This commit adds the option `--continue` which is used to resume
rebase after merge conflicts. The code tries to stay as close to
the equivalent shell scripts found in `git-legacy-rebase.sh` as
possible.

When continuing a rebase, the state variables are read from state_dir.
Some of the state variables are not actually stored there, such as
`upstream`. The shell script version simply does not set them, but for
consistency, we unset them in the builtin version.

Signed-off-by: Pratik Karki <predatoram...@gmail.com>
---
 builtin/rebase.c | 115 +++++++++++++++++++++++++++++++++++++++++++++--
 strbuf.c         |   9 ++++
 strbuf.h         |   3 ++
 3 files changed, 123 insertions(+), 4 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b2ddfa8dbf..10da4c978b 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -91,6 +91,7 @@ struct rebase_options {
                REBASE_INTERACTIVE_EXPLICIT = 1<<4,
        } flags;
        struct strbuf git_am_opt;
+       const char *action;
 };
 
 static int is_interactive(struct rebase_options *opts)
@@ -115,6 +116,62 @@ static const char *state_dir_path(const char *filename, 
struct rebase_options *o
        return path.buf;
 }
 
+/* Read one file, then strip line endings */
+static int read_one(const char *path, struct strbuf *buf)
+{
+       if (strbuf_read_file(buf, path, 0) < 0)
+               return error_errno(_("could not read '%s'"), path);
+       strbuf_trim_trailing_newline(buf);
+       return 0;
+}
+
+/* Initialize the rebase options from the state directory. */
+static int read_basic_state(struct rebase_options *opts)
+{
+       struct strbuf head_name = STRBUF_INIT;
+       struct strbuf buf = STRBUF_INIT;
+       struct object_id oid;
+
+       if (read_one(state_dir_path("head-name", opts), &head_name) ||
+           read_one(state_dir_path("onto", opts), &buf))
+               return -1;
+       opts->head_name = starts_with(head_name.buf, "refs/") ?
+               xstrdup(head_name.buf) : NULL;
+       strbuf_release(&head_name);
+       if (get_oid(buf.buf, &oid))
+               return error(_("could not get 'onto': '%s'"), buf.buf);
+       opts->onto = lookup_commit_or_die(&oid, buf.buf);
+
+       /*
+        * We always write to orig-head, but interactive rebase used to write to
+        * head. Fall back to reading from head to cover for the case that the
+        * user upgraded git with an ongoing interactive rebase.
+        */
+       strbuf_reset(&buf);
+       if (file_exists(state_dir_path("orig-head", opts))) {
+               if (read_one(state_dir_path("orig-head", opts), &buf))
+                       return -1;
+       } else if (read_one(state_dir_path("head", opts), &buf))
+               return -1;
+       if (get_oid(buf.buf, &opts->orig_head))
+               return error(_("invalid orig-head: '%s'"), buf.buf);
+
+       strbuf_reset(&buf);
+       if (read_one(state_dir_path("quiet", opts), &buf))
+               return -1;
+       if (buf.len)
+               opts->flags &= ~REBASE_NO_QUIET;
+       else
+               opts->flags |= REBASE_NO_QUIET;
+
+       if (file_exists(state_dir_path("verbose", opts)))
+               opts->flags |= REBASE_VERBOSE;
+
+       strbuf_release(&buf);
+
+       return 0;
+}
+
 static int finish_rebase(struct rebase_options *opts)
 {
        struct strbuf dir = STRBUF_INIT;
@@ -168,12 +225,13 @@ static int run_specific_rebase(struct rebase_options 
*opts)
        add_var(&script_snippet, "state_dir", opts->state_dir);
 
        add_var(&script_snippet, "upstream_name", opts->upstream_name);
-       add_var(&script_snippet, "upstream",
-                                oid_to_hex(&opts->upstream->object.oid));
+       add_var(&script_snippet, "upstream", opts->upstream ?
+               oid_to_hex(&opts->upstream->object.oid) : NULL);
        add_var(&script_snippet, "head_name",
                opts->head_name ? opts->head_name : "detached HEAD");
        add_var(&script_snippet, "orig_head", oid_to_hex(&opts->orig_head));
-       add_var(&script_snippet, "onto", oid_to_hex(&opts->onto->object.oid));
+       add_var(&script_snippet, "onto", opts->onto ?
+               oid_to_hex(&opts->onto->object.oid) : NULL);
        add_var(&script_snippet, "onto_name", opts->onto_name);
        add_var(&script_snippet, "revisions", opts->revisions);
        add_var(&script_snippet, "restrict_revision", opts->restrict_revision ?
@@ -189,6 +247,7 @@ static int run_specific_rebase(struct rebase_options *opts)
                opts->flags & REBASE_FORCE ? "t" : "");
        if (opts->switch_to)
                add_var(&script_snippet, "switch_to", opts->switch_to);
+       add_var(&script_snippet, "action", opts->action ? opts->action : "");
 
        switch (opts->type) {
        case REBASE_AM:
@@ -400,12 +459,16 @@ int cmd_rebase(int argc, const char **argv, const char 
*prefix)
                .git_am_opt = STRBUF_INIT,
        };
        const char *branch_name;
-       int ret, flags, in_progress = 0;
+       int ret, flags, total_argc, in_progress = 0;
        int ok_to_skip_pre_rebase = 0;
        struct strbuf msg = STRBUF_INIT;
        struct strbuf revisions = STRBUF_INIT;
        struct strbuf buf = STRBUF_INIT;
        struct object_id merge_base;
+       enum {
+               NO_ACTION,
+               ACTION_CONTINUE,
+       } action = NO_ACTION;
        struct option builtin_rebase_options[] = {
                OPT_STRING(0, "onto", &options.onto_name,
                           N_("revision"),
@@ -427,6 +490,8 @@ int cmd_rebase(int argc, const char **argv, const char 
*prefix)
                OPT_BIT(0, "no-ff", &options.flags,
                        N_("cherry-pick all commits, even if unchanged"),
                        REBASE_FORCE),
+               OPT_CMDMODE(0, "continue", &action, N_("continue"),
+                           ACTION_CONTINUE),
                OPT_END(),
        };
 
@@ -480,14 +545,55 @@ int cmd_rebase(int argc, const char **argv, const char 
*prefix)
        if (options.type != REBASE_UNSPECIFIED)
                in_progress = 1;
 
+       total_argc = argc;
        argc = parse_options(argc, argv, prefix,
                             builtin_rebase_options,
                             builtin_rebase_usage, 0);
 
+       if (action != NO_ACTION && total_argc != 2) {
+               usage_with_options(builtin_rebase_usage,
+                                  builtin_rebase_options);
+       }
+
        if (argc > 2)
                usage_with_options(builtin_rebase_usage,
                                   builtin_rebase_options);
 
+       switch (action) {
+       case ACTION_CONTINUE: {
+               struct object_id head;
+               struct lock_file lock_file = LOCK_INIT;
+               int fd;
+
+               options.action = "continue";
+
+               /* Sanity check */
+               if (get_oid("HEAD", &head))
+                       die(_("Cannot read HEAD"));
+
+               fd = hold_locked_index(&lock_file, 0);
+               if (read_index(the_repository->index) < 0)
+                       die(_("could not read index"));
+               refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL,
+                             NULL);
+               if (0 <= fd)
+                       update_index_if_able(the_repository->index,
+                                            &lock_file);
+               rollback_lock_file(&lock_file);
+
+               if (has_unstaged_changes(1)) {
+                       puts(_("You must edit all merge conflicts and then\n"
+                              "mark them as resolved using git add"));
+                       exit(1);
+               }
+               if (read_basic_state(&options))
+                       exit(1);
+               goto run_rebase;
+       }
+       default:
+               die("TODO");
+       }
+
        /* Make sure no rebase is in progress */
        if (in_progress) {
                const char *last_slash = strrchr(options.state_dir, '/');
@@ -719,6 +825,7 @@ int cmd_rebase(int argc, const char **argv, const char 
*prefix)
 
        options.revisions = revisions.buf;
 
+run_rebase:
        ret = !!run_specific_rebase(&options);
 
 cleanup:
diff --git a/strbuf.c b/strbuf.c
index 030556111d..fdc0ffbafb 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -120,6 +120,15 @@ void strbuf_trim_trailing_dir_sep(struct strbuf *sb)
        sb->buf[sb->len] = '\0';
 }
 
+void strbuf_trim_trailing_newline(struct strbuf *sb)
+{
+       if (sb->len > 0 && sb->buf[sb->len - 1] == '\n') {
+               if (--sb->len > 0 && sb->buf[sb->len - 1] == '\r')
+                       --sb->len;
+               sb->buf[sb->len] = '\0';
+       }
+}
+
 void strbuf_ltrim(struct strbuf *sb)
 {
        char *b = sb->buf;
diff --git a/strbuf.h b/strbuf.h
index 60a35aef16..b7aea8a966 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -190,6 +190,9 @@ extern void strbuf_ltrim(struct strbuf *);
 /* Strip trailing directory separators */
 extern void strbuf_trim_trailing_dir_sep(struct strbuf *);
 
+/* Strip trailing LF or CR/LF */
+extern void strbuf_trim_trailing_newline(struct strbuf *sb);
+
 /**
  * Replace the contents of the strbuf with a reencoded form.  Returns -1
  * on error, 0 on success.
-- 
2.18.0

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