refs: introduce a "ref paranoia" flag

Most operations that iterate over refs are happy to ignore
broken cruft. However, some operations should be performed
with knowledge of these broken refs, because it is better
for the operation to choke on a missing object than it is to
silently pretend that the ref did not exist (e.g., if we are
computing the set of reachable tips in order to prune
objects).

These processes could just call for_each_rawref, except that
ref iteration is often hidden behind other interfaces. For
instance, for a destructive "repack -ad", we would have to
inform "pack-objects" that we are destructive, and then it
would in turn have to tell the revision code that our
"--all" should include broken refs.

It's much simpler to just set a global for "dangerous"
operations that includes broken refs in all iterations.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -1026,6 +1026,17 @@ GIT_ICASE_PATHSPECS::
 	variable when it is invoked as the top level command by the
 	end user, to be recorded in the body of the reflog.
 
+`GIT_REF_PARANOIA`::
+	If set to `1`, include broken or badly named refs when iterating
+	over lists of refs. In a normal, non-corrupted repository, this
+	does nothing. However, enabling it may help git to detect and
+	abort some operations in the presence of broken refs. Git sets
+	this variable automatically when performing destructive
+	operations like linkgit:git-prune[1]. You should not need to set
+	it yourself unless you want to be paranoid about making sure
+	an operation has touched every ref (e.g., because you are
+	cloning a repository to make a backup).
+
 
 Discussion[[Discussion]]
 ------------------------

cache.h

@@ -613,6 +613,14 @@ extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
 
+/*
+ * Include broken refs in all ref iterations, which will
+ * generally choke dangerous operations rather than letting
+ * them silently proceed without taking the broken ref into
+ * account.
+ */
+extern int ref_paranoia;
+
 /*
  * The character that begins a commented line in user-editable file
  * that is subject to stripspace.

environment.c

@@ -24,6 +24,7 @@ int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;
+int ref_paranoia = -1;
 int repository_format_version;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;

refs.c

@@ -1907,6 +1907,11 @@ static int do_for_each_ref(struct ref_cache *refs, const char *base,
 	data.fn = fn;
 	data.cb_data = cb_data;
 
+	if (ref_paranoia < 0)
+		ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
+	if (ref_paranoia)
+		data.flags |= DO_FOR_EACH_INCLUDE_BROKEN;
+
 	return do_for_each_entry(refs, base, do_one_ref, &data);
 }