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sdjournal: don't close libsystemd on Close()
There is a symbol cache (map from function names to their addresses) so we don't need to call dlsym everytime we call a function from libsystemd if it was already called before. Every time we open a new Journal, we call dlopen(), and every time we close it, we call dlclose(). Since the map was global and we weren't clearing it up after closing Journal, we were reusing the addresses from the previous invocation. This is wrong because libsystemd can end up in a different address the second time we dlopen() it. Instead, just don't call dlclose() when we close the Journal so libsystemd remains in the process address space for the whole life of it. This means the symbol cache will be shared between different Journal instances. According to dlopen(3): If the same shared object is loaded again with dlopen(), the same object handle is returned. The dynamic linker maintains reference counts for object handles, so a dynamically loaded shared object is not deallocated until dlclose() has been called on it as many times as dlopen() has succeeded on it. So we don't worry about calling dlopen() on an already opened library.
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