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user_id=1534394

I found the cause of this bug:

A libc FILE* (used by python file objects) may hold a
different file offset than the underlying OS file
descriptor.  The posix version of Popen._get_handles does
not take this into account, resulting in this bug.

The following patch against svn trunk fixes the problem.  I
don't have permission to attach files to this item, so I'll
have to paste the patch here:

Index: subprocess.py
===================================================================
--- subprocess.py       (revision 51581)
+++ subprocess.py       (working copy)
@@ -907,6 +907,12 @@
             else:
                 # Assuming file-like object
                 p2cread = stdin.fileno()
+                # OS file descriptor's file offset does not
necessarily match
+                # the file offset in the file-like object,
so do an lseek:
+                try:
+                    os.lseek(p2cread, stdin.tell(), 0)
+                except OSError:
+                    pass # file descriptor does not support
seek/tell

             if stdout is None:
                 pass
@@ -917,6 +923,12 @@
             else:
                 # Assuming file-like object
                 c2pwrite = stdout.fileno()
+                # OS file descriptor's file offset does not
necessarily match
+                # the file offset in the file-like object,
so do an lseek:
+                try:
+                    os.lseek(c2pwrite, stdout.tell(), 0)
+                except OSError:
+                    pass # file descriptor does not support
seek/tell

             if stderr is None:
                 pass
@@ -929,6 +941,12 @@
             else:
                 # Assuming file-like object
                 errwrite = stderr.fileno()
+                # OS file descriptor's file offset does not
necessarily match
+                # the file offset in the file-like object,
so do an lseek:
+                try:
+                    os.lseek(errwrite, stderr.tell(), 0)
+                except OSError:
+                    pass # file descriptor does not support
seek/tell

             return (p2cread, p2cwrite,
                     c2pread, c2pwrite,