nshlib: fix infinite loop on broken stdout in nsh_catfile

When stdout is broken (e.g. PTY master closed), nsh_catfile could spin
forever: write() fails, error logging generates syslog output to
/dev/log, next read() picks it up, write() fails again, ad infinitum.

Two fixes:
- nsh_console: drop the _err() in nsh_consolewrite entirely.  The risk
  is errno-agnostic (EPIPE / EIO / ENOSPC / ...): any failure logged
  here can be re-injected by the syslog backend when OUTFD is bound to
  /dev/log.  Callers already see the failure via the negative return
  value and errno, so on-failure logging at this layer is redundant.
- nsh_fsutils: jump straight from the inner write-failure path to a
  single errout: cleanup label instead of using a two-level break +
  flag check, so a failed write cannot fall through to another read().

Assisted-by: GitHubCopilot:claude-4.7-opus
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

nshlib/nsh_console.c

@@ -131,18 +131,19 @@ static ssize_t nsh_consolewrite(FAR struct nsh_vtbl_s *vtbl,
                                 FAR const void *buffer, size_t nbytes)
 {
   FAR struct console_stdio_s *pstate = (FAR struct console_stdio_s *)vtbl;
-  ssize_t ret;
 
-  /* Write the data to the output stream */
+  /* Write the data to the output stream.
+   *
+   * Errors are reported to the caller via the negative return value and
+   * errno; do NOT _err() here.  OUTFD may itself be wired to the syslog
+   * backend (e.g. nsh_catfile dumping /dev/log on dmesg), in which case
+   * logging on write failure would produce new bytes, get picked up by
+   * the next read(), and lock the shell into an infinite loop at the
+   * highest priority — independent of which errno (EPIPE/EIO/ENOSPC/...)
+   * the underlying device returns.
+   */
 
-  ret = write(OUTFD(pstate), buffer, nbytes);
-  if (ret < 0)
-    {
-      _err("ERROR: [%d] Failed to send buffer: %d\n",
-          OUTFD(pstate), errno);
-    }
-
-  return ret;
+  return write(OUTFD(pstate), buffer, nbytes);
 }
 
 /****************************************************************************

nshlib/nsh_fsutils.c

@@ -228,8 +228,14 @@ int nsh_catfile(FAR struct nsh_vtbl_s *vtbl, FAR const char *cmd,
                                  NSH_ERRNO_OF(errcode));
                     }
 
+                  /* Jump straight to cleanup.  On a broken stdout (e.g.
+                   * closed PTY master) we must not keep reading: the
+                   * outer loop would otherwise drain /dev/log forever
+                   * when cat is dumping it on dmesg.
+                   */
+
                   ret = ERROR;
-                  break;
+                  goto errout;
                 }
               else
                 {
@@ -255,6 +261,7 @@ int nsh_catfile(FAR struct nsh_vtbl_s *vtbl, FAR const char *cmd,
 
   /* Close the input file and return the result */
 
+errout:
   close(fd);
   free(buffer);
   return ret;