generate-alt-random: wait for SIGWINCH if width/height is 0

If width/height (that is, the *pixel* values) are 0, that means we are
early (or that foot is slow) - we’ve managed to reach this point
before the foot window has been mapped.

Or rather, before foot has loaded the primary fonts and calculated the
cell geometry. But that, and the initial mapping of the window is
tightly coupled.

To handle this case, detect when width or height is 0, an then wait
for SIGWINCH before trying again.

scripts/generate-alt-random-writes.py

@@ -2,8 +2,9 @@
 import argparse
 import enum
 import fcntl
-import struct
 import random
+import signal
+import struct
 import sys
 import termios
 
@@ -38,11 +39,28 @@ def main():
     out = opts.out if opts.out is not None else sys.stdout
 
     try:
-        lines, cols, height, width = struct.unpack(
-            'HHHH',
-            fcntl.ioctl(sys.stdout.fileno(),
-                        termios.TIOCGWINSZ,
-                        struct.pack('HHHH', 0, 0, 0, 0)))
+        def dummy(*args):
+            """Need a handler installed for sigwait() to trigger."""
+            pass
+        signal.signal(signal.SIGWINCH, dummy)
+
+        while True:
+            lines, cols, height, width = struct.unpack(
+                'HHHH',
+                fcntl.ioctl(sys.stdout.fileno(),
+                            termios.TIOCGWINSZ,
+                            struct.pack('HHHH', 0, 0, 0, 0)))
+
+            if width > 0 and height > 0:
+                break
+
+            # We’re early; the foot window hasn’t been mapped yet. Or,
+            # to be more precise, fonts haven’t yet been loaded,
+            # meaning it doesn’t have any cell geometry yet.
+            signal.sigwait([signal.SIGWINCH])
+
+        signal.signal(signal.SIGWINCH, signal.SIG_DFL)
+
     except OSError:
         lines = None
         cols = None