a999debae2
In all branches back to 8.3, this patch fixes a questionable assumption in CompactCheckpointerRequestQueue/CompactBgwriterRequestQueue that there are no uninitialized pad bytes in the request queue structs. This would only cause trouble if (a) there were such pad bytes, which could happen in 8.4 and up if the compiler makes enum ForkNumber narrower than 32 bits, but otherwise would require not-currently-planned changes in the widths of other typedefs; and (b) the kernel has not uniformly initialized the contents of shared memory to zeroes. Still, it seems a tad risky, and we can easily remove any risk by pre-zeroing the request array for ourselves. In addition to that, we need to establish a coding rule that struct RelFileNode can't contain any padding bytes, since such structs are copied into the request array verbatim. (There are other places that are assuming this anyway, it turns out.) In 9.1 and up, the risk was a bit larger because we were also effectively assuming that struct RelFileNodeBackend contained no pad bytes, and with fields of different types in there, that would be much easier to break. However, there is no good reason to ever transmit fsync or delete requests for temp files to the bgwriter/checkpointer, so we can revert the request structs to plain RelFileNode, getting rid of the padding risk and saving some marginal number of bytes and cycles in fsync queue manipulation while we are at it. The savings might be more than marginal during deletion of a temp relation, because the old code transmitted an entirely useless but nonetheless expensive-to-process ForgetRelationFsync request to the background process, and also had the background process perform the file deletion even though that can safely be done immediately. In addition, make some cleanup of nearby comments and small improvements to the code in CompactCheckpointerRequestQueue/CompactBgwriterRequestQueue.
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