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89c64e179e
Fix issue where updating the size of the Local File Cache could lead to invalid reads: ## Problem LFC cache can get re-enabled when lfc_max_size is set, e.g. through an autoscaler configuration, or PostgreSQL not liking us setting the variable. 1. initialize: LFC enabled (lfc_size_limit > 0; lfc_desc = 0) 2. Open LFC file fails, lfc_desc = -1. lfc_size_limit is set to 0; 3. lfc_size_limit is updated by autoscaling to >0 4. read() now thinks LFC is enabled (size_limit > 0) and lfc_desc is valid, but doesn't try to read from the invalid file handle and thus doesn't update the buffer content with the page's data, but does think the data was read... Any buffer we try to read from local file cache is essentially uninitialized memory. Those are likely 0-bytes, but might potentially be any old buffer that was previously read from or flushed to disk. Fix this by adding a more definitive disable flag, plus better invalid state handling.