lang/funcs: File hashing functions stream data from disk #28681
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Previously our file hashing functions were backed by the same "read file into memory" function we use for situations like "file" and "templatefile", meaning that they'd read the entire file into memory first and then calculate the hash from that buffer. All of the hash implementations we use here can calculate hashes from a sequence of smaller buffer writes though, so there's no actual need for us to create a file-sized temporary buffer here. This, then, is a small refactoring of our underlying function into two parts, where one is responsible for deciding the actual filename to load opening it, and the other is responsible for buffering the file into memory. Our hashing functions can then use only the first function and skip the second. This then allows us to use io.Copy to stream from the file into the hashing function in smaller chunks, possibly of a size chosen by the hash function if it happens to implement io.ReaderFrom. The new implementation is functionally equivalent to the old but should use less temporary memory if the user passes a large file to one of the hashing functions.
alisdair
approved these changes
May 12, 2021
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Neat fix. I didn't realize that this is how io.Copy works, that's fascinating.
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Backported to v0.15 as 7843c1a |
This was referenced May 12, 2021
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@apparentlymart - Thanks for this fix :) Solves the issue I raised #23890 . |
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I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions. |
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Previously our file hashing functions were backed by the same "read file into memory" function we use for situations like
fileandtemplatefile, meaning that they'd read the entire file into memory first and then calculate the hash from that buffer.All of the hash implementations we use here can calculate hashes from a sequence of smaller buffer writes though, so there's no actual need for us to create a file-sized temporary buffer here.
This, then, is a small refactoring of our underlying function into two parts, where one is responsible for deciding the actual filename to load opening it, and the other is responsible for buffering the file into memory. Our hashing functions can then use only the first function and skip the second.
This then allows us to use
io.Copyto stream from the file into the hashing function in smaller chunks, possibly of a size chosen by the hash function if it happens to implementio.ReaderFrom.The new implementation is functionally equivalent to the old but should use less temporary memory if the user passes a large file to one of the hashing functions.
This might help with #28678, but we've not root-caused that yet. Either way, this seems like a reasonable optimization to implement.