libutil: move killUser innards to libexec

Change-Id: Iddd3099c31c91b18c946f7ba2db79c221e7686fe

lix/libexec/kill-user.cc

@@ -0,0 +1,60 @@
+#include "common.hh"
+#include <charconv>
+#include <cstring>
+#include <format>
+#include <signal.h>
+#include <unistd.h>
+
+#if __APPLE__
+#include <sys/syscall.h>
+#endif
+
+LIBEXEC_HELPER(1)
+
+int helperMain(const char * name, std::span<char *> args) noexcept
+{
+    std::string_view uidArg = args[0];
+    uid_t uid;
+
+    if (auto res = std::from_chars(uidArg.begin(), uidArg.end(), uid);
+        res.ptr != uidArg.end() || res.ec != std::errc())
+    {
+        die("invalid uid argument");
+    }
+
+    /* The system call kill(-1, sig) sends the signal `sig' to all
+       users to which the current process can send signals.  So we
+       switch to that uid and send a mass kill once we've done so. */
+
+    if (setuid(uid) == -1) {
+        die(std::format("setuid(): {}", strerror(errno)));
+    }
+
+    while (true) {
+#ifdef __APPLE__
+        /* OSX's kill syscall takes a third parameter that, among
+           other things, determines if kill(-1, signo) affects the
+           calling process. In the OSX libc, it's set to true,
+           which means "follow POSIX", which we don't want here */
+        if (syscall(SYS_kill, -1, SIGKILL, false) == 0) {
+            break;
+        }
+#else
+        if (kill(-1, SIGKILL) == 0) {
+            break;
+        }
+#endif
+        if (errno == ESRCH || errno == EPERM) {
+            break; /* no more processes */
+        }
+        if (errno != EINTR) {
+            die(std::format("cannot kill processes for uid {}: {}", uid, strerror(errno)));
+        }
+    }
+
+    /* !!! We should really do some check to make sure that there are
+       no processes left running under `uid', but there is no portable
+       way to do so (I think).  The most reliable way may be `ps -eo
+       uid | grep -q $uid'. */
+    return 0;
+}

lix/libexec/meson.build

@@ -1,3 +1,10 @@
+kill_user = executable(
+  'kill-user',
+  files('kill-user.cc'),
+  install : true,
+  install_dir : libexecdir / 'lix',
+)
+
 run_pager = executable(
   'run-pager',
   files('run-pager.cc'),

lix/libutil/processes.cc

@@ -130,42 +130,7 @@ void killUser(uid_t uid)
 
     assert(uid != 0); /* just to be safe... */
 
-    /* The system call kill(-1, sig) sends the signal `sig' to all
-       users to which the current process can send signals.  So we
-       fork a process, switch to uid, and send a mass kill. */
-
-    Pid pid{startProcess([&]() {
-
-        if (setuid(uid) == -1)
-            throw SysError("setting uid");
-
-        while (true) {
-#ifdef __APPLE__
-            /* OSX's kill syscall takes a third parameter that, among
-               other things, determines if kill(-1, signo) affects the
-               calling process. In the OSX libc, it's set to true,
-               which means "follow POSIX", which we don't want here
-                 */
-            if (syscall(SYS_kill, -1, SIGKILL, false) == 0) break;
-#else
-            if (kill(-1, SIGKILL) == 0) break;
-#endif
-            if (errno == ESRCH || errno == EPERM) break; /* no more processes */
-            if (errno != EINTR)
-                throw SysError("cannot kill processes for uid '%1%'", uid);
-        }
-
-        _exit(0);
-    })};
-
-    int status = pid.wait();
-    if (status != 0)
-        throw Error("cannot kill processes for uid '%1%': %2%", uid, statusToString(status));
-
-    /* !!! We should really do some check to make sure that there are
-       no processes left running under `uid', but there is no portable
-       way to do so (I think).  The most reliable way may be `ps -eo
-       uid | grep -q $uid'. */
+    runHelper("kill-user", {.args = {std::to_string(uid)}}).waitAndCheck();
 }