/JENKINS-49112 Run.getLog() calls
+        // loadFile() to fully read the generated log file. This file might
+        // contain unmappable and/or malformed byte sequences. We need to make
+        // sure that in such cases, no CharacterCodingException is thrown.
         //
-        // As reported at https://issues.jenkins-ci.org/browse/JENKINS-49112
-        // Run.getLog() calls loadFile() to fully read the generated log file.
-        // Until charset handling is resolved (e.g. by implementing
-        // https://issues.jenkins-ci.org/browse/JENKINS-48923 ), malformed
-        // bytes will need to be tolerated.
-        try (InputStream rawIn = Files.newInputStream(fileToPath(logfile));
-             Reader r = new BufferedReader(new InputStreamReader(rawIn, charset))) {
-            char[] buf = new char[1024];
-            int len;
-            while ((len = r.read(buf, 0, buf.length)) > 0)
-                str.append(buf, 0, len);
+        // One approach that cannot be used is to call Files.newBufferedReader()
+        // because there is a difference in how an InputStreamReader constructed
+        // from a Charset and the reader returned by Files.newBufferedReader()
+        // handle malformed and unmappable byte sequences for the specified
+        // encoding; the latter is more picky and will throw an exception.
+        // See: https://issues.jenkins-ci.org/browse/JENKINS-49060?focusedCommentId=325989&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-325989
+        try {
+            return FileUtils.readFileToString(logfile, charset);
+        } catch (FileNotFoundException e) {
+            return "";
         } catch (Exception e) {
-            throw new IOException("Failed to fully read " + logfile + " using charset " + charset.name(), e);
+            throw new IOException("Failed to fully read " + logfile, e);
         }
-
-        return str.toString();
     }
 
     /**