Make PDO connection construction single-shot#22874
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Constructing a PDO handle over an already constructed one re-ran connection setup; for a persistent connection the second pass freed the pemalloc'd dbh with efree() and corrupted the heap. Besides a plain second __construct(), the same sink was reachable by reentering during a uri: DSN stream open, by retrying a persistent construct that failed after swapping in the persistent handle, and by a subclass destructor reentering while a failed persistent connect() unwinds. Track construction with an is_constructing flag, set as soon as the handle exists for both __construct() and connect(), carried onto the persistent handle and cleared only on success; construction is rejected once the driver is attached or a construction is in progress. A construct that fails now leaves the handle unusable instead of allowing a retry.
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Constructing a PDO over an already constructed handle re-runs connection setup and, for a persistent connection, frees the pemalloc'd dbh with efree(), corrupting the heap. The same sink is also reachable by reentering __construct() from a uri: DSN stream wrapper or a failed connect()'s destructor, and by retrying a persistent construct that failed after swapping in the handle. This makes construction single-shot: an is_constructing flag (an ABI-neutral bit taken from _reserved_flags, sizeof(pdo_dbh_t) unchanged) is set as soon as the handle exists for both __construct() and connect(), and construction is rejected once the driver is attached or one is already in progress. Behavior change: a construct that fails now leaves the handle unusable instead of allowing a retry.