[PATCH] taskset: Accept 0 pid for current process

This is useful to print the current mask without using `$$`: `taskset -p 0`.

It is also helpful to test taskset: `taskset -c 1-4 taskset -p 0`.
This is not easy with `$$`.

sched_setaffinity(2)/sched_getaffinity(2) accept 0 for the calling
thread, so this seems consistent.

As an implementation detail, we replace 0 with getpid(), so the existing
pid != 0 <==> "will exec" logic continues to work unchanged.

A reasonable alternative would be to interpret just `taskset` (currently
an error) as printing the current mask.  This seems less orthogonal,
and a better use may be found for plain `taskset` in the future.

https://github.com/util-linux/util-linux/pull/3637

Signed-off-by: Jesse Rosenstock <[email protected]>
---
 schedutils/taskset.1.adoc |  8 ++++++++
 schedutils/taskset.c      | 10 +++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/schedutils/taskset.1.adoc b/schedutils/taskset.1.adoc
index 9773303f73..ebd9fdafe8 100644
--- a/schedutils/taskset.1.adoc
+++ b/schedutils/taskset.1.adoc
@@ -77,6 +77,7 @@

 *-p*, *--pid*::
 Operate on an existing PID and do not launch a new task.
+If PID is zero, then operate on the *taskset* process.

 include::man-common/help-version.adoc[]

@@ -134,6 +135,13 @@
 $ echo $? +
 1 +

+== EXAMPLES
+
+Print the current CPU affinity.
+
+$ taskset -pc 0 +
+pid 1355988's current affinity list: 0-47 +
+
 == AUTHORS

 Written by Robert M. Love.
diff --git a/schedutils/taskset.c b/schedutils/taskset.c
index b52cd4338b..40ccbb7342 100644
--- a/schedutils/taskset.c
+++ b/schedutils/taskset.c
@@ -186,7 +186,15 @@
                        all_tasks = 1;
                        break;
                case 'p':
-                       pid = strtopid_or_err(argv[argc - 1],
_("invalid PID argument"));
+                       // Like strtopid_or_err() but accept 0 for this process,
+                       // like sched_getaffinity()/sched_setaffinity() do.
+                       pid = (pid_t) str2num_or_err(
+                               argv[argc - 1], 10, _("invalid PID argument"),
+                               0, SINT_MAX(pid_t));
+                       if (pid == 0)
+                               pid = getpid();
+                       // After this point, pid == 0 means "no pid" and that
+                       // we will exec a command.
                        break;
                case 'c':
                        ts.use_list = 1;