[PATCH v4] pager: fix crash when pager program doesn't exist

When prepare_cmd() fails for, e.g., pager process setup, child_process_clear() frees the memory in pager_process.args, but .argv was pointed to pager_process.args.v earlier in start_command(), so it's now a dangling pointer.

setup_pager() is then called a second time, from cmd_log_init_finish() in this case, and any further operations using its .argv, e.g. strvec_*, will use the dangling pointer and eventually crash. According to trivial tests, setup_pager() is not called twice if the first call is successful.

This patch makes sure that pager_process is properly initialized on setup_pager(). Drop CHILD_PROCESS_INIT from its declaration since it's no longer really necessary.

Add a test to catch possible regressions.

Reproducer: $ git config pager.show INVALID_PAGER $ git show $VALID_COMMIT error: cannot run INVALID_PAGER: No such file or directory [1] 3619 segmentation fault (core dumped) git show $VALID_COMMIT

Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
---
Changes to v3:
 - change test description, as per Eric's suggestion
 - drop CHILD_PROCESS_INIT, as per Jeff's suggestion

Changes to v2:
 - move child_process_init() to setup_pager(), as other callers of
   child_process_clear() might've not expected .argv to change
 - add a test case with the reproducer of the original bug

Changes to v1:
 * Implement all of Jeff's suggestions:
   - remove double frees to .argv
   - discard the idea of falling back to DEFAULT_PAGER
   - replace memset() in child_process_clear() by child_process_init()
   - update/improve commit message

 pager.c          | 4 +++-
 t/t7006-pager.sh | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)