pidfile: Make checking binary path in virPidFileRead optional

This patch changes behavior of virPidFileRead to enable passing NULL as
path to the binary the pid file should be checked against to skip this
check. This enables using this function for reading files that have same
semantics as pid files, but belong to unknown processes.

src/util/virpidfile.c

@@ -193,6 +193,9 @@ int virPidFileRead(const char *dir,
  * resolves to @binpath. This adds protection against
  * recycling of previously reaped pids.
  *
+ * If @binpath is NULL the check for the executable path
+ * is skipped.
+ *
  * Returns -errno upon error, or zero on successful
  * reading of the pidfile. If the PID was not still
  * alive, zero will be returned, but @pid will be
@@ -218,16 +221,18 @@ int virPidFileReadPathIfAlive(const char *path,
     }
 #endif
 
-    if (virAsprintf(&procpath, "/proc/%d/exe", *pid) < 0) {
-        *pid = -1;
-        return -1;
-    }
+    if (binpath) {
+        if (virAsprintf(&procpath, "/proc/%d/exe", *pid) < 0) {
+            *pid = -1;
+            return -1;
+        }
 
-    if (virFileIsLink(procpath) &&
-        virFileLinkPointsTo(procpath, binpath) == 0)
-        *pid = -1;
+        if (virFileIsLink(procpath) &&
+            virFileLinkPointsTo(procpath, binpath) == 0)
+            *pid = -1;
 
-    VIR_FREE(procpath);
+        VIR_FREE(procpath);
+    }
 
     return 0;
 }