Fix WriteUTF8FileAtomic to preserve symlinks

[Don-Vito]

WriteUTF8FileAtomic overrides the content of the file "atomically"
by creating a temp file and then renaming it to the original path.
The problem arises when the original path is symbolic link,
as the link itself gets overridden by a file (rather than the link target).
This PR introduces a special handling of the symlinks:
if the path as a symlink we resolve the path and use:

1. target's directory to create a temp-file in
2. target itself to be replaced with the tempfile.

Symlink resolution is problematic when the target path does not exist,
as there is no good utility that resolves such link (canonical() fails).
In this corner case we skip the "atomic" approach of renaming the file
and write the link target directly.

Closes #10787

[github-actions]

@check-spelling-bot Report

Unrecognized words, please review:

• symlink

Previously acknowledged words that are now absent

DECDLD SPACEBAR Unregister

To accept these unrecognized words as correct (and remove the previously acknowledged and now absent words), run the following commands

... in a clone of the git@github.com:Don-Vito/terminal.git repository
on the 10787-preserve-settings-symlink branch:

update_files() {
perl -e '
my @expect_files=qw('".github/actions/spelling/expect/alphabet.txt
.github/actions/spelling/expect/expect.txt
.github/actions/spelling/expect/web.txt"');
@ARGV=@expect_files;
my @stale=qw('"$patch_remove"');
my $re=join "|", @stale;
my $suffix=".".time();
my $previous="";
sub maybe_unlink { unlink($_[0]) if $_[0]; }
while (<>) {
if ($ARGV ne $old_argv) { maybe_unlink($previous); $previous="$ARGV$suffix"; rename($ARGV, $previous); open(ARGV_OUT, ">$ARGV"); select(ARGV_OUT); $old_argv = $ARGV; }
next if /^(?:$re)(?:(?:\r|\n)*$| .*)/; print;
}; maybe_unlink($previous);'
perl -e '
my $new_expect_file=".github/actions/spelling/expect/c55888f88d6b59ae88afb50a806df0dec8fb3a3b.txt";
use File::Path qw(make_path);
use File::Basename qw(dirname);
make_path (dirname($new_expect_file));
open FILE, q{<}, $new_expect_file; chomp(my @words = <FILE>); close FILE;
my @add=qw('"$patch_add"');
my %items; @items{@words} = @words x (1); @items{@add} = @add x (1);
@words = sort {lc($a)."-".$a cmp lc($b)."-".$b} keys %items;
open FILE, q{>}, $new_expect_file; for my $word (@words) { print FILE "$word\n" if $word =~ /\w/; };
close FILE;
system("git", "add", $new_expect_file);
'
}

comment_json=$(mktemp)
curl -L -s -S \
  --header "Content-Type: application/json" \
  "https://github.com/gitapi/repos/microsoft/terminal/issues/comments/895478626" > "$comment_json"
comment_body=$(mktemp)
jq -r .body < "$comment_json" > $comment_body
rm $comment_json

patch_remove=$(perl -ne 'next unless s{^</summary>(.*)</details>$}{$1}; print' < "$comment_body")
  

patch_add=$(perl -e '$/=undef;
$_=<>;
s{<details>.*}{}s;
s{^#.*}{};
s{\n##.*}{};
s{(?:^|\n)\s*\*}{}g;
s{\s+}{ }g;
print' < "$comment_body")
  
update_files
rm $comment_body
git add -u

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^ refers to the file's path from the root of the repository, so ^README\.md$ would exclude README.md (on whichever branch you're using).

[lhecker]

@eryksun What's the default value of std::error_code?
Doesn't this mean if (ec) is always true if it's not a symlink, forcing non-atomic writes for non-symlink targets?

[Don-Vito]

@eryksun What's the default value of std::error_code?
Doesn't this mean if (ec) is always true if it's not a symlink, forcing non-atomic writes for non-symlink targets?

@lhecker - the default for error code is 0 (success). Which means that if (ec) is always false for non-symlinks.

[lhecker]

@Don-Vito std::error_code defines an operator bool which returns true if the value is 0.

[eryksun]

@Don-Vito std::error_code defines an operator bool which returns true if the value is 0.

It's the other way around. It's true when there's a non-zero error value and otherwise false: std::error_code operator bool "returns a value convertible to true only if value() is not equal to zero".

[lhecker]

Ah thanks!

[Don-Vito]

You are right 😄 and this is exactly what I meant when saying that initial error code is 0 (success). Which means that if the symlink was not invoked then the value remains success, and thus if(ec) returns false. I just didn't mention is that this conversion done by bool() operator. And of course I also tested all these scenarios after each code change.

[DHowett]

I'm v. cool with this; thanks for doing it! 😄

[DHowett]

@msftbot merge this in 5 minutes

[ghost]

Hello @DHowett!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Thu, 12 Aug 2021 16:46:53 GMT, which is in 5 minutes

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[DHowett]

Also? Welcome back! 👏🏻

[Don-Vito]

I'm v. cool with this; thanks for doing it! 😄

All the credit goes to @lhecker and @eryksun :)

[Don-Vito]

Also? Welcome back! 👏🏻

Thanks! :) Hope to finally get some vacation soon (aka best time for coding), so I am warming up :)

[ghost]

🎉Windows Terminal Preview v1.10.2383.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download

[ghost]

🎉Windows Terminal Preview v1.11.2421.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download