8373647: Avoid unnecessary fstat when opening file for write

src/java.base/unix/native/libjava/io_util_md.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -27,6 +27,7 @@
 #include "jvm.h"
 #include "io_util.h"
 #include "io_util_md.h"
+#include <fcntl.h>
 #include <string.h>
 #include <unistd.h>
 
@@ -75,20 +76,27 @@ FD
 handleOpen(const char *path, int oflag, int mode) {
     FD fd;
     RESTARTABLE(open(path, oflag, mode), fd);
-    if (fd != -1) {
-        struct stat buf;
-        int result;
-        RESTARTABLE(fstat(fd, &buf), result);
-        if (result != -1) {
-            if (S_ISDIR(buf.st_mode)) {
-                close(fd);
-                errno = EISDIR;
-                fd = -1;
-            }
-        } else {
+    // Fast-path, either it is an error or if it's
+    // not a read access mode then the Unix standard
+    // guarantees to have failed with EISDIR
+    if (fd == -1 || ((oflag & O_ACCMODE) != O_RDONLY) != 0) {
+        return fd;
+    }
+
+    // Slow-path, while Unix allow for directories
+    // to be read, Java don't
+    struct stat buf;
+    int result;
+    RESTARTABLE(fstat(fd, &buf), result);
+    if (result != -1) {
+        if (S_ISDIR(buf.st_mode)) {
             close(fd);
+            errno = EISDIR;
             fd = -1;
         }
+    } else {
+        close(fd);
+        fd = -1;
     }
     return fd;
 }

test/micro/org/openjdk/bench/java/io/FileWriteStress.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package org.openjdk.bench.java.io;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.util.concurrent.TimeUnit;
+
+import org.openjdk.jmh.annotations.*;
+
+/**
+ * Tests the overheads of I/O syscalls
+ */
+@State(Scope.Benchmark)
+@Warmup(iterations = 3, time = 2)
+@Measurement(iterations = 5, time = 5)
+@BenchmarkMode(Mode.SampleTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Threads(1)
+
+@Fork(value = 10)
+public class FileWriteStress {
+    final byte[] payload = "something".getBytes();
+    final String path = System.getProperty("os.name", "unknown").toLowerCase().contains("win") ? "NUL" : "/dev/null";
+
+    @Benchmark
+    public void test() throws IOException {
+        try (FileOutputStream f = new FileOutputStream(path)) {
+            f.write(payload);
+        }
+    }
+
+}