Add the GetMasterfilesStageLock() function

So that CFEngine components can make sure masterfiles updates
don't happen at wrong times or as multiple runs at the same
times.

Ticket: ENT-9386
Changelog: None

Author: vpodzime

libpromises/policy.c

@@ -43,6 +43,8 @@
 #include <audit.h>
 #include <logging.h>
 #include <expand.h>
+#include <file_lib.h>                   /* FileLock */
+#include <known_dirs.h>                 /* GetStateDir() */
 
 static const char *const POLICY_ERROR_BUNDLE_NAME_RESERVED =
     "Use of a reserved container name as a bundle name \"%s\"";
@@ -100,6 +102,29 @@ const char *NamespaceDefault(void)
 
 /*************************************************************************/
 
+#define MASTERFILES_STAGE_LOCK_FNAME "masterfiles-stage.lock"
+
+bool GetMasterfilesStageLock(FileLock *lock, bool exclusive, bool wait)
+{
+    assert(lock != NULL);
+    char path[PATH_MAX];
+    NDEBUG_UNUSED size_t ret = StringCopy(GetStateDir(), path, sizeof(path));
+    assert(ret < sizeof(path));
+    NDEBUG_UNUSED char *path_ret = JoinPaths(path, sizeof(path), MASTERFILES_STAGE_LOCK_FNAME);
+    assert(path_ret != NULL);
+
+    if (exclusive)
+    {
+        return (ExclusiveFileLockPath(lock, path, wait) == 0);
+    }
+    else
+    {
+        return (SharedFileLockPath(lock, path, wait) == 0);
+    }
+}
+
+/*************************************************************************/
+
 Policy *PolicyNew(void)
 {
     Policy *policy = xcalloc(1, sizeof(Policy));

libpromises/policy.h

@@ -31,6 +31,7 @@
 #include <sequence.h>
 #include <json.h>
 #include <set.h>
+#include <file_lib.h>                   /* FileLock */
 
 typedef enum
 {
@@ -141,6 +142,18 @@ struct Constraint_
     SourceOffset offset;
 };
 
+/**
+ * Get lock for masterfiles stage (update)
+ * @param lock       file lock to initialize (not-%NULL)
+ * @param exclusive  %true to get exclusive lock (for write access),
+ *                   %false for a shared lock (read access)
+ * @param wait       whether to wait for the lock (blocks) or give up immediately
+ * @return           whether the lock was successfully obtained or not
+ * @note             Unlock the lock with ExclusiveFileUnlock() or SharedFileUnlock()
+ *                   when done.
+ */
+bool GetMasterfilesStageLock(FileLock *lock, bool exclusive, bool wait);
+
 const char *NamespaceDefault(void);
 
 Policy *PolicyNew(void);