[WasmFS] Implement support for opening with O_APPEND (#17201)

We were previously incorrectly ignoring O_APPEND. There were not already tests
for O_APPEND, so add new tests.

Author: tlively

system/lib/wasmfs/syscalls.cpp

@@ -101,28 +101,29 @@ static __wasi_errno_t writeAtOffset(OffsetHandling setOffset,
     return __WASI_ERRNO_BADF;
   }
 
-  auto lockedOpenFile = openFile->locked();
-
-  if (setOffset == OffsetHandling::OpenFileState) {
-    offset = lockedOpenFile.getPosition();
-  }
-
   if (iovs_len < 0 || offset < 0) {
     return __WASI_ERRNO_INVAL;
   }
 
-  // TODO: Check open file access mode for write permissions.
-
+  auto lockedOpenFile = openFile->locked();
   auto file = lockedOpenFile.getFile()->dynCast<DataFile>();
-
-  // If file is nullptr, then the file was not a DataFile.
-  // TODO: change to add support for symlinks.
   if (!file) {
     return __WASI_ERRNO_ISDIR;
   }
 
   auto lockedFile = file->locked();
 
+  if (setOffset == OffsetHandling::OpenFileState) {
+    if (lockedOpenFile.getFlags() & O_APPEND) {
+      offset = lockedFile.getSize();
+      lockedOpenFile.setPosition(offset);
+    } else {
+      offset = lockedOpenFile.getPosition();
+    }
+  }
+
+  // TODO: Check open file access mode for write permissions.
+
   size_t bytesWritten = 0;
   for (size_t i = 0; i < iovs_len; i++) {
     const uint8_t* buf = iovs[i].buf;

tests/test_other.py

@@ -11740,6 +11740,10 @@ def test_wasmfs_dup(self):
   def test_unistd_open(self):
     self.do_run_in_out_file_test('wasmfs/wasmfs_open.c')
 
+  @also_with_wasmfs
+  def test_unistd_open_append(self):
+    self.do_run_in_out_file_test('wasmfs/wasmfs_open_append.c')
+
   @also_with_wasmfs
   def test_unistd_stat(self):
     self.do_runf(test_file('wasmfs/wasmfs_stat.c'))

tests/wasmfs/wasmfs_open_append.c

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2022 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+const char* file = "/foo.txt";
+
+off_t getPos(int fd) {
+  errno = 0;
+  off_t pos = lseek(fd, 0, SEEK_CUR);
+  assert(pos != (off_t)-1);
+  assert(errno == 0);
+  return pos;
+}
+
+void setPos(int fd, off_t pos) {
+  errno = 0;
+  pos = lseek(fd, pos, SEEK_SET);
+  assert(pos != (off_t)-1);
+  assert(errno == 0);
+}
+
+int main() {
+  int fd = open(file, O_RDWR | O_CREAT | O_EXCL | O_APPEND, 0777);
+  assert(fd > 0);
+
+  off_t pos = getPos(fd);
+  assert(pos == 0);
+
+  ssize_t nwritten = write(fd, "hello", 5);
+  assert(nwritten == 5);
+
+  pos = getPos(fd);
+  assert(pos == 5);
+
+  setPos(fd, 0);
+  pos = getPos(fd);
+  assert(pos == 0);
+
+  nwritten = write(fd, "", 0);
+  assert(nwritten == 0);
+
+  pos = getPos(fd);
+  assert(pos == 5);
+
+  setPos(fd, 0);
+  pos = getPos(fd);
+  assert(pos == 0);
+
+  nwritten = write(fd, ", world!", 8);
+  assert(nwritten == 8);
+
+  pos = getPos(fd);
+  assert(pos == 13);
+
+  setPos(fd, 42);
+  pos = getPos(fd);
+  assert(pos == 42);
+
+  nwritten = write(fd, "!!", 2);
+  assert(nwritten == 2);
+
+  pos = getPos(fd);
+  assert(pos == 15);
+
+  setPos(fd, 0);
+
+  char buf[100] = {};
+  ssize_t nread = read(fd, buf, 100);
+  assert(nread == 15);
+  assert(strcmp(buf, "hello, world!!!") == 0);
+
+  printf("ok\n");
+}

tests/wasmfs/wasmfs_open_append.out

@@ -0,0 +1 @@
+ok