cbits: Ensure that fork pipe doesn't shadow std fds

Previously we would assume that the pipe used to communicate
errors from the child back to the parent did not shadow the standard
descriptors (stdin, stdout, and stderr). Somewhat surprisingly, this
appears to hold most platforms. However, OpenBSD appears to be a notable
exception. This lead to the failure of the `processT251` test.

Avoid relying on this assumption by `dup`ing the pipe fds until they end
up out of the standard fd range.

Closes #266.

Author: bgamari

cbits/posix/fork_exec.c

@@ -101,6 +101,32 @@ setup_std_handle_fork(int fd,
     }
 }
 
+/* We must ensure that the fork communications pipe does not inhabit fds 0
+ * through 2 since we will need to manipulate these fds in
+ * setup_std_handle_fork while keeping the pipe available so that it can report
+ * errors. See #266.
+ */
+int unshadow_pipe_fd(int fd, char **failed_doing) {
+    if (fd <= 2) {
+        int fd2 = dup(fd);
+        if (fd2 == -1) {
+            *failed_doing = "dup(unshadow)";
+            return -1;
+        }
+
+        // This should recurse at most three times
+        int fd3 = unshadow_pipe_fd(fd2, failed_doing);
+        if (close(fd2) == -1) {
+            *failed_doing = "close(unshadow)";
+            return -1;
+        }
+
+        return fd3;
+    } else {
+        return fd;
+    }
+}
+
 /* Try spawning with fork. */
 ProcHandle
 do_spawn_fork (char *const args[],
@@ -119,6 +145,16 @@ do_spawn_fork (char *const args[],
         return -1;
     }
 
+    // Ensure that the pipe fds don't shadow stdin/stdout/stderr
+    forkCommunicationFds[0] = unshadow_pipe_fd(forkCommunicationFds[0], failed_doing);
+    if (forkCommunicationFds[0] == -1) {
+        return -1;
+    }
+    forkCommunicationFds[1] = unshadow_pipe_fd(forkCommunicationFds[1], failed_doing);
+    if (forkCommunicationFds[1] == -1) {
+        return -1;
+    }
+
     // Block signals with Haskell handlers.  The danger here is that
     // with the threaded RTS, a signal arrives in the child process,
     // the RTS writes the signal information into the pipe (which is