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The compressed format is using a slighty modified variant of the extern format
(term_to_binary). To not worsen key lookup's too much, the top tuple itself
and the key element are not compressed. Table objects with only immediate
non-key elements will therefor not gain anything (but actually consume one
extra word for "alloc_size").
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Alignment of trailing data in messages has been adjusted.
This in order to be able to pass data of any type as
trailing data in the future.
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Store Erlang terms in 32-bit entities on the heap, expanding the
pointers to 64-bit when needed. This works because all terms are stored
on addresses in the 32-bit address range (the 32 most significant bits
of pointers to term data are always 0).
Introduce a new datatype called UWord (along with its companion SWord),
which is an integer having the exact same size as the machine word
(a void *), but might be larger than Eterm/Uint.
Store code as machine words, as the instructions are pointers to
executable code which might reside outside the 32-bit address range.
Continuation pointers are stored on the 32-bit stack and hence must
point to addresses in the low range, which means that loaded beam code
much be placed in the low 32-bit address range (but, as said earlier,
the instructions themselves are full words).
No Erlang term data can be stored on C stacks (enforced by an
earlier commit).
This version gives a prompt, but test cases still fail (and dump core).
The loader (and emulator loop) has instruction packing disabled.
The main issues has been in rewriting loader and actual virtual
machine. Subsystems (like distribution) does not work yet.
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* jv/binary_to_term-opts:
document ErtsExternalDist flags and CON_ID mask
add options to binary_to_term
OTP-8367 There is new erlang:binary_to_binary/2 BIF that takes an option
list. The option safe can be used to prevent creation of
resources that are not garbage collected (such as atoms). (Thanks
to Jayson Vantuyl.)
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In the ErtsExternalDist structure, the flags field holds a combination of flags
(tagged into the high bits) and the connection ID (in the low bits). This
wasn't clearing indicated anywhere. This patch adds a comment before the flags
and mask that indicates their use and relation to each other. This will help
guide people through the code and reduce the likelihood that someone will add a
flag without adjusting the mask.
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term_to_binary and binary_to_term are powerful tools that can be used easily in
lieu of a custom binary network protocol. Unfortunately, carefully crafted
data can be used to exhaust the memory in an Erlang node by merely attempting
to decode binaries. This makes it unsafe to receive data from untrusted
sources.
This is possible because binary_to_term/1 will allocate new atoms and new
external function references. These data structures are not garbage collected.
This patch implements the new form of binary_to_term that takes a list of
options, and a simple option called 'safe'. If specified, this option will
cause decoding to fail with a badarg error if an atom or external function
reference would be allocated.
In the general case, it will happily decode any Erlang term other than those
containing new atoms or new external function references. However, fun, pid,
and ref data types can embed atoms. They might fail to decode if one of these
embedded atoms is new to the node. This may be an issue if encoded binaries
are transferred between nodes or persisted between invocations of Erlang.
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