Support reading "special files" that lie about their size

[graebm]

Issue:
aws_byte_buf_init_from_file() didn't work with "special files", which don't accurately report their size. For example, on my Amazon Linux 2:

• /proc/cpuinfo: size is 0, but contents are several KB of data.
• /sys/devices/virtual/dmi/id/product_name: size is 4096, but contents are "c5.2xlarge"

Description of changes:

• aws_byte_buf_init_from_file() no longer 100% trusts the reported size. Size is used as a hint, but it always reads now until EOF, growing the buffer if necessary.
• New function aws_byte_buf_init_from_file_with_size_hint() lets users pass a hint, rather than querying the OS for it.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bretambrose]

maybe with_size_hint() rather than "special" which isn't very meaningful

[graebm]

maybe with_size_hint() rather than "special" which isn't very meaningful

I also just encountered this term yesterday for the first time, but it came up in 2 different contexts! Apparently it's the official term: https://www.google.com/search?q=special+files

So I assumed it was something everyone but me already knew. But if you and me were both thrown by the term, then probably not.

So yeah, I'll change it

[codecov-commenter]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (d09b75e) 82.19% compared to head (7f8efed) 82.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
+ Coverage   82.19%   82.73%   +0.53%     
==========================================
  Files          52       52              
  Lines        5646     5664      +18     
==========================================
+ Hits         4641     4686      +45     
+ Misses       1005      978      -27     

Files	Coverage Δ
source/file.c	77.11% <63.15%> (+14.11%)	⬆️

... and 2 files with indirect coverage changes

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[waahm7]

Debatable / Code Clarity: There are two different ways to use this function, which is a bit confusing. I suggest we change this function to

Suggested change

	bool use_file_size_as_hint,
	size_t size_hint) {
	s_byte_buf_init_from_file(
	struct aws_byte_buf *out_buf,
	struct aws_allocator *alloc,
	const char *filename,
	size_t size_hint)

and let aws_byte_buf_init_from_file calculate size_hint.

[graebm]

I tried writing it that way first, but it was more confusing IMHO.

You ended up with 3 functions that needed to deal with error-handling instead of just 1, and scrambled logic around instead of putting it all in 1 place.