gitmodules(5) sayeth:

   submodule.<name>.branch
       A remote branch name for tracking updates in the upstream
       submodule. If the option is not specified, it defaults to master.

This doesn't allow having a "pinned" submodule that should not be updated
from upstream. We should change this to have no default --- if branch is
not specified, don't update that submodule, just like in Gerrit's
corresponding feature[1].

[1] 
https://gerrit-review.googlesource.com/Documentation/user-submodules.html#_defining_the_submodule_branch


However changing defaults is difficult as it may surprise the user,
Jonathan came up with a 4 step plan:
Step 0: introduce

        # current behavior:
        git submodule update --remote --remote-default-to-master

        # new behavior:
        git submodule update --remote --remote-pinned

and treat plain "git submodule update --remote" as --default-to-master.

Step 1: when neither --default-to-master nor --no-default-to-master
has been passed, warn when encountering a submodule with no branch
and treat it as "master".

Step 2: when neither --default-to-master nor --no-default-to-master
has been passed, error out when encountering a submodule with no
branch.

Step 3: when neither --default-to-master nor --no-default-to-master
has been passed, warn when encountering a submodule with no branch
and treat it as pinned.

Step 4: eliminate the warning.

This implements step 0 and 1.

Signed-off-by: Stefan Beller <sbel...@google.com>
---

Sorry that this took so long, this is a rough patch for steps 0 & 1,
I'd still need to update docs.

Stefan


 Documentation/gitmodules.txt | 11 ++++++-----
 builtin/submodule--helper.c  | 36 +++++++++++++++++++++++++++++-------
 git-submodule.sh             | 34 +++++++++++++++++++++++++---------
 t/t7406-submodule-update.sh  | 29 +++++++++++++++++++++++++++++
 4 files changed, 89 insertions(+), 21 deletions(-)

diff --git a/Documentation/gitmodules.txt b/Documentation/gitmodules.txt
index 4d63def206..fe42dbdb3e 100644
--- a/Documentation/gitmodules.txt
+++ b/Documentation/gitmodules.txt
@@ -50,11 +50,12 @@ submodule.<name>.update::
 
 submodule.<name>.branch::
        A remote branch name for tracking updates in the upstream submodule.
-       If the option is not specified, it defaults to 'master'.  A special
-       value of `.` is used to indicate that the name of the branch in the
-       submodule should be the same name as the current branch in the
-       current repository.  See the `--remote` documentation in
-       linkgit:git-submodule[1] for details.
+       A special value of `.` is used to indicate that the name of the
+       branch in the submodule should be the same name as the current
+       branch in the current repository.  See the `--remote` documentation
+       in linkgit:git-submodule[1] for details.
+       If not set, the default for `git submodule update --remote` is
+       to update the submodule to the superproject's recorded object id.
 
 submodule.<name>.fetchRecurseSubmodules::
        This option can be used to control recursive fetching of this
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 40844870cf..6413f2b410 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1889,7 +1889,7 @@ static int resolve_relative_path(int argc, const char 
**argv, const char *prefix
        return 0;
 }
 
-static const char *remote_submodule_branch(const char *path)
+static const char *remote_submodule_branch(const char *path, int 
default_pinned)
 {
        const struct submodule *sub;
        const char *branch = NULL;
@@ -1904,8 +1904,12 @@ static const char *remote_submodule_branch(const char 
*path)
                branch = sub->branch;
        free(key);
 
-       if (!branch)
-               return "master";
+       if (!branch) {
+               if (default_pinned)
+                       return "";
+               else
+                       return "master";
+       }
 
        if (!strcmp(branch, ".")) {
                const char *refname = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
@@ -1932,12 +1936,30 @@ static int resolve_remote_submodule_branch(int argc, 
const char **argv,
 {
        const char *ret;
        struct strbuf sb = STRBUF_INIT;
-       if (argc != 2)
-               die("submodule--helper remote-branch takes exactly one 
arguments, got %d", argc);
+       int default_pinned = 0;
+
+       struct option remote_options[] = {
+               OPT_SET_INT(0, "default-master", &default_pinned,
+                               N_("unconfigured submodules default to master 
branch"), 0),
+               OPT_SET_INT(0, "default-pinned", &default_pinned,
+                               N_("unconfigured submodules default to 
superproject object id"), 1),
+               OPT_END()
+       };
+
+       const char *const git_submodule_helper_usage[] = {
+               N_("git submodule--helper remote-branch [--default-{master, 
pinned}]  -- <path>"),
+               NULL
+       };
+
+       argc = parse_options(argc, argv, prefix, remote_options,
+                            git_submodule_helper_usage, 0);
+
+       if (argc != 1)
+               die("submodule--helper remote-branch takes exactly one path, 
got %d", argc);
 
-       ret = remote_submodule_branch(argv[1]);
+       ret = remote_submodule_branch(argv[0], default_pinned);
        if (!ret)
-               die("submodule %s doesn't exist", argv[1]);
+               die("submodule %s doesn't exist", argv[0]);
 
        printf("%s", ret);
        strbuf_release(&sb);
diff --git a/git-submodule.sh b/git-submodule.sh
index 1b568e29b9..829b90ea97 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -431,6 +431,7 @@ fetch_in_submodule () (
 #
 cmd_update()
 {
+       remote_default_option=
        # parse $args after "submodule ... update".
        while test $# -ne 0
        do
@@ -450,6 +451,12 @@ cmd_update()
                --remote)
                        remote=1
                        ;;
+               --remote-default-to-master)
+                       remote_default_option=--default-master
+                       ;;
+               --remote-pinned)
+                       remote_default_option=--default-pinned
+                       ;;
                -N|--no-fetch)
                        nofetch=1
                        ;;
@@ -555,17 +562,26 @@ cmd_update()
 
                if test -n "$remote"
                then
-                       branch=$(git submodule--helper remote-branch "$sm_path")
-                       if test -z "$nofetch"
+                       if test -z $remote_default_option
                        then
-                               # Fetch remote before determining tracking $sha1
-                               fetch_in_submodule "$sm_path" $depth ||
-                               die "$(eval_gettext "Unable to fetch in 
submodule path '\$sm_path'")"
+                               say "--remote needs clarification: 
--remote-pinned or --remote-default-to-master?"
+                               say "assuming --remote-default-to-master for 
now"
+                               remote_default_option=--default-master
+                       fi
+                       branch=$(git submodule--helper remote-branch 
${remote_default_option} -- "$sm_path")
+                       if test -n "$branch"
+                       then
+                               if test -z "$nofetch"
+                               then
+                                       # Fetch remote before determining 
tracking $sha1
+                                       fetch_in_submodule "$sm_path" $depth ||
+                                       die "$(eval_gettext "Unable to fetch in 
submodule path '\$sm_path'")"
+                               fi
+                               remote_name=$(sanitize_submodule_env; cd 
"$sm_path" && get_default_remote)
+                               sha1=$(sanitize_submodule_env; cd "$sm_path" &&
+                                       git rev-parse --verify 
"${remote_name}/${branch}") ||
+                               die "$(eval_gettext "Unable to find current 
\${remote_name}/\${branch} revision in submodule path '\$sm_path'")"
                        fi
-                       remote_name=$(sanitize_submodule_env; cd "$sm_path" && 
get_default_remote)
-                       sha1=$(sanitize_submodule_env; cd "$sm_path" &&
-                               git rev-parse --verify 
"${remote_name}/${branch}") ||
-                       die "$(eval_gettext "Unable to find current 
\${remote_name}/\${branch} revision in submodule path '\$sm_path'")"
                fi
 
                if test "$subsha1" != "$sha1" || test -n "$force"
diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index 10dc91620a..3019978211 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -260,6 +260,35 @@ test_expect_success 'submodule update --remote should 
fetch upstream changes wit
        )
 '
 
+test_expect_success 'submodule update --remote should not fetch upstream when 
no branch is set' '
+       (
+               cd super &&
+               test_might_fail git config --unset -f .gitmodules 
submodule.submodule.branch &&
+               git add .gitmodules &&
+               git commit --allow-empty -m "submodules: pin in superproject 
branch" &&
+               git -C ../submodule checkout -f master
+       ) &&
+       (
+               cd submodule &&
+               echo line4b >>file &&
+               git add file &&
+               test_tick &&
+               git commit -m "upstream line4b"
+       ) &&
+       (
+               cd super &&
+
+               git submodule update --remote-pinned --remote --force submodule 
&&
+               git status --porcelain --untracked=no --ignore-submodules=none 
>actual &&
+               test_must_be_empty actual &&
+
+               git submodule update --remote-default-to-master --remote 
--force submodule &&
+               git -C submodule log -1 --oneline >actual &&
+               git -C ../submodule log -1 --oneline >expect &&
+               test_cmp expect actual
+       )
+'
+
 test_expect_success 'local config should override .gitmodules branch' '
        (cd submodule &&
         git checkout test-branch &&
-- 
2.19.0

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