misc: Add options to constrain autostop's bloodthirsty rampage.

Autostop is quite handy for keeping your system under control, but when things
go pear shaped the guidance provided by Linux's OOM score doesn't necessarily
align with what the user cares about. In particular, it can take down parts of
your windowing subsystem or a harmless browser tab going about its business.
This commit adds command line options to give autostop a list of processes
which are the only ones you would like stopped. Existing behaviour remains
unchanged.

misc/autostop/stop.c

@@ -22,6 +22,7 @@
  * 2012 David Greenaway
  */
 
+#define _GNU_SOURCE /* for asprintf */
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -73,9 +74,33 @@ fatal(const char *str)
     exit(1);
 }
 
+/* Get the name of a process from its PID. */
+int
+name_of(int pid, char *output, size_t len) {
+    char *path;
+    if (asprintf(&path, "/proc/%d/cmdline", pid) == -1)
+        return -1;
+
+    /* Open the process's command line details from /proc. */
+    FILE *f = fopen(path, "r");
+    free(path);
+    if (f == NULL)
+        return -1;
+
+    /* Here we potentially read too much, but cmdline entries are NUL delimited
+     * so the resulting data is a valid C string of just the first argument as
+     * desired.
+     */
+    int r = fread(output, len, 1, f);
+    (void)r;
+    fclose(f);
+
+    return 0;
+}
+
 /* Iterate through processes in the system. */
 void
-iterate_processes(void (*proc_fn)(int, void *), void *data)
+iterate_processes(char **limit, void (*proc_fn)(int, void *), void *data)
 {
     /* Open /proc */
     DIR *proc_dir = opendir("/proc");
@@ -97,8 +122,44 @@ iterate_processes(void (*proc_fn)(int, void *), void *data)
 
         /* Process? */
         int p = atoi(e->d_name);
-        if (p != 0)
-            proc_fn(p, data);
+        if (p == 0)
+            continue;
+
+        if (limit != NULL) {
+            int skip = 1;
+
+            /* Find the name of the process we're looking at. */
+            char name[PATH_MAX];
+            if (name_of(p, name, PATH_MAX) != 0)
+                /* This process doesn't have a name. Poor thing. */
+                continue;
+
+            /* Determine if this process matches any of the processes we should
+             * be considering.
+             */
+            char **l;
+            for (l = limit; *l != NULL; l++) {
+                if (!strcmp(name, *l)) {
+                    skip = 0;
+                    break;
+                }
+                char *last_slash = strrchr(name, '/');
+                if (last_slash != NULL && !strcmp(last_slash + 1, *l)) {
+                    skip = 0;
+                    break;
+                }
+            }
+
+            if (skip == 1)
+                /* No match. */
+                continue;
+
+#if DEBUG
+            printf("Considering %s...\n", name);
+#endif
+        }
+
+        proc_fn(p, data);
     }
 
     /* Cleanup. */
@@ -264,10 +325,11 @@ int parse_signal(const char *input, int *signal, const char **signame)
 void usage(int argc, char **argv)
 {
     printf("\n"
-        "usage: %s [<SIGNAL>]\n\n"
+        "usage: %s [<SIGNAL>] [<processes>]\n\n"
         "Monitors the system for high load and sends a signal to (hopefully)\n"
         "the culprit process.\n\n"
-        "<SIGNAL> must be either SIGKILL or SIGSTOP.\n\n",
+        "<SIGNAL> must be either SIGKILL or SIGSTOP.\n"
+        "If you don't pass a list of candidate processes, all are considered.\n\n",
         argc > 0 ? argv[0] : "autostop");
 }
 
@@ -277,6 +339,7 @@ int main(int argc, char **argv)
     long long last_fault_count = 0;
     int signal;
     const char *signame;
+    char **suspects;
 
     /* Determine which signal to send. */
     if (argc < 2) {
@@ -290,6 +353,9 @@ int main(int argc, char **argv)
         }
     }
 
+    /* Determine the list of candidate processes. */
+    suspects = argc > 2 ? &argv[2] : NULL;
+
     /* Set our scheduling priority higher. */
     (void)setpriority(PRIO_PROCESS, 0, SCHED_PRIO);
 
@@ -300,7 +366,7 @@ int main(int argc, char **argv)
             .worst_oom_score = MIN_OOM_SCORE,
             .total_faults = 0,
         };
-        iterate_processes(test_process, &d);
+        iterate_processes(suspects, test_process, &d);
 
         /* Determine if things are looking bad and we haven't recently stoped something. */
         if (is_system_unstable(last_fault_count, d.total_faults)) {