From: Ben Pfaff <blp@cs.stanford.edu>
Date: Thu, 16 Dec 2010 05:42:12 +0000 (-0800)
Subject: squish-pty: Flush buffered data from pty to stdout when pty's slave closed
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9f4d95bb41b5fa8c75449187d352ad28bd499a2a;p=pintos-anon

squish-pty: Flush buffered data from pty to stdout when pty's slave closed

When the slave side of the pty is closed, the "read" system call returns
EIO.  In this situation, squish-pty failed to flush any output that it
already had in its buffer (up to 16 bytes) to stdout.  This could cause
the "Powering off..." message printed by Pintos just before exiting to
be missing from the output.

This usually didn't happen, because there are two other exit paths from
the relay() function that does most of squish-pty's work.  The most common
exit path was (evidently) the one which detected that the child process
had died, which did properly flush the buffer.  (The third path was wrong
too, and this patch fixes that one too.)

This patch fixes the problem by using a single exit path from relay() that
always reads any remaining input from the pty (if it is open) and flushes
it to stdout.

Reported by Borja Sotomayor <borja@cs.uchicago.edu>.
---

diff --git a/src/utils/squish-pty.c b/src/utils/squish-pty.c
index c8375a5..e81043d 100644
--- a/src/utils/squish-pty.c
+++ b/src/utils/squish-pty.c
@@ -77,9 +77,8 @@ make_nonblocking (int fd, bool nonblocking)
 /* Handle a read or write on *FD, which is the pty if FD_IS_PTY
    is true, that returned end-of-file or error indication RETVAL.
    The system call is named CALL, for use in error messages.
-   Returns true if processing may continue, false if we're all
-   done. */
-static bool
+   Sets *FD to -1 if the fd is no longer readable or writable. */
+static void
 handle_error (ssize_t retval, int *fd, bool fd_is_pty, const char *call)
 {
   if (fd_is_pty)
@@ -89,13 +88,11 @@ handle_error (ssize_t retval, int *fd, bool fd_is_pty, const char *call)
           if (errno == EIO)
             {
               /* Slave side of pty has been closed. */
-              return false;
+              *fd = -1;
             }
           else
             fail_io (call); 
         }
-      else
-        return true;
     }
   else 
     {
@@ -103,7 +100,6 @@ handle_error (ssize_t retval, int *fd, bool fd_is_pty, const char *call)
         {
           close (*fd);
           *fd = -1;
-          return true;
         }
       else
         fail_io (call);
@@ -142,7 +138,7 @@ relay (int pty, int dead_child_fd)
   pipes[1].in = pty;
   pipes[1].out = STDOUT_FILENO;
   
-  while (pipes[0].in != -1 || pipes[1].in != -1)
+  while (pipes[1].in != -1)
     {
       fd_set read_fds, write_fds;
       int retval;
@@ -176,34 +172,7 @@ relay (int pty, int dead_child_fd)
         fail_io ("select");
 
       if (FD_ISSET (dead_child_fd, &read_fds))
-        {
-          /* Child died.  Do final relaying. */
-          struct pipe *p = &pipes[1];
-          if (p->out == -1)
-            return;
-          make_nonblocking (STDOUT_FILENO, false);
-          for (;;) 
-            {
-              ssize_t n;
-                  
-              /* Write buffer. */
-              while (p->size > 0) 
-                {
-                  n = write (p->out, p->buf + p->ofs, p->size);
-                  if (n < 0)
-                    fail_io ("write");
-                  else if (n == 0)
-                    fail_io ("zero-length write");
-                  p->ofs += n;
-                  p->size -= n;
-                }
-              p->ofs = 0;
-
-              p->size = n = read (p->in, p->buf, sizeof p->buf);
-              if (n <= 0)
-                return;
-            }
-        }
+        break;
 
       for (i = 0; i < 2; i++) 
         {
@@ -222,8 +191,8 @@ relay (int pty, int dead_child_fd)
                       p->ofs = 0;
                     }
                 }
-              else if (!handle_error (n, &p->in, p->in == pty, "read"))
-                return;
+              else
+                handle_error (n, &p->in, p->in == pty, "read");
             }
           if (p->out != -1 && FD_ISSET (p->out, &write_fds)) 
             {
@@ -235,11 +204,38 @@ relay (int pty, int dead_child_fd)
                   if (p->size == 0)
                     p->ofs = 0;
                 }
-              else if (!handle_error (n, &p->out, p->out == pty, "write"))
-                return;
+              else
+                handle_error (n, &p->out, p->out == pty, "write");
             }
         }
     }
+
+    if (pipes[1].out == -1)
+      return;
+
+    make_nonblocking (STDOUT_FILENO, false);
+    for (;;)
+      {
+        struct pipe *p = &pipes[1];
+        ssize_t n;
+
+        /* Write buffer. */
+        while (p->size > 0) 
+          {
+            n = write (p->out, p->buf + p->ofs, p->size);
+            if (n < 0)
+              fail_io ("write");
+            else if (n == 0)
+              fail_io ("zero-length write");
+            p->ofs += n;
+            p->size -= n;
+          }
+        p->ofs = 0;
+
+        p->size = n = read (p->in, p->buf, sizeof p->buf);
+        if (n <= 0)
+          return;
+      }
 }
 
 static int dead_child_fd;