From: Derrick Stolee <redacted>

The 'read_replace_objects' constant is initialized by git_default_config
(if core.useReplaceRefs is disabled) and within setup_git_env (if
GIT_NO_REPLACE_OBJECTS) is set. To ensure that this variable cannot be
set accidentally in other places, wrap it in a replace_refs_enabled()
method.

This allows us to remove the global from being recognized outside of
replace-objects.c.

Further, a future change will help prevent the variable from being read
before it is initialized. Centralizing its access is an important first
step.

Signed-off-by: Derrick Stolee <redacted>
---
 commit-graph.c   |  4 +---
 environment.c    |  1 -
 log-tree.c       |  2 +-
 replace-object.c |  7 +++++++
 replace-object.h | 15 ++++++++++++++-
 5 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 43558b4d9b0..95873317bf7 100644
--- a/commit-graph.c
+++ b/commit-graph.c

@@ -203,14 +203,12 @@ static struct commit_graph *alloc_commit_graph(void)
 	return g;
 }
 
-extern int read_replace_refs;
-
 static int commit_graph_compatible(struct repository *r)
 {
 	if (!r->gitdir)
 		return 0;
 
-	if (read_replace_refs) {
+	if (replace_refs_enabled(r)) {
 		prepare_replace_object(r);
 		if (hashmap_get_size(&r->objects->replace_map->map))
 			return 0;

diff --git a/replace-object.c b/replace-object.c
index ceec81c940c..cf91c3ba456 100644
--- a/replace-object.c
+++ b/replace-object.c

@@ -85,7 +85,14 @@ const struct object_id *do_lookup_replace_object(struct repository *r,
 	die(_("replace depth too high for object %s"), oid_to_hex(oid));
 }
 
+static int read_replace_refs = 1;
+
 void disable_replace_refs(void)
 {
 	read_replace_refs = 0;
 }
+
+int replace_refs_enabled(struct repository *r)
+{
+	return read_replace_refs;
+}

diff --git a/replace-object.h b/replace-object.h
index 7786d4152b0..b141075023e 100644
--- a/replace-object.h
+++ b/replace-object.h

@@ -27,6 +27,19 @@ void prepare_replace_object(struct repository *r);
 const struct object_id *do_lookup_replace_object(struct repository *r,
 						 const struct object_id *oid);
 
+
+/*
+ * Some commands disable replace-refs unconditionally, and otherwise each
+ * repository could alter the core.useReplaceRefs config value.
+ *
+ * Return 1 if and only if all of the following are true:
+ *
+ *  a. disable_replace_refs() has not been called.
+ *  b. GIT_NO_REPLACE_OBJECTS is unset or zero.
+ *  c. the given repository does not have core.useReplaceRefs=false.
+ */
+int replace_refs_enabled(struct repository *r);