commit ef3beaf664347f7fce1f3475fe65de729320837e
Author: Henrik Grimler <henrik@grimler.se>
Date:   Tue May 11 09:36:52 2021 +0200

    Use setlinebuf to make stderr line buffered
    
    If atomic writes are supported.  Android >= 10 has a file descriptor
    sanitizer which errors if a stream is closed while there are still
    fd's associated with it. In emacs stderr is opened twice if line
    buffered streams are supported.  Rather than opening a copy we can
    make stderr line buffered with setlinebuf instead.

diff --git a/src/sysdep.c b/src/sysdep.c
index d940acc4e0..77e1a0be3c 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -232,10 +232,6 @@ force_open (int fd, int flags)
     }
 }
 
-/* A stream that is like stderr, except line buffered.  It is NULL
-   during startup, or if line buffering is not in use.  */
-static FILE *buferr;
-
 /* Make sure stdin, stdout, and stderr are open to something, so that
    their file descriptors are not hijacked by later system calls.  */
 void
@@ -249,12 +245,10 @@ init_standard_fds (void)
   force_open (STDOUT_FILENO, O_RDONLY);
   force_open (STDERR_FILENO, O_RDONLY);
 
-  /* Set buferr if possible on platforms defining _PC_PIPE_BUF, as
+  /* Make stderr line buffered on platforms defining _PC_PIPE_BUF, as
      they support the notion of atomic writes to pipes.  */
   #ifdef _PC_PIPE_BUF
-    buferr = fdopen (STDERR_FILENO, "w");
-    if (buferr)
-      setvbuf (buferr, NULL, _IOLBF, 0);
+    setlinebuf (stderr);
   #endif
 }
 
@@ -2650,11 +2644,11 @@ safe_strsignal (int code)
 static FILE *
 errstream (void)
 {
-  FILE *err = buferr;
-  if (!err)
-    return stderr;
+#ifdef _PC_PIPE_BUF
+  /* flush line buffered stderr */
   fflush_unlocked (stderr);
-  return err;
+#endif
+  return stderr;
 }
 
 /* These functions are like fputc, vfprintf, and fwrite,
@@ -2696,9 +2690,12 @@ close_output_streams (void)
       _exit (EXIT_FAILURE);
     }
 
+  bool err;
+#ifdef _PC_PIPE_BUF
+  err = (fflush (stderr) != 0 || ferror (stderr));
+#endif
   /* Do not close stderr if addresses are being sanitized, as the
      sanitizer might report to stderr after this function is invoked.  */
-  bool err = buferr && (fflush (buferr) != 0 || ferror (buferr));
   if (err | (ADDRESS_SANITIZER
 	     ? fflush (stderr) != 0 || ferror (stderr)
 	     : close_stream (stderr) != 0))