[FS] Fix partial write case when using _fd_write/doWritev/writev (#22261)

doWritev iterates through the iovs structure writing the data for each
entry. The bug fixed here is that if a write does not write all data,
the loop continues. If the subsequent write also accepts data, it has
accepted data out of order.

Fixes #22258

Author: jonahgraham

src/library_wasi.js

@@ -223,6 +223,10 @@ var WasiLibrary = {
       var curr = FS.write(stream, HEAP8, ptr, len, offset);
       if (curr < 0) return -1;
       ret += curr;
+      if (curr < len) {
+        // No more space to write.
+        break;
+      }
       if (typeof offset != 'undefined') {
         offset += curr;
       }

test/fs/test_writev_partial_write.c

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2024 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 <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/uio.h>
+#include <unistd.h>
+#include <emscripten.h>
+
+EM_JS_DEPS(main, "$ERRNO_CODES");
+
+void setup(void) {
+ EM_ASM(
+    var device = FS.makedev(80, 0);
+    FS.registerDevice(device, {
+      write: function(stream, buffer, offset, length, pos) {
+        if (length === 0) {
+          return 0;
+        }
+        // Only do a partial write of one byte each time
+        out('TO DEVICE: ' + JSON.stringify(String.fromCharCode(buffer[offset])));
+        return 1;
+      }
+    });
+    FS.mkdev('/device', device);
+  );
+}
+
+// Run the test with writev directly
+void test_writev_direct(void) {
+  int fd = open("/device", O_WRONLY);
+  assert(fd);
+  struct iovec iovs[] = {{.iov_base = "ABC", .iov_len = 3},
+                         {.iov_base = "XYZ", .iov_len = 3}};
+  struct iovec* iov = iovs;
+
+  size_t rem = iov[0].iov_len + iov[1].iov_len;
+  int iovcnt = 2;
+  ssize_t cnt;
+  for (;;) {
+    cnt = writev(fd, iov, iovcnt);
+    assert(cnt >= 0);
+    if (cnt == rem) {
+      // All data written
+      break;
+    }
+    rem -= cnt;
+    if (cnt > iov[0].iov_len) {
+      cnt -= iov[0].iov_len;
+      iov++;
+      iovcnt--;
+    }
+    iov[0].iov_base = (char*)iov[0].iov_base + cnt;
+    iov[0].iov_len -= cnt;
+  }
+
+  close(fd);
+}
+
+// Run the test using stdio, this test is dependent on specific buffering used
+// and is included here as this code most closely matches the original bug
+// report
+void test_via_stdio(void) {
+  // XXX: We open in append mode because truncating JS based files is not
+  // supported yet. See #22262
+#ifdef WASMFS
+  FILE* f = fopen("/device", "a");
+#else
+  FILE* f = fopen("/device", "w");
+#endif
+  assert(f);
+  // Use line buffering. The bug is exposed with line buffering because with
+  // line buffering two entries in __stdio_write's iovs are used.
+  setvbuf(f, NULL, _IOLBF, 0);
+  fputs("abc", f);
+  fputs("\n", f);
+  fflush(f);
+  fclose(f);
+}
+
+int main() {
+  setup();
+  test_writev_direct();
+  test_via_stdio();
+  printf("done\n");
+  return 0;
+}

test/fs/test_writev_partial_write.out

@@ -0,0 +1,11 @@
+TO DEVICE: "A"
+TO DEVICE: "B"
+TO DEVICE: "C"
+TO DEVICE: "X"
+TO DEVICE: "Y"
+TO DEVICE: "Z"
+TO DEVICE: "a"
+TO DEVICE: "b"
+TO DEVICE: "c"
+TO DEVICE: "\n"
+done

test/test_other.py

@@ -15028,3 +15028,8 @@ def test_extra_struct_info(self):
     stderr = self.run_process([EMCC, test_file('hello_world.c'), '--js-library', test_file('other/test_extra_struct_info.js')], stderr=PIPE).stderr
     self.assertContained('(before) AF_INET=2', stderr)
     self.assertContained('(after) AF_INET=42', stderr)
+
+  @also_with_wasmfs
+  def test_fs_writev_partial_write(self):
+    self.set_setting('FORCE_FILESYSTEM')
+    self.do_run_in_out_file_test('fs/test_writev_partial_write.c')