A program that could buffer stdin or file

You can do this with sponge from moreutils. sponge will "soak up standard input and write to a file". With no arguments, that file is standard output. Input given to this command is stored in memory until EOF, and then written out all at once.

For writing to a normal file, you can just give the filename:

cmd | sponge filename

The main purpose of sponge is to allow reading and writing from the same file within a pipeline, but it does what you want as well.

A poor man's sponge using awk:

awk '{a[NR] = $0} END {for (i = 1; i <= NR; i++) print a[i]}'

If you have tac, you can misuse it too:

... | tac | tac

As long as your input is ASCII text (contains no NUL 0x0 bytes until the end), then sed -z does what you want:

$ sed -z ''
Line 1
Line 2
Line 3
^D
Line 1
Line 2
Line 3
$ 

The -z causes sed to treat the NUL byte as a line delimiter instead of the usual newline. So as long as your input is regular text with no NUL bytes, then sed will continue reading the whole input into its pattern buffer until EOF is reached. sed then does no processing on the buffer and outputs it.

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If NUL bytes are present in your input, then you can do this instead:

sed ':l;N;bl'