libstore: remove impersonate-linux-26 setting

it's ancient, it does not do what it was added for (improving
determinism), and it probably hasn't been needed for anything
in a very long time. we can probably get away with killing it

Change-Id: I66f5c841964d9ad62726cb2dae8d6fc42095399f

doc/manual/rl-next/impersonate-linux-26-removed.md

@@ -0,0 +1,14 @@
+---
+synopsis: "`impersonate-linux-26` setting removed"
+cls: [5047]
+category: "Miscellany"
+credits: [horrors]
+---
+
+Linux 3.0 was released 15 years ago. The `impersonate-linux-26` setting was added
+14 years ago with no mention of it being necessary to build anything, only saying
+that it improves determinism—which isn't accurate since impersonating Linux 2.6.x
+still allows the version string to change, and the final component of the version
+does still change with each Linux release. Since this setting should be no longer
+necessary in modern systems and workarounds for building old code exist (by using
+e.g. `setarch --uname-2.6` to wrap builds) we are removing this setting from Lix.

lix/libstore/build/personality.cc

@@ -28,13 +28,6 @@ void setPersonality(std::string_view system)
                 throw SysError("cannot set 32-bit personality");
         }
 
-        /* Impersonate a Linux 2.6 machine to get some determinism in
-           builds that depend on the kernel version. */
-        if ((system == "i686-linux" || system == "x86_64-linux") && settings.impersonateLinux26) {
-            int cur = personality(0xffffffff);
-            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
-        }
-
         /* Disable address space randomization for improved
            determinism. */
         int cur = personality(0xffffffff);

lix/libstore/build/personality.hh

@@ -1,10 +1,8 @@
 #pragma once
 ///@file
 
-#include <string>
+#include <string_view>
 
 namespace nix {
-
 void setPersonality(std::string_view system);
-
 }

lix/libstore/meson.build

@@ -100,7 +100,6 @@ libstore_setting_definitions = files(
   'settings/hashed-mirrors.md',
   'settings/id-count.md',
   'settings/ignored-acls.md',
-  'settings/impersonate-linux-26.md',
   'settings/keep-build-log.md',
   'settings/keep-derivations.md',
   'settings/keep-env-derivations.md',

lix/libstore/settings/impersonate-linux-26.md

@@ -1,9 +0,0 @@
----
-name: impersonate-linux-26
-internalName: impersonateLinux26
-platforms: [linux]
-type: bool
-default: false
-aliases: [build-impersonate-linux-26]
----
-Whether to impersonate a Linux 2.6 machine on newer kernels.