PoC: Do not export A

Instead of passing A as part of the process environment, we pass it
via a file. Since A is usually the greatest contributor to the process
environment, removing it from the process environment significantly
helps running into MAX_ARG_STRLEN when spawning a new child process.

This means that A is now unexported in the ebuild. However, A is
mostly used as part of the default_src_unpack function. And there A
does not need to be exported.

Proof-of-concept for https://bugs.gentoo.org/721088

Bug: https://bugs.gentoo.org/721088
Signed-off-by: Florian Schmaus <flow@gentoo.org>

bin/ebuild.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Copyright 1999-2021 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # Prevent aliases from causing portage to act inappropriately.
@@ -10,6 +10,11 @@ unalias -a
 unset BASH_COMPAT
 declare -F ___in_portage_iuse >/dev/null && export -n -f ___in_portage_iuse
 
+if [[ -v PORTAGE_EBUILD_EXTRA_SOURCE ]]; then
+	source "${PORTAGE_EBUILD_EXTRA_SOURCE}" || exit 1
+	unset PORTAGE_EBUILD_EXTRA_SOURCE
+fi
+
 source "${PORTAGE_BIN_PATH}/isolated-functions.sh" || exit 1
 
 # Set up the bash version compatibility level.  This does not disable
@@ -114,7 +119,7 @@ export PORTAGE_BZIP2_COMMAND=${PORTAGE_BZIP2_COMMAND:-bzip2}
 
 # These two functions wrap sourcing and calling respectively.  At present they
 # perform a qa check to make sure eclasses and ebuilds and profiles don't mess
-# with shell opts (shopts).  Ebuilds/eclasses changing shopts should reset them 
+# with shell opts (shopts).  Ebuilds/eclasses changing shopts should reset them
 # when they are done.
 
 __qa_source() {

lib/portage/package/ebuild/doebuild.py

@@ -2131,9 +2131,40 @@ def spawn(
         logname_backup = mysettings.configdict["env"].get("LOGNAME")
         mysettings.configdict["env"]["LOGNAME"] = logname
 
+    eapi = mysettings["EAPI"]
+
+    # TODO: Move into eapi.py
+    def eapi_does_not_export_a(eapi):
+        return eapi == "9"
+
+    dont_export_a = "dont-export-a" in mysettings.features or eapi_does_not_export_a(
+        eapi
+    )
+
+    if dont_export_a:
+        orig_env = mysettings.environ()
+        # Copy since we are potentially removing keys from the dict.
+        env = orig_env.copy()
+
+        t = env["T"]
+        if not os.path.isdir(t):
+            os.makedirs(t)
+
+        ebuildExtraSource = os.path.join(t, "portage-ebuild-extra-source")
+        with open(ebuildExtraSource, mode="w") as f:
+            for name, value in orig_env.items():
+                if name != "A":
+                    continue
+                f.write(f"{name}='{value}'\n")
+                del env[name]
+
+        env["PORTAGE_EBUILD_EXTRA_SOURCE"] = str(ebuildExtraSource)
+    else:
+        env = mysettings.environ()
+
     try:
         if keywords.get("returnpid") or keywords.get("returnproc"):
-            return spawn_func(mystring, env=mysettings.environ(), **keywords)
+            return spawn_func(mystring, env=env, **keywords)
 
         proc = EbuildSpawnProcess(
             background=False,