From 4be4341ccc98be0bc2767ba73c654790c7db38f5 Mon Sep 17 00:00:00 2001
From: Nick Johnson <sylvyrfysh@gmail.com>
Date: Sun, 12 Jul 2026 12:46:54 -0500
Subject: [PATCH] =?UTF-8?q?=E2=9C=A8=20Enable=20building=20mold=20with=20g?=
 =?UTF-8?q?it=20reftable=20backend?=
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The git reftable backend mirrors the structure of the old backend, but
only uses a binary table. This means that mold's current checks detect
the git repository as intact, buta ttempt to read refs that do not
exist.

To fix this, first check if the HEAD points to "refs/heads/.invalid",
which is how reftable is indicated without using the git CLI. Once we
know we are in a reftable repo, use a new option, MOLD_NO_GIT, to gate
attempts to find and use the Git executable. This enables us to attempt
to use git while maintaing the same fallback paths of no hash if git is
not present.

Resolves: #1621
---
 lib/update-git-hash.cmake | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/lib/update-git-hash.cmake b/lib/update-git-hash.cmake
index 42fc95378e..e54015849d 100644
--- a/lib/update-git-hash.cmake
+++ b/lib/update-git-hash.cmake
@@ -6,7 +6,37 @@ if(EXISTS "${SOURCE_DIR}/.git/HEAD")
 
   if(HASH MATCHES "^ref: (.*)")
     set(HEAD "${CMAKE_MATCH_1}")
-    if(EXISTS "${SOURCE_DIR}/.git/${HEAD}")
+    if(HEAD STREQUAL "refs/heads/.invalid")
+      # Git v2.45+ with config `feature.experimental`=true, config `extensions.refstorage`=reftable,
+      # or `--ref-format=reftable`, uses the new binary reftable format, which does not expose
+      # plaintext files and breaks our no-Git wishes as the HEAD hash is not directly stored.
+      #
+      # Provide an option to disable Git and fall back to no hash if desired. Else, best-effort
+      # attempt to find ans use Git.
+      option(MOLD_NO_GIT "Do not look for Git if the new reftable format is in use" OFF)
+      if(MOLD_NO_GIT)
+        set(HASH "")
+      else()
+        find_package(Git QUIET)
+        if (NOT Git_FOUND)
+          message(WARNING "In a git repo with a reftable format, but could not find Git. No hash will be embedded in the binary.")
+          set(HASH "")
+        else()
+          execute_process(
+            COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
+            OUTPUT_VARIABLE git_output
+            RESULT_VARIABLE git_result
+            ERROR_QUIET
+            OUTPUT_STRIP_TRAILING_WHITESPACE
+          )
+          if(git_result EQUAL 0)
+            set(HASH "${git_output}")
+          else()
+            message(WARNING "Git returned ${git_result} when querying the HEAD revision")
+          endif()
+        endif()
+      endif()
+    elseif(EXISTS "${SOURCE_DIR}/.git/${HEAD}")
       file(READ "${SOURCE_DIR}/.git/${HEAD}" HASH)
       string(STRIP "${HASH}" HASH)
     else()