Use "opened" Xenstore key instead of magic file in /etc

Changing behavior based on whether or not a magic file exists in /etc is
a bad idea.  Instead, have the kernel create an "opened" key (with the
value 0) to indicate that it supports autocleared devices and diskseqs
in Xenstore, and use the existence of that key to determine behavior.

NOT TESTED.

not-script/not-script.c

@@ -44,13 +44,6 @@ static int open_loop_dev(uint32_t devnum, bool writable)
     return res == -1 ? -errno : res;
 }
 
-static bool use_autoclear(void) {
-    int const status = access("/etc/use-autoclear", F_OK);
-    if (!(status == 0 || (status == -1 && errno == ENOENT)))
-        err(1, "access(\"/etc/use-autoclear\")");
-    return status == 0;
-}
-
 static int setup_loop(struct loop_context *ctx,
                       uint32_t fd,
                       uint64_t offset,
@@ -227,6 +220,27 @@ static bool strict_strtoul_hex(char *p, char **endp, char expected, unsigned lon
     return **endp == expected;
 }
 
+static const char *const opened_key = "opened";
+
+static bool get_opened(struct xs_handle *const h, char *const extra_path,
+                       const char *const xenstore_path_buffer, char expected)
+{
+    strcpy(extra_path, opened_key);
+    unsigned int value_len;
+    char *value = xs_read(h, 0, xenstore_path_buffer, &value_len);
+    if (value) {
+        if ((value_len != 1) || (*value != expected))
+            err(1, "Expected \"opened\" entry to be missing or contain \"%c\", but it contained %s",
+                expected, value);
+        free(value);
+        return true;
+    } else {
+        if (errno != ENOENT) {
+            err(1, "Error reading Xenstore key %s", xenstore_path_buffer);
+        }
+        return false;
+    }
+}
 
 static void remove_device(struct xs_handle *const h, char *xenstore_path_buffer,
                           char *extra_path)
@@ -237,9 +251,6 @@ static void remove_device(struct xs_handle *const h, char *xenstore_path_buffer,
     uint64_t actual_diskseq;
     unsigned long _major, _minor;
 
-    if (use_autoclear())
-        return;
-
     /*
      * Order matters here: the kernel only cares about "physical-device" for
      * now, so ensure that it gets removed first.
@@ -405,9 +416,12 @@ int main(int argc, char **argv)
     /* Buffer to copy extra data into */
     char *const extra_path = xenstore_path_buffer + xs_path_len + 1;
     unsigned int len, path_len;
+    bool const autoclear = get_opened(h, extra_path, xenstore_path_buffer,
+                                      action == REMOVE ? '1' : '0');
 
     if (action == REMOVE) {
-        remove_device(h, xenstore_path_buffer, extra_path);
+        if (!autoclear)
+            remove_device(h, xenstore_path_buffer, extra_path);
         return 0;
     }
 
@@ -456,7 +470,6 @@ int main(int argc, char **argv)
     if ((fd = open(data, (writable ? O_RDWR : O_RDONLY) | O_NOCTTY | O_CLOEXEC | O_NONBLOCK)) < 0)
         err(1, "open(%s)", data);
     char phys_dev[18], hex_diskseq[17];
-    bool const autoclear = use_autoclear();
 
     process_blk_dev(fd, data, writable, &dev, &diskseq, permissive, autoclear);
     unsigned const int l =
@@ -468,10 +481,9 @@ int main(int argc, char **argv)
     if (snprintf(hex_diskseq, sizeof(hex_diskseq), "%016llx", (unsigned long long)diskseq) != 16)
         err(1, "snprintf");
 
-    const char *const entry_to_watch = "opened";
     if (autoclear) {
-        strcpy(extra_path, entry_to_watch);
-        if (!xs_watch(h, xenstore_path_buffer, entry_to_watch))
+        strcpy(extra_path, opened_key);
+        if (!xs_watch(h, xenstore_path_buffer, opened_key))
             err(1, "Cannot setup XenStore watch on %s", xenstore_path_buffer);
     }
 
@@ -500,7 +512,7 @@ int main(int argc, char **argv)
     }
 
     if (autoclear) {
-        strcpy(extra_path, entry_to_watch);
+        strcpy(extra_path, opened_key);
         for (;;) {
             unsigned int num, state_len;
             char *value, **watch_res = xs_read_watch(h, &num);