file_readv_compat/file_writev_compat: Fix partial success handling

The problem has been inherited from the original libc readv/writev
implementation. However, now it's exposed in more situations because
this implemenation is used to back read/write as well.

I expect this fixes the regressions observed on the Espressif CI.
https://github.com/apache/nuttx/pull/13498#issuecomment-2448031197

Note that, even with this fix, these "compat" readv/writev
implementations are still inheritedly broken. (E.g. consider that
a data boundary happens to match one of iovec boundaries) However,
this fix is enough for read/write, where iovcnt is always 1.

fs/vfs/fs_read.c

@@ -68,42 +68,36 @@ static ssize_t file_readv_compat(FAR struct file *filep,
     {
       /* Ignore zero-length reads */
 
-      if (iov[i].iov_len > 0)
+      if (iov[i].iov_len == 0)
         {
-          buffer    = iov[i].iov_base;
+          continue;
-          remaining = iov[i].iov_len;
-
-          /* Read repeatedly as necessary to fill buffer */
-
-          do
-            {
-              nread = inode->u.i_ops->read(filep, (void *)buffer,
-                                                  remaining);
-
-              /* Check for a read error */
-
-              if (nread < 0)
-                {
-                  return ntotal ? ntotal : nread;
-                }
-
-              /* Check for an end-of-file condition */
-
-              else if (nread == 0)
-                {
-                  return ntotal;
-                }
-
-              /* Update pointers and counts in order to handle partial
-               * buffer reads.
-               */
-
-              buffer    += nread;
-              remaining -= nread;
-              ntotal    += nread;
-            }
-          while (remaining > 0);
         }
+
+      buffer    = iov[i].iov_base;
+      remaining = iov[i].iov_len;
+
+      nread = inode->u.i_ops->read(filep, (void *)buffer, remaining);
+
+      /* Check for a read error */
+
+      if (nread < 0)
+        {
+          return ntotal ? ntotal : nread;
+        }
+
+      ntotal += nread;
+
+      /* Check for a parital success condition, including an end-of-file */
+
+      if (nread < remaining)
+        {
+          return ntotal;
+        }
+
+      /* Update the pointer */
+
+      buffer    += nread;
+      remaining -= nread;
     }
 
   return ntotal;

fs/vfs/fs_write.c

@@ -68,35 +68,36 @@ static ssize_t file_writev_compat(FAR struct file *filep,
     {
       /* Ignore zero-length writes */
 
-      if (iov[i].iov_len > 0)
+      if (iov[i].iov_len == 0)
         {
-          buffer    = iov[i].iov_base;
+          continue;
-          remaining = iov[i].iov_len;
-
-          /* Write repeatedly as necessary to write the entire buffer */
-
-          do
-            {
-              nwritten = inode->u.i_ops->write(filep, (void *)buffer,
-                                                      remaining);
-
-              /* Check for a write error */
-
-              if (nwritten < 0)
-                {
-                  return ntotal ? ntotal : nwritten;
-                }
-
-              /* Update pointers and counts in order to handle partial
-               * buffer writes.
-               */
-
-              buffer    += nwritten;
-              remaining -= nwritten;
-              ntotal    += nwritten;
-            }
-          while (remaining > 0);
         }
+
+      buffer    = iov[i].iov_base;
+      remaining = iov[i].iov_len;
+
+      nwritten = inode->u.i_ops->write(filep, (void *)buffer, remaining);
+
+      /* Check for a write error */
+
+      if (nwritten < 0)
+        {
+          return ntotal ? ntotal : nwritten;
+        }
+
+      ntotal += nwritten;
+
+      /* Check for a parital success condition */
+
+      if (nwritten < remaining)
+        {
+          return ntotal;
+        }
+
+      /* Update the pointer */
+
+      buffer    += nwritten;
+      remaining -= nwritten;
     }
 
   return ntotal;