Re: [PATCH 2/2] Revert "repository: pre-initialize hash algo pointer"

This reverts commit e26f7f19b6c7485f04234946a59ab8f4fd21d6d1. The root
problem, git clone not setting up the_hash_algo, has been fixed in the
previous patch.

As a result of the revert, some code paths that use the_hash_algo
without initialization is revealed and fixed here. It's basically
commands that are allowed to run without a repository. The fix here is
not the best. We probably could figure out the hash algorithm from input
somehow.

Since this is a dangerous move and could potentially break stuff after
release (and leads to workaround like the reverted commit), the
workaround technically remains, but is hidden behind a new environment
variable GIT_HASH_FIXUP. This should let the users continue to use git
while we fix the problem.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 builtin/index-pack.c             | 5 +++++
 common-main.c                    | 4 ++++
 diff-no-index.c                  | 5 +++++
 repository.c                     | 2 +-
 t/helper/test-dump-split-index.c | 2 ++
 5 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 7e3e1a461c..8ee935504e 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c

@@ -1673,6 +1673,11 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
        if (prefix && chdir(prefix))
                die(_("Cannot come back to cwd"));

+       if (!the_hash_algo) {
+               warning(_("Running without a repository, assuming SHA-1 hash"));
+               repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+       }
+
        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];

diff --git a/common-main.c b/common-main.c
index 6a689007e7..12aec36794 100644
--- a/common-main.c
+++ b/common-main.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "exec_cmd.h"
 #include "attr.h"
+#include "repository.h"

 /*
  * Many parts of Git have subprograms communicate via pipe, expect the

@@ -40,5 +41,8 @@ int main(int argc, const char **argv)

        restore_sigpipe_to_default();

+       if (getenv("GIT_HASH_FIXUP"))
+               repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
        return cmd_main(argc, argv);
 }

diff --git a/diff-no-index.c b/diff-no-index.c
index 0ed5f0f496..f038f665bc 100644
--- a/diff-no-index.c
+++ b/diff-no-index.c

@@ -241,6 +241,11 @@ void diff_no_index(struct rev_info *revs,
        struct strbuf replacement = STRBUF_INIT;
        const char *prefix = revs->prefix;

+       if (!the_hash_algo) {
+               warning(_("Running without a repository, assuming SHA-1 hash"));
+               repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+       }
+
        diff_setup(&revs->diffopt);
        for (i = 1; i < argc - 2; ) {
                int j;

diff --git a/repository.c b/repository.c
index 4ffbe9bc94..0d715f4fdb 100644
--- a/repository.c
+++ b/repository.c

@@ -5,7 +5,7 @@

 /* The main repository */
 static struct repository the_repo = {
-       NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, &the_index, &hash_algos[GIT_HASH_SHA1], 0, 0
+       NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, &the_index, NULL, 0, 0