[PATCH v3] pager: fix crash when pager program doesn't exist

[Enzo Matsumiya <ematsumiya@suse.de>]

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().

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 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          | 2 ++
 t/t7006-pager.sh | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/pager.c b/pager.c
index 52f27a6765c8..d93304527d62 100644
--- a/pager.c
+++ b/pager.c
@@ -124,6 +124,8 @@ void setup_pager(void)
 
 	setenv("GIT_PAGER_IN_USE", "true", 1);
 
+	child_process_init(&pager_process);
+
 	/* spawn the pager */
 	prepare_pager_args(&pager_process, pager);
 	pager_process.in = -1;
diff --git a/t/t7006-pager.sh b/t/t7006-pager.sh
index 0e7cf75435ec..0be9bcba49a8 100755
--- a/t/t7006-pager.sh
+++ b/t/t7006-pager.sh
@@ -786,4 +786,9 @@ test_expect_success TTY 'git returns SIGPIPE on propagated signals from pager' '
 	test_path_is_file pager-used
 '
 
+test_expect_success TTY 'handle attempt to run an invalid pager' '
+	test_config pager.show invalid-pager &&
+	test_terminal git show
+'
+
 test_done
-- 
2.33.1