[PATCH v3] tee: handle EAGAIN returned from fwrite/fclose()

[Kamil Dudka]

'tee' expected the output file descriptors to operate in blocking mode
but this assumption might be invalidated by other programs connected to
the same terminal, as in the following example:

$ telnet ... | tee log_file

telnet calls ioctl(stdin, FIONBIO, [1]), which causes the O_NONBLOCK
flag to be set on tee's output, which is connected to the same terminal.

This patch has zero impact unless EAGAIN returns from fwrite/fclose().
In that case 'tee' waits for the underlying file descriptor to become
writable again and then it writes the remaining data.
---
 src/tee.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 73 insertions(+), 2 deletions(-)

diff --git a/src/tee.c b/src/tee.c
index dae0f1e50..264560e81 100644
--- a/src/tee.c
+++ b/src/tee.c
@@ -17,6 +17,8 @@
 /* Mike Parker, Richard M. Stallman, and David MacKenzie */
 
 #include <config.h>
+#include <assert.h>
+#include <poll.h>
 #include <sys/types.h>
 #include <signal.h>
 #include <getopt.h>
@@ -176,6 +178,75 @@ main (int argc, char **argv)
   return ok ? EXIT_SUCCESS : EXIT_FAILURE;
 }
 
+/* On EAGAIN, wait for the underlying file descriptor to become writable.
+   Return true, if EAGAIN has been successfully handled. */
+static bool
+handle_eagain_on_write (FILE *f)
+{
+  struct pollfd pfd;
+  if (errno != EAGAIN)
+    /* non-recoverable write error */
+    return false;
+
+  /* obtain the file descriptor we are writing to */
+  pfd.fd = fileno (f);
+  if (pfd.fd == -1)
+    goto fail;
+
+  /* wait for the file descriptor to become writable */
+  pfd.events = POLLOUT;
+  pfd.revents = 0;
+  if (poll (&pfd, 1, -1) != 1)
+    goto fail;
+  if (!(pfd.revents & POLLOUT))
+    goto fail;
+
+  /* successfully waited for the descriptor to become writable */
+  clearerr(f);
+  return true;
+
+fail:
+  errno = EAGAIN;
+  return false;
+}
+
+/* wrapper of fwrite() that handles EAGAIN properly in case the O_NONBLOCK
+   flag was set on the output file descriptor */
+static bool
+write_buf (const char *buf, ssize_t size, FILE *f)
+{
+  for (;;)
+    {
+      const size_t written = fwrite (buf, 1, size, f);
+      size -= written;
+      assert (size >= 0);
+      if (size <= 0)
+        /* everything written */
+        return true;
+
+      if (!handle_eagain_on_write (f))
+        return false;
+
+      /* update position in the write buffer */
+      buf += written;
+    }
+}
+
+/* wrapper of fclose() that handles EAGAIN properly in case the O_NONBLOCK
+   flag was set on the output file descriptor */
+static bool
+close_file (FILE *f)
+{
+  for (;;)
+    {
+      if (fclose (f) == 0)
+        return true;
+
+      if (!handle_eagain_on_write (f))
+        return false;
+    }
+}
+
 /* Copy the standard input into each of the NFILES files in FILES
    and into the standard output.  As a side effect, modify FILES[-1].
    Return true if successful.  */
@@ -238,7 +309,7 @@ tee_files (int nfiles, char **files)
          Standard output is the first one.  */
       for (i = 0; i <= nfiles; i++)
         if (descriptors[i]
-            && fwrite (buffer, bytes_read, 1, descriptors[i]) != 1)
+            && !write_buf (buffer, bytes_read, descriptors[i]))
           {
             int w_errno = errno;
             bool fail = errno != EPIPE || (output_error == output_error_exit
@@ -266,7 +337,7 @@ tee_files (int nfiles, char **files)
 
   /* Close the files, but not standard output.  */
   for (i = 1; i <= nfiles; i++)
-    if (descriptors[i] && fclose (descriptors[i]) != 0)
+    if (descriptors[i] && !close_file (descriptors[i]))
       {
         error (0, errno, "%s", quotef (files[i]));
         ok = false;
-- 
2.17.1