mitogen: Send first stage parameters as argv (796 bytes -> 822)

Benefit: The base64 lump is now static for a given Mitogen version, and the
variable parts are more visible. This will make debugging, auditting, and
allow-listing a bit easier.
Potential benefit: generate the base64 once, at build time or startup. Rather
than once per connection.
Cost: Bootstrap command is 26 bytes longer.

```
➜  mitogen git:(boot-cmd--argv) ✗ ./preamble_size.py
SSH command size: 822
Preamble (mitogen.core + econtext) size: 18230 (17.80KiB)

                        Original           Minimized           Compressed
mitogen.core         152237 148.7KiB  68453 66.8KiB 45.0%  18130 17.7KiB 11.9%
mitogen.parent        98746  96.4KiB  51215 50.0KiB 51.9%  12922 12.6KiB 13.1%
mitogen.fork           8445   8.2KiB   4139  4.0KiB 49.0%   1652  1.6KiB 19.6%
mitogen.ssh           10847  10.6KiB   6913  6.8KiB 63.7%   2102  2.1KiB 19.4%
mitogen.sudo          12089  11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select        12325  12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB  7.8%
mitogen.service       41581  40.6KiB  22398 21.9KiB 53.9%   5847  5.7KiB 14.1%
mitogen.fakessh       15753  15.4KiB   8135  7.9KiB 51.6%   2672  2.6KiB 17.0%
mitogen.master        52891  51.7KiB  27586 26.9KiB 52.2%   7129  7.0KiB 13.5%
```

docs/changelog.rst

@@ -21,6 +21,9 @@ To avail of fixes in an unreleased version, please download a ZIP file
 In progress (unreleased)
 ------------------------
 
+* :gh:issue:`1243` :mod:`mitogen`: Pass first stage, context name, & preamble
+  size as seperate **argv** arguments
+
 
 v0.3.31 (2025-11-05)
 --------------------

docs/howitworks.rst

@@ -27,14 +27,13 @@ Python Command Line
 ###################
 
 The Python command line sent to the host is a :mod:`zlib`-compressed [#f2]_ and
-base64-encoded copy of the :py:meth:`mitogen.master.Stream._first_stage`
-function, which has been carefully optimized to reduce its size. Prior to
-compression and encoding, ``CONTEXT_NAME`` is replaced with the desired context
-name in the function's source code.
+base64-encoded copy of :py:meth:`mitogen.parent.Connection._first_stage`,
+which is carefully written to maximize it compatibility and minimize its size.
+A simplified illustration of the bootstrap command is
 
 .. code::
 
-    python -c 'exec "xxx".decode("base64").decode("zlib")'
+    python -c 'exec(sys.argv[1].decode("base64").decode("zlib"))' <base64> ...
 
 The command-line arranges for the Python interpreter to decode the base64'd
 component, decompress it and execute it as Python code. Base64 is used since
@@ -71,8 +70,8 @@ of the large base64-encoded first stage parameter, and to replace **argv[0]**
 with something descriptive.
 
 After configuring its ``stdin`` to point to the read end of the pipe, the
-parent half of the fork re-executes Python, with **argv[0]** taken from the
-``CONTEXT_NAME`` variable earlier substituted into its source code. As no
+fork parent re-executes Python with **argv[0]** composed of the Python
+interpreter path and a remote name supplied by the Mitogen parent.  As no
 arguments are provided to this new execution of Python, and since ``stdin`` is
 connected to a pipe (whose write end is connected to the first stage), the
 Python interpreter begins reading source code to execute from the pipe

mitogen/parent.py

@@ -1396,10 +1396,6 @@ class Connection(object):
     # with a custom argv.
     #   * Optimized for minimum byte count after minification & compression.
     #     The script preamble_size.py measures this.
-    #   * 'CONTEXT_NAME' and 'PREAMBLE_COMPRESSED_LEN' are substituted with
-    #     their respective values.
-    #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
-    #     order to allow virtualenvs to detect their install prefix.
     #
     # macOS tweaks for Python 2.7 must be kept in sync with the the Ansible
     # module test_echo_module, used by the integration tests.
@@ -1439,10 +1435,10 @@ class Connection(object):
             if os.uname()[0]+os.uname()[2][:2]+sys.version[:3]=='Darwin202.7':os.environ['PYTHON_LAUNCHED_FROM_WRAPPER']='1'
             if os.uname()[0]+os.uname()[2][:2]+sys.version[:3]=='Darwin212.7':os.environ['PYTHON_LAUNCHED_FROM_WRAPPER']='1'
             os.environ['ARGV0']=sys.executable
-            os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
+            os.execl(sys.executable,sys.executable+'(mitogen:%s)'%sys.argv[2])
         os.write(1,'MITO000\n'.encode())
         C=''.encode()
-        while PREAMBLE_COMPRESSED_LEN-len(C)and select.select([0],[],[]):C+=os.read(0,PREAMBLE_COMPRESSED_LEN-len(C))
+        while int(sys.argv[3])-len(C)and select.select([0],[],[]):C+=os.read(0,int(sys.argv[3])-len(C))
         C=zlib.decompress(C)
         f=os.fdopen(W,'wb',0)
         f.write(C)
@@ -1470,10 +1466,6 @@ class Connection(object):
         source = inspect.getsource(self._first_stage)
         source = textwrap.dedent('\n'.join(source.strip().split('\n')[2:]))
         source = source.replace('    ', ' ')
-        source = source.replace('CONTEXT_NAME', self.options.remote_name)
-        preamble_compressed = self.get_preamble()
-        source = source.replace('PREAMBLE_COMPRESSED_LEN',
-                                str(len(preamble_compressed)))
         compressor = zlib.compressobj(
             zlib.Z_BEST_COMPRESSION, zlib.DEFLATED, -zlib.MAX_WBITS,
         )
@@ -1490,6 +1482,8 @@ class Connection(object):
             'import binascii,os,select,zlib;'
             'exec(zlib.decompress(binascii.a2b_base64(sys.argv[1]),-15))',
             encoded.decode(),
+            self.options.remote_name,
+            str(len(self.get_preamble())),
         ]
 
     def get_econtext_config(self):

tests/first_stage_test.py

@@ -26,7 +26,7 @@ class CommandLineTest(testlib.RouterMixin, testlib.TestCase):
         # preamble from stdin, then execute it.
 
         # This test attaches /dev/zero to stdin to create a specific failure
-        # 1. Fork child reads PREAMBLE_COMPRESSED_LEN bytes of junk (all `\0`)
+        # 1. Fork child reads <compressed preamble size> bytes of NUL (`b'\0'`)
         # 2. Fork child crashes (trying to decompress the junk data)
         # 3. Fork child's file descriptors (write pipes) are closed by the OS
         # 4. Fork parent does `dup(<read pipe>, <stdin>)` and `exec(<python>)`