modules: load modules from versioned /var/run dir

On upgrades the old .so files usually are replaced. But on the other
hand since a qemu process represents a guest instance it is usually kept
around.

That makes late addition of dynamic features e.g. 'hot-attach of a ceph
disk' fail by trying to load a new version of e.f. block-rbd.so into an
old still running qemu binary.

This adds a fallback to also load modules from a versioned directory in the
temporary /var/run path. That way qemu is providing a way for packaging
to store modules of an upgraded qemu package as needed until the next reboot.

An example how that can then be used in packaging can be seen in:
https://git.launchpad.net/~paelzer/ubuntu/+source/qemu/log/?h=bug-1847361-miss-old-so-on-upgrade-UBUNTU

Fixes: 1847361
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200310145806.18335-2-christian.ehrhardt@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/module.c

@@ -19,6 +19,9 @@
 #endif
 #include "qemu/queue.h"
 #include "qemu/module.h"
+#ifdef CONFIG_MODULE_UPGRADES
+#include "qemu-version.h"
+#endif
 
 typedef struct ModuleEntry
 {
@@ -170,6 +173,9 @@ bool module_load_one(const char *prefix, const char *lib_name)
 #ifdef CONFIG_MODULES
     char *fname = NULL;
     char *exec_dir;
+#ifdef CONFIG_MODULE_UPGRADES
+    char *version_dir;
+#endif
     const char *search_dir;
     char *dirs[4];
     char *module_name;
@@ -201,6 +207,14 @@ bool module_load_one(const char *prefix, const char *lib_name)
     dirs[n_dirs++] = g_strdup_printf("%s", CONFIG_QEMU_MODDIR);
     dirs[n_dirs++] = g_strdup_printf("%s/..", exec_dir ? : "");
     dirs[n_dirs++] = g_strdup_printf("%s", exec_dir ? : "");
+
+#ifdef CONFIG_MODULE_UPGRADES
+    version_dir = g_strcanon(g_strdup(QEMU_PKGVERSION),
+                             G_CSET_A_2_Z G_CSET_a_2_z G_CSET_DIGITS "+-.~",
+                             '_');
+    dirs[n_dirs++] = g_strdup_printf("/var/run/qemu/%s", version_dir);
+#endif
+
     assert(n_dirs <= ARRAY_SIZE(dirs));
 
     g_free(exec_dir);