mkmf: Stabilize temporary filenames

[gemmaro]

Hello,

This closes mkmf generates a non-deterministic log file.
Related pull request and issue: ruby/rubygems#2481 and ruby/rubygems#6259

Thank you,

---

TODO to remove draft mark:

Details

• draft patch

From 85f034594e93ef4186ad8ec99d7d7ba5fd1759d4 Mon Sep 17 00:00:00 2001
From: gemmaro <gemmaro.dev@gmail.com>
Date: Mon, 13 Apr 2026 14:15:14 +0900
Subject: [PATCH] mkmf: Stabilize temporary filenames

---
 lib/mkmf.rb           | 12 ++++++++++++
 test/mkmf/test_log.rb | 18 ++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 test/mkmf/test_log.rb

diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 37ee4a70d9..e9cf1d7149 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -515,6 +515,17 @@ def create_tmpsrc(src)
 
   # :stopdoc:
 
+  # Try to stabilize temporary filenames as far as compilers (such as
+  # GCC or Clang) support it.  Without this, object files generated
+  # from conftest.c during feature checks may have randomized names
+  # (e.g. /tmp/ccXXXXXX.o), making the content of mkmf.log
+  # non-deterministic.
+  def stabilize_filenames
+    defined? @stabilized_filenames and return
+    @stabilized_filenames = true
+    append_cflags('-save-temps')
+  end
+
   def have_devel?
     unless defined? $have_devel
       $have_devel = true
@@ -524,6 +535,7 @@ def have_devel?
   end
 
   def try_do(src, command, **opts, &b)
+    stabilize_filenames
     unless have_devel?
       raise <<MSG
 The compiler failed to generate an executable file.
diff --git a/test/mkmf/test_log.rb b/test/mkmf/test_log.rb
new file mode 100644
index 0000000000..2030e583d1
--- /dev/null
+++ b/test/mkmf/test_log.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: false
+require_relative 'base'
+
+class TestMkmfLog < TestMkmf
+  def test_mkmf_log_is_deterministic
+    %w[gcc clang].include?(RbConfig::MAKEFILE_CONFIG["CC"]) \
+      or omit 'Only supported with GCC or Clang'
+
+    have_func('non_existent_function')
+    have_func('non_existent_function')
+
+    # deterministic: ld: conftest.o: in function `t':
+    # non-deterministic: ld: /path/to/ccXXXXXX.o: in function `t':
+    ld1, ld2 = MKMFLOG[].scan(/^ld: .*?[.]o: in function `t':/)
+
+    assert_equal ld1, ld2
+  end
+end
-- 
2.52.0

• omnibus compilation number 5 C99

    compiling /github/workspace/src/ext/-test-/RUBY_ALIGNOF/cpp.cpp
  c.i:1:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 1 "/github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c"
        |     ^
  /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 1 "<built-in>" 1
        |     ^
  /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 1 "/github/workspace/src/include/ruby.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  /github/workspace/src/include/ruby.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 38 "/github/workspace/src/include/ruby.h"
        |      ^
  /github/workspace/src/include/ruby.h:38:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     38 | # 1 "/github/workspace/src/include/ruby/ruby.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  /github/workspace/src/include/ruby/ruby.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 15 "/github/workspace/src/include/ruby/ruby.h"
        |      ^
  /github/workspace/src/include/ruby/ruby.h:15:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     15 | # 1 "/github/workspace/src/include/ruby/internal/config.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  /github/workspace/src/include/ruby/internal/config.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 22 "/github/workspace/src/include/ruby/internal/config.h"
        |      ^
  /github/workspace/src/include/ruby/internal/config.h:22:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     22 | # 1 "../../../.ext/include/x86_64-linux/ruby/config.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:22:
  ../../../.ext/include/x86_64-linux/ruby/config.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 23 "/github/workspace/src/include/ruby/internal/config.h" 2
        |      ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  /github/workspace/src/include/ruby/internal/config.h:25:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     25 | # 1 "./extconf.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:25:
  ./extconf.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 26 "/github/workspace/src/include/ruby/internal/config.h" 2
        |      ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  /github/workspace/src/include/ruby/internal/config.h:28:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     28 | # 1 "/github/workspace/src/include/ruby/internal/compiler_since.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:28:
  /github/workspace/src/include/ruby/internal/compiler_since.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 23 "/github/workspace/src/include/ruby/internal/compiler_since.h"
        |      ^
  /github/workspace/src/include/ruby/internal/compiler_since.h:23:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     23 | # 1 "/github/workspace/src/include/ruby/internal/compiler_is.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:28:
  In file included from /github/workspace/src/include/ruby/internal/compiler_since.h:23:
  /github/workspace/src/include/ruby/internal/compiler_is.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 32 "/github/workspace/src/include/ruby/internal/compiler_is.h"
        |      ^
  /github/workspace/src/include/ruby/internal/compiler_is.h:32:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     32 | # 1 "/github/workspace/src/include/ruby/internal/compiler_is/apple.h" 1
        |     ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:28:
  In file included from /github/workspace/src/include/ruby/internal/compiler_since.h:23:
  In file included from /github/workspace/src/include/ruby/internal/compiler_is.h:32:
  /github/workspace/src/include/ruby/internal/compiler_is/apple.h:1:6: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
      1 | # 33 "/github/workspace/src/include/ruby/internal/compiler_is.h" 2
        |      ^
  In file included from /github/workspace/src/ext/-test-/RUBY_ALIGNOF/c.c:1:
  In file included from /github/workspace/src/include/ruby.h:38:
  In file included from /github/workspace/src/include/ruby/ruby.h:15:
  In file included from /github/workspace/src/include/ruby/internal/config.h:28:
  In file included from /github/workspace/src/include/ruby/internal/compiler_since.h:23:
  /github/workspace/src/include/ruby/internal/compiler_is.h:33:5: error: this style of line directive is a GNU extension [-Werror,-Wgnu-line-marker]
     33 | # 1 "/github/workspace/src/include/ruby/internal/compiler_is/clang.h" 1
        |     ^
  fatal error: too many errors emitted, stopping now [-ferror-limit=]
  linking shared-object -test-/arith_seq/beg_len_step.so
  linking shared-object -test-/arith_seq/extract.so
  linking shared-object -test-/array/to_ary_concat.so
  make[2]: Leaving directory '/tmp/tmp.Gm1VrLZN8s/ext/-test-/arith_seq/beg_len_step'
  make[2]: Leaving directory '/tmp/tmp.Gm1VrLZN8s/ext/-test-/arith_seq/extract'
  make[2]: Entering directory '/tmp/tmp.Gm1VrLZN8s/ext/-test-/array/resize'
  compiling /github/workspace/src/ext/-test-/array/resize/resize.c
  make[2]: Entering directory '/tmp/tmp.Gm1VrLZN8s/ext/-test-/bignum'
  compiling /github/workspace/src/ext/-test-/bignum/big2str.c
  20 errors generated.
  make[2]: *** [Makefile:257: c.o] Error 1
  

Memo 1: Adding empty commit passes CI, so the patch I added surely causes CI failing.
Memo 2: Targeting only GCC (excluding Clang) doesn't mitigate the issue -- in the contrary, it fails more.