Cleanup exception handling in IOUtils (#29069)

When we copied IOUtils into the Elasticsearch codebase from Lucene, we
brought with it its handling of throwables which are out of whack with
how we handle throwables in our codebase. This commit modifies our copy
of IOUtils to be consistent with how we handle throwables today: do not
catch them. We take advantage of this cleanup to simplify IOUtils.

libs/elasticsearch-core/src/main/java/org/elasticsearch/core/internal/io/IOUtils.java

@@ -58,23 +58,29 @@ public final class IOUtils {
      * @see #close(Closeable...)
      */
     public static void close(final Iterable<? extends Closeable> objects) throws IOException {
-        Throwable th = null;
+        Exception ex = null;
 
         for (final Closeable object : objects) {
             try {
                 if (object != null) {
                     object.close();
                 }
-            } catch (final Throwable t) {
-                addSuppressed(th, t);
-                if (th == null) {
-                    th = t;
+            } catch (final IOException | RuntimeException e) {
+                if (ex == null) {
+                    ex = e;
+                } else {
+                    ex.addSuppressed(e);
                 }
             }
         }
 
-        if (th != null) {
-            throw rethrowAlways(th);
+        if (ex != null) {
+            if (ex instanceof IOException) {
+                throw (IOException) ex;
+            } else {
+                // since we only assigned an IOException or a RuntimeException to ex above, in this case ex must be a RuntimeException
+                throw (RuntimeException) ex;
+            }
         }
     }
 
@@ -101,65 +107,12 @@ public final class IOUtils {
                 if (object != null) {
                     object.close();
                 }
-            } catch (final Throwable t) {
+            } catch (final IOException | RuntimeException e) {
 
             }
         }
     }
 
-    /**
-     * Adds a {@link Throwable} to the list of suppressed {@link Exception}s of the first {@link Throwable}.
-     *
-     * @param exception  the exception to add a suppression to, if non-null
-     * @param suppressed the exception to suppress
-     */
-    private static void addSuppressed(final Throwable exception, final Throwable suppressed) {
-        if (exception != null && suppressed != null) {
-            exception.addSuppressed(suppressed);
-        }
-    }
-
-    /**
-     * This utility method takes a previously caught (non-null) {@link Throwable} and rethrows either the original argument if it was a
-     * subclass of the {@link IOException} or an {@link RuntimeException} with the cause set to the argument.
-     * <p>
-     * This method <strong>never returns any value</strong>, even though it declares a return value of type {@link Error}. The return
-     * value declaration is very useful to let the compiler know that the code path following the invocation of this method is unreachable.
-     * So in most cases the invocation of this method will be guarded by an {@code if} and used together with a {@code throw} statement, as
-     * in:
-     * <p>
-     * <pre>{@code
-     *   if (t != null) throw IOUtils.rethrowAlways(t)
-     * }
-     * </pre>
-     *
-     * @param th the throwable to rethrow; <strong>must not be null</strong>
-     * @return this method always results in an exception, it never returns any value; see method documentation for details and usage
-     * example
-     * @throws IOException      if the argument was an instance of {@link IOException}
-     * @throws RuntimeException with the {@link RuntimeException#getCause()} set to the argument, if it was not an instance of
-     *                          {@link IOException}
-     */
-    private static Error rethrowAlways(final Throwable th) throws IOException, RuntimeException {
-        if (th == null) {
-            throw new AssertionError("rethrow argument must not be null.");
-        }
-
-        if (th instanceof IOException) {
-            throw (IOException) th;
-        }
-
-        if (th instanceof RuntimeException) {
-            throw (RuntimeException) th;
-        }
-
-        if (th instanceof Error) {
-            throw (Error) th;
-        }
-
-        throw new RuntimeException(th);
-    }
-
     /**
      * Deletes all given files, suppressing all thrown {@link IOException}s. Some of the files may be null, if so they are ignored.
      *
@@ -180,7 +133,7 @@ public final class IOUtils {
                 // noinspection EmptyCatchBlock
                 try {
                     Files.delete(name);
-                } catch (final Throwable ignored) {
+                } catch (final IOException ignored) {
 
                 }
             }