Ticket #4480: Read the entire working directory from the subshell

If the subshell writes the working directory slowly, previously we could
read its beginning and stop there.

Signed-off-by: Egmont Koblinger <egmont@gmail.com>

Author: egmontkob

src/subshell/common.c

@@ -919,10 +919,23 @@ feed_subshell (int how, gboolean fail_on_error)
         }
 
         else if (FD_ISSET (subshell_pipe[READ], &read_set))
-        // Read the subshell's CWD and capture its prompt
+        // Read the subshell's CWD and later capture its prompt.
+        // We're inside an "else" branch to checking data on subshell_pty, and CWD is only written
+        // to subshell_pipe after completing the subshell command, therefore we have already read
+        // the entire output of the command.
         {
+            // CWD in subshell_pipe may not be complete yet (ticket #4480). Wait until the shell
+            // completes writing it.
+            // FIXME this deadlocks if pwd is longer than a Unix pipe's buffer size, but this
+            // should not be a problem in practice.
+            synchronize ();
+
             const ssize_t bytes = read (subshell_pipe[READ], subshell_cwd, sizeof (subshell_cwd));
 
+            // FIXME if CWD is longer than our buffer size then there are two bugs. We silently
+            // chop the value and use that directory, instead of explicitly raising an error.
+            // We also don't flush the remaining data from the pipe, breaking the next CWD read.
+
             if (bytes <= 0)
             {
                 tcsetattr (STDOUT_FILENO, TCSANOW, &shell_mode);
@@ -933,8 +946,6 @@ feed_subshell (int how, gboolean fail_on_error)
 
             subshell_cwd[(size_t) bytes - 1] = '\0';  // Squash the final '\n'
 
-            synchronize ();
-
             clear_subshell_prompt_string ();
             should_read_new_subshell_prompt = TRUE;
             subshell_ready = TRUE;