The expected way to pass data into the callback is to pass them via
the customizable callback pointer. The error reporting in
default_{start_failure, task_finished} is not user friendly enough, that
we want to encourage using the child data for such purposes.

Furthermore the struct child data is cleaned by the run-command API,
before we access them in the callbacks, leading to use-after-free
situations.

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

 Thanks Johannes for reviewing and double checking. I squashed in your
 proposed documentation change.
 
 This applies on 2.8.0-rc0.
 
 Thanks,
 Stefan

 run-command.c      | 24 +++---------------------
 run-command.h      |  9 +++------
 submodule.c        |  7 +++----
 test-run-command.c |  1 -
 4 files changed, 9 insertions(+), 32 deletions(-)

diff --git a/run-command.c b/run-command.c
index 51fd72c..8e3ad07 100644
--- a/run-command.c
+++ b/run-command.c
@@ -902,35 +902,18 @@ struct parallel_processes {
        struct strbuf buffered_output; /* of finished children */
 };
 
-static int default_start_failure(struct child_process *cp,
-                                struct strbuf *err,
+static int default_start_failure(struct strbuf *err,
                                 void *pp_cb,
                                 void *pp_task_cb)
 {
-       int i;
-
-       strbuf_addstr(err, "Starting a child failed:");
-       for (i = 0; cp->argv[i]; i++)
-               strbuf_addf(err, " %s", cp->argv[i]);
-
        return 0;
 }
 
 static int default_task_finished(int result,
-                                struct child_process *cp,
                                 struct strbuf *err,
                                 void *pp_cb,
                                 void *pp_task_cb)
 {
-       int i;
-
-       if (!result)
-               return 0;
-
-       strbuf_addf(err, "A child failed with return code %d:", result);
-       for (i = 0; cp->argv[i]; i++)
-               strbuf_addf(err, " %s", cp->argv[i]);
-
        return 0;
 }
 
@@ -1048,8 +1031,7 @@ static int pp_start_one(struct parallel_processes *pp)
        pp->children[i].process.no_stdin = 1;
 
        if (start_command(&pp->children[i].process)) {
-               code = pp->start_failure(&pp->children[i].process,
-                                        &pp->children[i].err,
+               code = pp->start_failure(&pp->children[i].err,
                                         pp->data,
                                         &pp->children[i].data);
                strbuf_addbuf(&pp->buffered_output, &pp->children[i].err);
@@ -1117,7 +1099,7 @@ static int pp_collect_finished(struct parallel_processes 
*pp)
 
                code = finish_command(&pp->children[i].process);
 
-               code = pp->task_finished(code, &pp->children[i].process,
+               code = pp->task_finished(code,
                                         &pp->children[i].err, pp->data,
                                         &pp->children[i].data);
 
diff --git a/run-command.h b/run-command.h
index d5a57f9..6e17894 100644
--- a/run-command.h
+++ b/run-command.h
@@ -158,8 +158,7 @@ typedef int (*get_next_task_fn)(struct child_process *cp,
  * To send a signal to other child processes for abortion, return
  * the negative signal number.
  */
-typedef int (*start_failure_fn)(struct child_process *cp,
-                               struct strbuf *err,
+typedef int (*start_failure_fn)(struct strbuf *err,
                                void *pp_cb,
                                void *pp_task_cb);
 
@@ -178,7 +177,6 @@ typedef int (*start_failure_fn)(struct child_process *cp,
  * the negative signal number.
  */
 typedef int (*task_finished_fn)(int result,
-                               struct child_process *cp,
                                struct strbuf *err,
                                void *pp_cb,
                                void *pp_task_cb);
@@ -192,9 +190,8 @@ typedef int (*task_finished_fn)(int result,
  * (both stdout and stderr) is routed to stderr in a manner that output
  * from different tasks does not interleave.
  *
- * If start_failure_fn or task_finished_fn are NULL, default handlers
- * will be used. The default handlers will print an error message on
- * error without issuing an emergency stop.
+ * start_failure_fn and task_finished_fn can be NULL to omit any
+ * special handling.
  */
 int run_processes_parallel(int n,
                           get_next_task_fn,
diff --git a/submodule.c b/submodule.c
index b83939c..916fc84 100644
--- a/submodule.c
+++ b/submodule.c
@@ -705,8 +705,7 @@ static int get_next_submodule(struct child_process *cp,
        return 0;
 }
 
-static int fetch_start_failure(struct child_process *cp,
-                              struct strbuf *err,
+static int fetch_start_failure(struct strbuf *err,
                               void *cb, void *task_cb)
 {
        struct submodule_parallel_fetch *spf = cb;
@@ -716,8 +715,8 @@ static int fetch_start_failure(struct child_process *cp,
        return 0;
 }
 
-static int fetch_finish(int retvalue, struct child_process *cp,
-                       struct strbuf *err, void *cb, void *task_cb)
+static int fetch_finish(int retvalue, struct strbuf *err,
+                       void *cb, void *task_cb)
 {
        struct submodule_parallel_fetch *spf = cb;
 
diff --git a/test-run-command.c b/test-run-command.c
index fbe0a27..30a64a9 100644
--- a/test-run-command.c
+++ b/test-run-command.c
@@ -41,7 +41,6 @@ static int no_job(struct child_process *cp,
 }
 
 static int task_finished(int result,
-                        struct child_process *cp,
                         struct strbuf *err,
                         void *pp_cb,
                         void *pp_task_cb)
-- 
2.8.0.rc0.1.g68b4e3f

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