ci: run test round with parallel-checkout enabled

We already have tests for the basic parallel-checkout operations. But
this code can also run be executed by other commands, such as
git-read-tree and git-sparse-checkout, which are currently not tested
with multiple workers. To promote a wider test coverage without
duplicating tests:

1. Add the GIT_TEST_CHECKOUT_WORKERS environment variable, to optionally
   force parallel-checkout execution during the whole test suite.

2. Set this variable (with a value of 2) in the second test round of our
   linux-gcc CI job. This round runs `make test` again with some
   optional GIT_TEST_* variables enabled, so there is no additional
   overhead in exercising the parallel-checkout code here.

Note that tests checking out less than two parallel-eligible entries
will fall back to the sequential mode. Nevertheless, it's still a good
exercise for the parallel-checkout framework as the fallback codepath
also writes the queued entries using the parallel-checkout functions
(only without spawning any worker).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/run-build-and-tests.sh

@@ -25,6 +25,7 @@ linux-gcc)
 	export GIT_TEST_ADD_I_USE_BUILTIN=1
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
 	export GIT_TEST_WRITE_REV_INDEX=1
+	export GIT_TEST_CHECKOUT_WORKERS=2
 	make test
 	;;
 linux-clang)

parallel-checkout.c

@@ -35,6 +35,20 @@ static const int DEFAULT_NUM_WORKERS = 1;
 
 void get_parallel_checkout_configs(int *num_workers, int *threshold)
 {
+	char *env_workers = getenv("GIT_TEST_CHECKOUT_WORKERS");
+
+	if (env_workers && *env_workers) {
+		if (strtol_i(env_workers, 10, num_workers)) {
+			die("invalid value for GIT_TEST_CHECKOUT_WORKERS: '%s'",
+			    env_workers);
+		}
+		if (*num_workers < 1)
+			*num_workers = online_cpus();
+
+		*threshold = 0;
+		return;
+	}
+
 	if (git_config_get_int("checkout.workers", num_workers))
 		*num_workers = DEFAULT_NUM_WORKERS;
 	else if (*num_workers < 1)

t/README

@@ -436,6 +436,10 @@ and "sha256".
 GIT_TEST_WRITE_REV_INDEX=<boolean>, when true enables the
 'pack.writeReverseIndex' setting.
 
+GIT_TEST_CHECKOUT_WORKERS=<n> overrides the 'checkout.workers' setting
+to <n> and 'checkout.thresholdForParallelism' to 0, forcing the
+execution of the parallel-checkout code.
+
 Naming Tests
 ------------
 

t/lib-parallel-checkout.sh

@@ -1,5 +1,8 @@
 # Helpers for tests invoking parallel-checkout
 
+# Parallel checkout tests need full control of the number of workers
+unset GIT_TEST_CHECKOUT_WORKERS
+
 set_checkout_config () {
 	if test $# -ne 2
 	then