mitogen: first_stage: Break the while loop in case of EOF

The current implementation can cause an infinite loop, leading to a process that
hangs and consumes 100% CPU. This occurs because the EOF condition is not
handled properly, resulting in repeated select(...) and read(...) calls.

The fix is to properly handle the EOF condition and break out of the loop when
it occurs.

-SSH command size: 822
+SSH command size: 838

-mitogen.parent        98746  96.4KiB  51215 50.0KiB 51.9%  12922 12.6KiB 13.1%
+mitogen.parent        98827  96.5KiB  51219 50.0KiB 51.8%  12942 12.6KiB 13.1%

Fixes: #1348
Signed-off-by: Marc Hartmayer <[email protected]>

Author: mhartmay

mitogen/parent.py

@@ -1416,6 +1416,8 @@ def __repr__(self):
     #   r: read side of core_src FD.
     #   w: write side of core_src FD.
     #   C: the decompressed core source.
+    #   n: size of the compressed core source to be read
+    #   V: data chunk
 
     # Final os.close(STDOUT_FILENO) to avoid --py-debug build corrupting stream with
     # "[1234 refs]" during exit.
@@ -1437,8 +1439,8 @@ def _first_stage():
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:%s)'%sys.argv[2])
         os.write(1,'MITO000\n'.encode())
-        C=''.encode()
-        while int(sys.argv[3])-len(C)and select.select([0],[],[]):C+=os.read(0,int(sys.argv[3])-len(C))
+        n=int(sys.argv[3]);C=''.encode();V='V'
+        while n>len(C) and V:select.select([0],[],[]);V=os.read(0,n-len(C));C+=V
         C=zlib.decompress(C)
         f=os.fdopen(W,'wb',0)
         f.write(C)