fix SIGWINCH race condition during startup

Some programs (e.g. ncmpc) do not like receiving a terminal of unknown
size. Before we issue the resizepty method that would be the case. Since
there was no synchronisation between resizepty and execvp, a program
that starts quickly could run into this situation. Now we ensure that
resizepty is called before execvp using an extra pipe.

1 files changed, 33 insertions, 6 deletions

diff --git a/tcvt.py b/tcvt.py
index 0e1a528..a257c2b 100755
--- a/tcvt.py
+++ b/tcvt.py
@@ -732,32 +732,58 @@ class ForkPty(object):
         self.environ = environ
         self.pid = -1
         self.masterfd = -1
+        self.startpipew = -1
+        self.errpiper = -1
         self.exitcode = 255
 
     def __enter__(self):
         assert self.pid == -1
         assert self.masterfd == -1
-        errpiper, errpipew = os.pipe()
+        assert self.startpipew == -1
+        assert self.errpiper == -1
+        startpiper, self.startpipew = os.pipe()
+        self.errpiper, errpipew = os.pipe()
         set_cloexec(errpipew)
         self.pid, self.masterfd = pty.fork()
         if self.pid == 0: # child
-            os.close(errpiper)
+            os.close(self.startpipew)
+            os.close(self.errpiper)
             os.environ.update(self.environ)
+            # wait for the parent
+            os.read(startpiper, 1)
+            os.close(startpiper)
             try:
                 os.execvp(self.argv[0], self.argv)
             except OSError as err:
                 os.write(errpipew, "exec failed: %s" % (err,))
             sys.exit(255)
 
+        os.close(startpiper)
         os.close(errpipew)
-        data = os.read(errpiper, 1024)
-        os.close(errpiper)
+        return self.masterfd
+
+    def start(self):
+        """Allow the process to start executing.
+        @raises ExecutionError: when execvp in the child fails
+        """
+        assert self.startpipew >= 0
+        assert self.errpiper >= 0
+        # signal that execvp can proceed
+        os.write(self.startpipew, b"\0")
+        os.close(self.startpipew)
+        self.startpipew = -1
+        # check for execvp errors
+        data = os.read(self.errpiper, 1024)
+        os.close(self.errpiper)
+        self.errpiper = -1
         if data:
             raise ExecutionError(data)
 
-        return self.masterfd
-
     def __exit__(self, *_):
+        assert self.pid > 0
+        assert self.masterfd >= 0
+        assert self.startpipew == -1
+        assert self.errpiper == -1
         os.close(self.masterfd)
         status = os.waitpid(self.pid, 0)[1]
         if status & 0xff == 0: # not killed by a signal
@@ -779,6 +805,7 @@ def main():
         with process as masterfd:
             with Terminal(acsc, options.columns, reverse=options.reverse) as t:
                 t.resizepty(masterfd)
+                process.start()
                 while True:
                     timeout = 0 if t.refresh_needed() else None
                     try: