gh-138775: fix handle python -m base64 stdin correct with EOF signal

Lib/base64.py

@@ -601,7 +601,14 @@ def main():
         with open(args[0], 'rb') as f:
             func(f, sys.stdout.buffer)
     else:
-        func(sys.stdin.buffer, sys.stdout.buffer)
+        if sys.stdin.isatty():
+            # gh-138775: read input data at once when reading from stdin.
+            import io
+            data = sys.stdin.buffer.read()
+            func(io.BytesIO(data), sys.stdout.buffer)
+        else:
+            # keep the old behaviour for non-interactive input
+            func(sys.stdin.buffer, sys.stdout.buffer)
 
 
 if __name__ == '__main__':

Misc/NEWS.d/next/Library/2025-09-11-15-03-37.gh-issue-138775.w7rnSx.rst

@@ -0,0 +1 @@
+:mod:`base64`: fix reading from stdin for the command-line interface.